Ttooc Z TA245.7 B873 13-1727 NO.1727 498 =3‘ lton Soils: Distribution, Importance, Variabi ity, and Management The Texas Agricultural Experiment Station Edward A. Hiler, Director College Station, Texas in cooperation with i? United States Department of Agriculture Agricultural Research Service _ Soil Conservation Service Contents Introduction ............................................................................................................................................................ .. 1 Area Occupied by Olton Soils .......................................................................................................................... .. 1 p Objectives 0f the Study .................................................................................................................................... .. 1 ‘ History of the Olton Series .............................................................................................................. ............... .. 3 Physiography .................................................................................................................................................... .. 3 Uses and Importance of Olton Soils ................................................................................................................ .. 6 Typical Site for Olton Soils ..................................................................................................... ........................ .. 6 Present Water Management Systems ...................................................................................... ....................... .. 6 Experimental Procedure ........................................................................................................................................ .. 7 Site Selection .................................................................................................................................................... .. 7 Sampling Techniques ....................................................................................................................................... .. 7 Sample Preparation and Analyses .................................................................................................................... .. 8 Results and Discussion ........................................................................................................................................... .. 9 Profile Descriptions ......................................................................................................................................... .. 9 Particle Size Distribution ................................................................................................................................ .. 15 Bulk Density .................................................................................................................................. ............... .. 18 Organic Matter ............................................................................................................................................... .. 19 Calcium Carbonate (CaCO3) Equivalent ......................................................................................................... .. 19 pH ................................................................................................................................................................... .. 19 Cation Exchange Capacity (CEC) ................................................................................................................... .. 20 Water Retention .............................................................................................................................................. .. 2O Water Infiltration ............................................................................................................................................ .. 21 Implications for Management ............................................................................................................................... .. 23 Plant-Available Water (PAW) ........................................................................................................................... .. 23 Water Application .......................................................................................................................................... .. 25 Water Infiltration Variation ............................................................................................................................ .. 25 Crop Sequences .............................................................................................................................................. .. 26 Tillage and Cropping Practices ...................................................................................................................... .. 26 Ranching and Livestock Production .............................................................................................................. .. 28 Summary ............................................................................................................................................................... .. 30 Acknowledgment .................................................................................................................................................. .. 31 Literature Cited ..................................................................................................................................................... .. 31 ii Olton Soils—Distribution, Importance, Variability and Managementl Paul W. Unger and Fred B. Pringle Soil Scientists USDA, Agricultural Research Service, Bushland, Texas and Introduction Area Occupied by 0lton Soils Olton soilsz have been mapped extensively in twelve counties in the Southern High Plains of Texas and one county in eastern New Mexico (Figure 1) and to a lesser extent in three additional counties in Texas and one county in New Mexico (Fig- ure 2; Tables 2 and 3). The portions of different counties occupied by Olton soil range from 1.2 to 46.4 percent (Table 2). The area of Olton soils is bounded by the area of Pull- man soils on the north and east, Yellowhouse Creek on the south, the Caprock escarpment at the High Plains-Rolling Plains boundary on the southeast, and Blackwater Draw from near Earth, Texas, to Portales, New Mexico, on the southwest. Within this roughly crescent shaped area, Olton soils occupy about 19 percent of the land surface. The area of Olton soils ranges from about 101°10' to 105°50' West longitude and about 32°50‘ to 35°40‘ North latitude. Elevation of the sur- face of Olton soils ranges from about 2,600 to 4,200 feet above mean sea level. The area is in a semiarid climatic zone where average annual precipitation ranges from about 17 inches at the western edge to about 21 inches at the eastern edge (Table 5). Also listed in Table 5 are the average length and dates of the frost-free period, average daily maxi- ‘Contribution from the USDA, Agricultural Research Service, P.O. Drawer 10, Bushland, Texas 79012, and the USDA, Natural Resources Conservation Service, Amarillo, Texas 79121. 2 See Table 1 for classification of soils men- tioned in this report. USDA, Soil Conservation Service, Amarillo, Texas mum and minimum temperatures, and average precipitation in coun- ties in which Olton soils are found. Olton soils occupy about 1.26 mil- lion acres of land (Table 2) and are among the most extensive arable soils in Texas. Olton soils also oc- Figure 1. The approximate areas where Olton delineated by the solid line. The approximate locations of sampling sites are indicated by the numbered dots. cupy a small area of New Mexico. Other major arable soils in Texas are Pullman with 5.8 million acres, Amarillo with 2.5 million acres, and Houston Black with 1.5 million acres. soils have been mapped extensively are NORA PWIDIHG to: lures SANTA FE SM MIGUEL [LO GUADALUPS l IORRMICE 0E 848A UHCOLR CHAVES ailkuir-nn 11ml; TSHERMM HANSFORO OCHILTREE LIPSCOMB g UALLMI ' I I mun noon: nurcmnsou seams nsuwmu i OUAY _} f locum POTTER unsou wnzatn I I I GRAY I our sauna musraouc DONLEY ~¢=~~*~=*-="~l . I m. WON-l a I I 9mm CASTRO SWIS-‘IER aalsco: luu muons: 1 ' _ K. Roosmtr : wan was m: rtom norm com: “mm” I tom wu cocmu. nocxtn tuaaocx cnosar olcxzus mar: i m°""'\x w I vwuu mar mm mu um sroumu. mxzu ""'< I cunts oawsou aonosu um FISHER routs “"1" I I l TE A l I moms mun aowuzo uncuzu um: _! noun ranoa scion umuna swscocx srzntmc cox: nuunns comm WIPIKIIR _ cam: uPIoa 3mm A | ‘ton emu mo“ coacao blcfil rzcos \\_\ caocxzrr I SCHLEICHER umao I Figure 2. Counties of Texas and New Mexico in which Olton soils have been mapped are within the solid, heavy- lined area. Objectives of the Study Olton soils vary considerably in profile properties across the area. One property is depth to the calcic horizon, which varies from 34 to 54 inches. Because profile depth strong- ly influences plant rooting depth and thus the effective depth for storing Water, a knowledge of profile depth along With a characterization of other profile properties is important for improved water and crop man- agement. The objective of this study Was to determine the variation in depth, texture, bulk density, organic matter content, calcium carbonate Table 1. Classification of soils mentioned in the text and figures. Series Classification Acuff Fine-loamy, mixed, superactive, thermic, Aridic Paleustolls Amarillo Fine-loamy, mixed, superactive, thermic, Aridic Paleustalfs Berda Fine-loamy, mixed, superactive, thermic, Aridic Ustochrepts Bippus Fine-loamy, mixed, superactive, thermic, Cumulic Haplustolls Drake Fine-loamy, mixed (calcareous), thermic, Typic Ustorthents Estacado Fine-loamy, mixed, superactive, thermic, Calciargidic Paleustolls Houston Black Fine, montmorillonitic, thermic, Udic Pellusterts Mansker Fine-loamy, carbonatic, thermic, Calciargidic Paleustolls Mobeetie Coarse-loamy, mixed, superactive, thermic, Aridic Ustochrepts Olton Fine, mixed, superactive, thermic, Aridic Paleustolls Pep Fine-loamy, mixed, superactive, thermic, Aridic Calciustolls Posey Fine-loamy, mixed, thermic, Calciorthidic Paleustalfs Potter Loamy, carbonatic, thermic, shallow, Ustic Haplocalcids Pullman Fine, mixed, superactive, thermic, Torretic Paleustolls Randall Fine, smetitic, thermic, Ustic Epiaquerts Richfield Fine, smetitic, mesic, Aridic Argiustolls L35. Table 2. Areas occupied by Olton soil. Mapping Portion Total series Total Irrigated County, State Slope unit area of County area‘? cropland“ cropland Rangeland Other land‘ % acres % acres Bailey, TX 0-1 6,310 1.2 6,310 5,670 710 470 170 Briscoe, TX 0-1 4,4406 0.7 11,380 2,280 540 2,050 110 1-3 6,940 1.2 3,540 850 3,360 140 Castro, TX 0-1 132,030 23.4 163,930 104,300 68,840 24,430 3,300 1-3 31,230 5.5 24,670 16,280 5,780 780 3-5 670 0.1 660 10 Crosby, TX 0-1 109,810 18.8 124,770 87,850 45,680 19,220 2,750 1-3 14,960 2.6 11,970 6,220 2,620 370 Curry, NM 0-1 80,0006 8.9 120,000 63,200 32,860 14,800 2,000 1-3 40,0006 4.4 31,600 16,430 7,400 1,000 Deaf Smith, TX 0-1 44,1506 4.6 67,930 35,760 18,600 7,290 1,100 1-3 23,7706 2.4 19,260 10,210 3,930 590 Floyd, TX 0-1 10,450 1.6 21,280 8,360 3,350 1,830 260 1-3 10,830 1.7 8,660 3,470 1,900 270 Garza, TX 0-1 55,9206 9.6 59,860 50,880 4,070 3,540 1,500 1-3 3,9406 0.7 3,350 270 510 80 Hale, TX 0-1 84,600 13.5 125,500 63,450 38,070 19,030 2,120 1-3 40,900 6.5 30,680 18,410 9,200 1,020 Hockley, TX 0-1 21,100 3.7 21,100 16,880 8,780 3,690 530 Lamb, TX 0-1 75,680 11.5 81,080 77,780 39,940 900 2,040 1-3 5,400 0.8 2,180 2,860 220 150 Lubbock, TX 0-1 109,420 19.2 123,850 84,530 38,040 19,510 3,720 1-3 14,430 2.5 8,360 3,340 5,500 1,240 Lynn, TX 0-1 7,1306 1.2 9,500 5,920 1,270 1,150 130 1-3 2,3806 0.4 2,510 380 560 60 Parmer, TX 0-1 227,840 41.4 225,280 195,940 166,080 26,200 5,700 1-3 27,440 5.0 23,600 17,840 3,160 690 Randall, TX 0-1 10,120 1.7 25,430 4,770 1,550 5,100 250 1-3 14,150 2.4 4,480 790 9,320 350 3-5 1,160 0.2 140 1,000 20 Roosevelt, NM 0-1 24,680 1.6 24,680 8,140 2,040 16,160 370 Swisher, TX 0-1 4,900 0.8 20,270 3,870 1,240 910 120 1-3 15,380 2.7 8,610 2,760 6,380 380 Total 1,262,160 1,262,160 1,003,190 571,770 227,780 33,320 1 Includes total area for all slopes and conditions. 2Totals for the different slopes and conditions may not equal the total for the series because values are rounded to the nearest 10 acres. 6 includes land in Conservation Reserve Program (CRP). 4 Includes land in roads, towns, and other nonagricultural uses. 5 Calculated from General Soils Map. 6 Tentative. equivalent, pH, cation exchange ca- pacity, and water retention of the dif- ferent horizons of Olton soils as af- fected by location in the region. Water infiltration at locations se- lected also Was determined. The data obtained are discussed relative to managing the soil for efficient and effective water use for optimum crop production. History of the Olton Series The Olton series was established in the Soil Survey of Lamb County, Texas, in 1960 (Soil Conservation Service, 1962). It was named after the town of Olton in Lamb County. Before 1960, Olton soils were in- cluded in other series, mainly the Amarillo, Pullman, and Richfield series. The processes of inventory- ing and classifying soils on the High Plains began with publication of the Reconnaissance Soil Survey of the Panhandle Region of Texas in 1910. In this survey, Olton soils were called Richfield clay loam. The Richfield series was established in this survey and included all nonred- dish silty clay loams and clay loams on the Southern High Plains. As soil surveys and investigations continued, differences in the physi- cal and chemical properties of soils were noted. This led to the recog- nition of other soil series. Early soil surveys of Dickens, Lubbock, and Wheeler counties of the Texas High Plains included these soils in both the Amarillo and Richfield series. Further investigations plus the implementation of soil taxonomy re- sulted in refinements to series cri- teria. The Olton series was estab- lished for those soils of the South- Table 3. Elevation and climatic features in counties having Olton soils. Average annual Average daily Average lake Average tern erature‘ annual County, state, station Elev. evaporation growing season Max. Min. precip.‘ _fi_ in _d_ays_ period °F in ___ Bailey, TX, Muleshoe 3760 69 187 Apr 17-Oct 21 72.3 40.9 16.84 Briscoe, TX, Silverton 3280 68 189 Apr 15—Oct 20 71.0 42.6 21.67 Castro, TX, Dimmitt 3860 — 178 Apr 23-Oct 17 71.3 40.4 17.94 Crosby, TX, Crosbyton 3010 — 209 Apr 3—Oct 29 73.1 45.3 22.57 Curry, NM, Clovis 4290 — 192 Apr 16-Oct 24 71.4 42.2 17.44 Deaf Smith, TX, Hereford 3820 67 185 Apr 18—Oct 20 70.6 41.5 17.17 Floyd, TX, Floydada 3220 69 203 Apr 10-Oct 29 72.1 44.2 20.44 Garza, TX, Post 25502 71 225 Mar 29—Nov 9 75.1 48.1 20.92 Hale, TX, Plainview 3370 69 206 Apr 3-Oct 26 72.1 45.3 19.96 Hockley, TX, Levelland 3550 70 200 Apr 10-Oct 27 73.8 43.2 19.47 Lamb, TX, Littlefield 3510 70 200 Apr 7-Oct 24 72.4 43.6 19.07 Lubbock, TX, Lubbock 3250 69 210 Apr 5—Oct 31 73.4 46.7 18.66 Lynn, TX, Tahoka 3120 69 213 Apr 2—Nov 1 74.3 45.6 19.72 Parmer, TX, Friona 4030 68 187 Apr 16-Oct 20 70.9 41.7 16.88 Roosevelt, NM, Elida 4350 72 178 Apr 21-Oct 16 72.2 42.1 14.52 Swisher, TX, Tulia 3500 68 205 Apr 10—Nov 1 72.9 42.6 17.24 1 Average values for monthly maximum and minimum temperatures and precipitation are available in most published soil surveys. 2 Recording station not located in Olton series area of occurrence. ern High Plains that have dark col- ored surfaces and reddish brown subsoils with clay contents ranging from 55 to 40 percent, and a mean annual soil temperature greater than 59°F at a 20-inch depth. Physiography The topography consists of nearly level to gently sloping, smooth, tree- less plains (Figure 3). Surfaces are plane to convex and slopes range from 0 to 5 percent, but are mainly 0 to 2 percent. These broad plains are interrupted only by the numerous playas, or shallow lakes, containing other soils. Except for playas, the sur- face is remarkably smooth. Playas range from a few square yards to sev- eral square miles in surface area, and from a few inches to more than 50 feet in depth. The average grade of the High Plains is about 10 feet per mile to the southeast. Runoff follows a poorly defined pattern. Water flows mainly into the playas, from which there is rarely an outlet. Much of the water collected in shallow playas is lost by evaporation, but some is used for irrigation. In deeper playas, wa- ter percolates to depths greater than 80 inches and perhaps adds small amounts of water to the underlying aquifer. Other soils associated with the Olton series in its area of occurrence include Acuff, Amarillo, Drake, Estacado, Mansker, Pep, Pullman, and Randall (Figures 4, 5, 6, and 7). Acuff soils are similar in appearance to Olton but have less than 35 percent clay in the upper 20 inches of the subsoil. They occur on the same gen- eral landscape and in close associa- tion with Olton soils, but surfaces are slightly more convex. Amarillo soils have light colored, typically fine sandy loam surfaces and are found on Figure 3. Aerial photo showing the typical topography of the Olton soil region. 4 i: ‘J4 - .. “~13 mfifiskelr“ / ;-¢-—— i ‘**% u —__--‘,¢ -,_3 p’ ‘F! Pullman Figure 4. Illustration of the major soils and underlying formations in the area occupied by Olton soils. Reddieh Brown Clay Loom unconsolidated ""-‘=‘-.-._-_ * , calcareous _ ‘ ‘j'--_.__ Sands. Silt: j. . and Clays ~ U Yeliovrish Red Sandstorm Figure 5. Soil pattern on erosional surfaces near major drainageways in the northern area of occurrence for Olton soils. Reddish Brown - Clay LoomT __ 2.2; Unconsolidolcd .-; _ - _. -\Z.:-r-3"\~‘-'~;;. Sands. Sills ' ._ -- ‘ - -._ ,-— .---"" ** " l \ I - 0nd Clays I '1; _ /.- ‘.044,’ ".- tw I '3 a117,, a jjljs-Qk-i Figure 6. Typical soil pattern in the central area of occurrence for Olton soils. the same general landscape and in close association with Acuff soils. Estacado, Mansker, and Pep are cal- careous, loamy soils 0n low convex ridges, 0n sideslopes around playas, and along draws. Pullman soils are on nearly level smooth plains and are similar in appearance to Olton. How- ever, the Bt horizon of Pullman soil is dark brown, has a clay texture, and is less permeable. Randall soils are dark gray, have clay textures throughout, and occur on playa bottoms. Drake soils are weakly developed, calcare- ous, have loam or clay loam textures, and are more permeable. They occur on convex knolls or crescent-shaped dunes on the eastern rims of playas. Differences in morphological properties of the Olton series are re- lated to geographic location and land- scape position. These differences determine water storage capacity, which in turn affects water manage- ment on these soils. The morphologi- cal features are depth to strong calcic horizon [>30 percent calcium carbon- ate (CaCOQL depth to a layer of strongly contrasting material, soil tex- ture, and permeability. An analysis of soil survey field notes for nine coun- ties and additional recent profile ob- servations revealed that depth to a strong calcic horizon ranges from 34 to more than 54 inches. Observations by soil and plant scientists indicate that calcic horizons containing at least 3O percent lime (CaCO3) inhibit root development of most crops. Based on laboratory determinations using a simple volume calcimeter, the aver- age CaCOs content in the Btk hori- zon of Olton soils is about 45 percent. To present a clearer understand- ing of these soils as they relate to geo- graphic location, it is convenient to describe the landscape positions on which they are found. In the north- ern part of the region, Olton soils are on erosional plains adjacent to ma- jor drainageways that dissect the High Plains. These include Frio Draw, T ierra Blanco Creek, and Tule Creek in Castro, Deaf Smith, Parmer, Randall, and Swisher Counties in Texas. Also included are erosional surfaces along the margins of the High Plains in Briscoe, Randall, and Swisher Counties. Slopes are nearly level to gently sloping (Figure 5), and surfaces are smooth and slightly con- vex. In this part of the High Plains, Olton soils are intermingled with ar- eas of Estacado, Mansker, Pep, Pull- man, and Randall soils. Pullman soils occupy the same general landscape position as Olton soils, although the surfaces of Pullman are less sloping. Estacado, Mansker, and Pep soils are on low convex ridges, sideslopes around playas, and along draws. Within the area of occurrence, Olton soils comprise about 35 percent of the total area; Pullman soils comprise Hlgh Plain: Eollan Month 1,,“ - . i; ~ Unconsolidotod -.'~ ' Colcaroous '_ _- .. *-T'--.'f-=.-. Sands, Slit: W‘ _ - ',' .,_. b and Clays ~.‘ Recent Eolion Dune - _ iv’): I, Soft Caliche and Gray C 0y: Figure 7. Typical soil pattern in the western area of occurrence for Olton soils. 35 percent; and Estacado, Mansker, Pep, and Randall soils comprise the remaining 3O percent. Where Olton soils are closely associated with Pull- man soils, their clay contents of the upper subsoil are similar. They range from 35 to 42 percent and average about 39 percent. The depth to a strong calcic horizon averages about 49 inches. The CaCOg content of this layer ranges from 35 to 56 percent and averages about 45 percent. The primary area of occurrence for Olton soils includes the area south and west of a line extending from the High Plains escarpment on the Texas-New Mexico state line to White River Lake in Crosby County, Texas. It includes parts of Bailey, Crosby, Deaf Smith, Garza, Hale, Hockley, Lamb, and Lub- bock Counties in Texas. Also included are parts of Curry and Roosevelt Coun- ties in New Mexico. Slopes of Olton soils in this region are nearly level to gently sloping. The smooth surfaces are planar to slightly convex (Figures 6 and 7). Olton soils comprise about 25 percent of the total area. The re- mainder is mainly Acuff, Amarillo, Drake, Estacado, Mansker, Pep, Posey, and Randall soils. Acuff, Amarillo, and Estacado soils are on smooth plains with slightly convex surfaces. Mansker, Pep, and Posey soils are on sideslopes around playas and along draws. Drake soils are on low Aeolian dunes on the eastern rims of playas. Randall soils are on playa floors. When Olton soils are closely associated with Acuff soils, clay contents of the upper subsoil range from 33 to 40 percent and average about 36 percent. Surface textures of some pedons reflect the influence of winnowing by wind erosion. The depth to a strong calcic ranges from 34 to 54 inches, averaging about 43 inches. Calcium carbonate content of the calcic horizon ranges from about 35 to more than 55 percent and aver- ages about 42 percent. Uses and Importance of Olton Soils Olton soils are used primarily for agriculture with about 79 percent of their area being used for crop pro- duction and about 18 percent in rangeland. The remaining area is in roads, towns, and other nonagricul- tural uses. Of the cropland area, about 45 percent is irrigated and 55 percent is dryland (Table 2). The area of irrigated Olton soil represents about 11 percent of all irrigated crop- land in Texas (Texas Dept. Agric., 1995). Cotton (Gossypium hirsutum L.), grain sorghum [Sorghum bicolor (L.) Moench], corn (Zea mays L.), and wheat (Triticum aestivum L.) are the major field crops (Texas Dept. Agric., 1995). Other crops grown on smaller areas are oat (Avena sativa L.), barley (Hordeum vulgare L.), sugar beet (Beta vulgaris L.), soy- bean (Glycine max L.), forage sor- ghum (Sorghum sp.), alfalfa (Medic- ago sativa L.), sunflower (Helian- thus annuus L.), and vegetables. Because Olton soils are located in a semiarid region, yields of dryland crops are relatively low. Irrigation from the Ogallala Aquifer greatly in- creases yields, but the water supply is limited and is being depleted. Al- though the cost of energy for pump- ing water has decreased since the mid 1980s, it is still a major variable crop production cost. Surface water for ir- rigation is negligible. It is, therefore, essential that water be used as effi- ciently as possible so that economic 6 crop production can be maintained and the eventual return to dryland crop production can be delayed as , long as possible. When dryland farm- ing replaces irrigated farming, even if only on the Olton soils, a signifi- cant percentage of the total irrigated crop production in Texas will be lost. Typical Site for Olton Soils Olton soils developed in a rela- tively cool, subhumid to semiarid cli- mate from medium-textured sedi- ments largely or entirely of Aeolian origin. They occupy smooth areas that are nearly level to gently slop- ing. Surface slopes range from 0 to about 5 percent toward the playas or shallow basins, and on erosional sur- faces along draws. Although largely cultivated, the typical native vegeta- tion on Olton soils was short- grasses, principally blue grama (Bouteloua gracilis) and buffalograss (Buchloe dactyloides). Profiles of Olton soil are shown in Figures 8 and 9. The surface layer of a typical Olton soil is a brown to dark brown clay loam, but the texture ranges from loam to sandy clay loam. Thick- ness of the surface layer usually ranges from 6 to 8 inches, at which depth there is a clear boundary to a brown, reddish brown, or dark red- dish brown clay loam with moder- ate blocky structure. The soil may contain buried horizons of older soils at 3 to 5 feet below the surface. The buried horizons usually have a clay loam or sandy clay loam texture. The upper boundary of the calcic layer is clear and wavy. Although depth to the calcic layer often is con- sidered the effective depth of a soil for crop production purposes, win- ter wheat and especially sunflower use water from well into the calcic layer, based on observations and measurements on a similar soil (Pull- man clay loam) at Bushland, Texas (O. R. Jones, Bushland, Texas, un- published data; Unger, 1978a). Present Water Management Systems It is desirable to have a soil filled to capacity with water at planting time, especially for dryland crops, because 0f limited and erratic pre- cipitation during the growing sea- son in semiarid regions such as where the Olton soils occur. When the soil is filled to capacity at plant- ing time, crops usually experience less water stress during the grow- ing season than when it contains a limited amount of water. Crop yields usually are higher when growing season water stress is not severe. Based on data from the Texas A&M University Agricultural Research and Extension Center at Lubbock—Half- way and values published in soil sur- veys of Hale and Lubbock Counties, l. Figure 8. A relatively deep Olton soil profile from Hale County, Texas (USDA-NRCS photo). Texas (Soil Conservation Service, 1974, 1979), the Olton soil at these locations has a total water storage capacity of about 10.1 and 15.4 inches to 5- and 4-foot profile depths, respectively. Of the total, about 5.4 and 7.2 inches, respectively, are avail- able for plant use. The remainder (4.7 and 6.2 inches to 3- and 4-foot depths, respectively) is held at tensions (en- ergy levels) greater than those at which plants can extract the water. Although irrigation can provide water to crops, soil water content at planting is still important because any water stored from precipitation re- duces the amount required from irri- gation. When water storage from pre- cipitation is low, a preplant or emer- gence irrigation often is used to in- crease the soil water content. Be- cause the Olton soil is moderately slowly permeable, relatively short periods of water application must be used to avoid incurring deep drain- age losses. With furrow irrigation (Figures 1O and 1 1), considerable tailwater runoff is usually permitted so that adequate water is stored at the lower end of the field. Unless an ef- fective tailwater recovery system (Fig- ure 12) is used, tailwater runoff re- duces the efficiency of water use. In recent years, many center-pivot sprinkler systems (Figure 15) have been installed on Olton soils. These systems, when properly designed and operated, reduce runoff amounts compared to furrow irrigation, but require considerably more energy in- put than furrow systems. With all farming systems on both dryland and irrigated land, a knowledge of the water-holding capacity of the soil pro- file (Figure 14) is important for effec- tive water management. Experimental Procedure Site Selection To obtain samples that would represent a near-complete range in the expected variation in soil prop- erties, sites were selected at 12 widely separated locations across the region. The sampling sites were in Castro, Crosby, Hale, Lubbock, and Parmer Counties in Texas; and Curry County in New Mexico (Figure 1). Although the locations were widely separated, samples were not obtained near the margins of the region to avoid zones of transition to other soils. Likewise, locations of transition to other soils within the region were avoided. Sam- pling was restricted to “typical” Olton soil sites for the particular region. Brief descriptions of the locations are given with the profile descriptions in the Results and Discussion section. Sites 2, 4, 5, 6, 7, 8, 9, 10, 11, and 12 were in irrigated fields, and Sites 1 and 3 were in dryland fields. All sites ~ -=s;= L‘ k Figure 9. A relatively shallow Olton soil profile from Hale County, Texas (USDA- NRCS photo). were 0n nearly level uplands 0f the High Plains. Sampling Techniques At each site, loose soil 0f the plow layer, usually to the depth 0f the Ap horizon, was removed before ob- taining core samples with a hydrau- lically-operated, pickup-mounted core sampler. The inside diameter of the cutting tip was 1.625 inches. Two cores at each location were used for profile description. Several other cores were taken and sepa- rated into depth segments based on thickness of the different horizons to provide adequate material from each depth for determining bulk density, particle size distribution, organic matter content, pH, and cat- ion exchange capacity. Also, two bulk samples of surface soil were collected at each site. At a different time, three water infiltration deter- minations were made at each site using recorder-equipped, constant head, double-ring infiltrometers (Haise et al., 1956). The rings were seated into the most restrictive sub- surface layer, and a 1.5-inch head of \ water was maintained for the dura- tion of the test. Water surfaces were covered to prevent evaporation. Placement of individual infiltro- meters Was determined after exam- ining the field to determine tillage zone conditions at the time of test- mg. Sample Preparation and Analyses Bulk density was determined by drying the cores at 105°C, then Weighing them. Soil from these cores was retained and ground to pass a 2- mm sieve. Subsamples of this sieved soil were then used to determine or- ganic matter content by the Walkey- Black method (Jackson, 1958), pH (1 :1 soil:water ratio), particle size dis- tribution (mechanical analysis) by the hydrometer method (Day, 1965), and cation capacity by the USDA-SCS pro- cedure (Soil Conservation Service, 1984). Sand retained from the par- ticle size distribution analyses was subsequently sieved to determine the size distribution of the sand fraction. Soil water retention at -1/3 and -15 bars matric potentials was calculated by equations developed by Unger (1975). Samples of surface soil were air- dried, ground, and passed through a 2-mm sieve. Subsamples of the soil were used to determine particle size distribution, organic matter con- tent, pH, cation exchange capacity, and water retention by the methods outlined above. Relationships among various Ap, Btl, and Bt2 horizon characteristics; total water infiltration in 1O minutes and 20 hours; and infiltration rates at these times were investigated by mul- tiple linear regression analyses. Hori- zon characteristics investigated were thickness; sand, silt, clay, and organic matter content; and bulk density. For Ap, Btl, and Bt2 horizons, actual val- ues were used, except that densities of Ap and Btl horizons were mea- sured in the field where infiltration reduce furrow erosion. Figure 10. Furrow irrigation through gated pipes with ‘socks’ attached to the gates to ‘A w. “ob Figure 11. Furrow irrigation from an open ditch using syphon tubes. Water losses often are high when open ditches are used to deliver the water. measurements were made. Besides the partial regression coefficients and the coefficient of correlation (R2), standardized partial regression coef- ficients and t-values were also calcu- lated (Ezekial and Fox, 1959; Steel and Torrie, 1960). Based on the standard- ized coefficients, the independent variables were ranked numerically in order of their relative importance for influencing total infiltration or infil- tration rates. All independent vari- ables were used in the initial analysis for each set of data. In subsequent analyses, the lowest-ranking variable was excluded, which resulted in the last analysis being a simple linear re- gression analysis if only one variable was significant. Results and Discussion Profile Descriptions This section describes the pro- files at the 12 sites and their loca- tions. The descriptions are based on examination and determinations made in the field immediately after extracting the cores. Although data in subsequent sections are based mainly on horizons above the cal- cic horizon, the calcic horizon is included in the profile descriptions. Site N0. I Soil Type: Olton loam Location: Curry County, New Mexico; in a cultivated field 150 feet south of paved county road, 3.6 miles west and 6.0 miles north of N.M. Highway 241 at Bellview, 3.7 miles west of the Texas/New Mexico State line. Pedon description: Sample No. S9ONM009-1-(1-5) Ap-O to 8 inches; brown (7.5YR 4/2) loam, dark brown (7.5YR 3/2) moist; weak medium subangular blocky structure; slightly hard, friable; many fine and medium roots; common fine and medium pores; neu- tral; abrupt smooth boundary. Bt1—8 to 23 inches; brown (7.5YR 4/3) clay loam, dark brown (7.5YR 3/3) moist; moderate medium blocky structure; very hard, very firm; common fine roots; few fine pores; thin continuous clay films; neutral; gradual smooth boundary. Bt2—23 to 31 inches; brown (7.5YR 5/4) clay loam, dark brown (7.5YR 4/4) moist; moderate medium blocky structure; very hard, very firm; few fine roots; few fine pores; thin continuous clay films; few threads and films of calcium carbonate; calcareous; mildly alkaline; gradual smooth boundary. Bt3—31 to 38 inches; reddish brown (SYR 5/4) clay loam, yel- lowish brown (SYR 4/4) moist; moderate medium blocky struc- ture; very hard, very firm; few fine roots; few fine pores; thin continuous clay films; few threads, films, small concre- tions of calcium carbonate; cal- careous; mildly alkaline; clear smooth boundary. Btk—38 to 8O inches; pink (SYR 8/4) clay loam; pink (SYR 7/4) moist; moderate medium blocky structure; very hard, Figure 12. A lake pump in the tailwater recovery pit recycles runoff water to the cropland (USDA-NRCS photo). Figure 13. A sprinkler irrigation system equipped with drop hoses that deliver the water close to the soil surface, thus reducing evaporation. friable; common fine pores; about 53 percent of soil mass consists of soft masses and concretions of calcium carbon- ate; calcareous; moderately al- kaline. Site N0. 2 Soil Type: Olton loam Pedon description: Sample No. S90NMOO9-2-(1-5) Ap-O to 7 inches; brown (7.5YR 4/3) loam, dark brown (7.5YR 3/3) moist; Weak medium subangular blocky structure; slightly hard, friable; many fine and medium roots; common fine and medium pores; neu- tral; abrupt smooth boundary. Bt1—7 to 2O inches; brown (7.5YR 4/3) clay loam, dark brown (7.5YR 3/3) moist; moderate medium blocky structure; very hard, very firm; common fine roots; few Location: Curry County, New Mexico; in a cultivated field 2150 feet west of paved county road, 14.2 miles north of U.S. Highway 84, and 1.1 miles west of the Texas/New Mexico State line in Texico, New Mexico. 1O fine pores; thin continuous clay films; neutral; gradual smooth boundary. Bt2—2O to 28 inches; reddish brown (SYR 5/4) clay loam, reddish brown (7.5YR 4/4) moist; moderate medium blocky structure; very hard, very firm; few fine roots; few fine pores; thin continuous clay films; few threads and films of calcium carbonate; calcareous; mildly alkaline; gradual smooth boundary. Bt3—28 to 39 inches; yellowish red (SYR 5/6) clay loam, yel- lowish red (SYR 4/6) moist; moderate medium blocky structure; very hard, very firm; few fine roots; few fine pores; thin continuous clay films; few threads, films, small concretions of calcium carbon- ate; calcareous; mildly alka- line; clear smooth boundary. Btk—39 to 7O inches; pink (SYR 8/4) clay loam; pink (SYR 7/4) moist; moderate medium blocky structure; very hard, fri- able; common fine pores; about 53 percent of soil mass consists of soft masses and concretions of calcium carbonate; calcare- ous; moderately alkaline. Site N0. j’ Soil Type: Olton clay loam Location: Parmer County, Texas; in a cultivated field 700 feet south of unpaved county road, 0.7 miles west of Farm Road 2013, 7.4 miles northwest of its inter- section with Farm Road 1731, 9.7 miles west of Friona. Pedon description: Sample No. S9OTX569-1-(1-5) Ap-O to 6 inches; brown (7.5YR 4/2) clay loam, dark brown (7.5YR 3/2) moist; weak me- dium subangular blocky struc- ture; slightly hard, friable; many fine and medium roots; common fine and medium pores; neutral; abrupt smooth boundary. WATER-HOLDING cmmcnrv (Inches/foot of soil depth) SAND FINE SAND LOAM FINE SANDY SANDY LOAN 2 h: ga-i >5 ,>-: >- < ..|< <4 << <4 4 O —0 2.10 4g mug -| .1 w-l 40-! 0.: =0; O SOIL TEXTURE Figure 14. Typical water-holding capacities of soils with different textures (adapted from USDA, 1955). Bt1—6 t0 16 inches; brown (7.5YR 4/3) clay loam, dark brown (7.5YR 3/3) moist; moderate medium blocky structure; very hard, very firm; common fine roots; few fine pores; thin continuous clay films; neutral; gradual smooth boundary. Bt2—16 to 25 inches; reddish brown (5YR 5/ 5) clay loam, reddish brown (5YR 4/3) moist; moderate medium blocky structure; very hard, very firm; few fine roots; few fine pores; thin continuous clay films; few threads and films of calcium carbonate; cal- careous; mildly alkaline; grad- ual smooth boundary. Bt3—25 to 41 inches; yellowish red (5YR 5/6) clay loam, yel- lowish red (5YR 4/6) moist; moderate medium blocky structure; very hard, very firm; few fine roots; few fine pores; thin continuous clay films; few threads, films, small concretions of calcium carbon- ate; calcareous; mildly alka- line; clearsmooth boundary. Btk—41 to 72 inches; pink (5YR 8/4) clay loam, pink (5YR 7/4) moist; moderate medium blocky structure; very hard, fri- able; common fine pores; about 57 percent of soil mass consists of soft masses and concretions of calcium carbonate; calcare- ous; moderately alkaline. Site N0. 4 Soil Type: Olton clay loam Location: Parmer County, Texas; in a cultivated field 1800 feet south of unpaved county road, 1.9 miles west of Texas Hwy 214, 8.0 miles south of Friona. Pedon description: Sample No. S90TX369~2-(1-5) Ap-O to 8 inches; brown (7.5YR 4/3) clay loam, dark brown (7.5YR 3/3) moist; weak me- dium subangular blocky struc- ture; slightly hard, friable; many fine and medium roots; common fine and medium pores; neutral; abrupt smooth boundary. Bt1—8 to 24 inches; brown (7.5YR 4/3) clay loam, dark brown (7.5YR 3/3) moist; moderate medium blocky structure; very hard, very firm; common fine roots; few fine pores; thin continuous clay films; neutral; gradual smooth boundary. Bt2—24 to 37 inches; reddish brown (5YR 5/4) clay loam, reddish brown (5YR 4/4) moist; moderate medium blocky structure; very hard, 11 very firm; few fine roots; few fine pores; thin continuous clay films; few threads and films of calcium carbonate; calcareous; mildly alkaline; gradual smooth boundary. Bt3—37 to 48 inches; yellowish red (5YR 5/6) sandy clay loam, yellowish red (5YR 4/6) moist; weak medium blocky struc- ture; very hard, firm; few fine roots; few fine pores; thin patchy clay films; few threads, films, and small concretions of calcium carbonate; calcareous; mildly alkaline; clear smooth boundary. Btk—48 to 72 inches; pink (5YR 8/4) clay loam, pink (5YR 7/4) moist; moderate medium blocky structure; very hard, fri- able; common fine pores; about 55 percent of soil mass consists of soft masses and concretions of calcium carbonate; calcare- ous; moderately alkaline. Site N0. 5 Soil Type: Olton clay loam Location: Castro County, Texas; in a cultivated field 300 feet north of Farm Road 145, 7.5 miles west of its intersection with Farm Road 168 in Hart. Pedon description: Sample No. S90TX069-1-(1-5) Ap-O to 6 inches; brown (7.5YR 4/3) clay loam, dark brown (7.5YR 3/3) moist; weak me- dium subangular blocky struc- ture; slightly hard, friable; many fine and medium roots; common fine and medium pores; neutral; abrupt smooth boundary. Bt1—6 to 17 inches; dark brown (7.5YR 4/2) clay loam, dark brown (7.5YR 3/2) moist; weak coarse prismatic struc- ture, parting to weak medium blocky structure; hard, friable; common fine roots; few fine pores; thin patchy clay films; neutral; gradual smooth boundary. Bt2—17 to 33 inches; reddish brown (5YR 5/4) clay loam, reddish brown (5YR 4/4) moist; moderate medium blocky structure; very hard, firm; few fine roots; few fine pores; thin patchy clay films; few threads and films 0f cal- cium carbonate; calcareous; moderately alkaline; gradual smooth boundary. Bt3—33 to 45 inches; yellowish red (5YR 5/6) clay loam, yel- lowish red (5YR 4/6) moist; weak medium blocky struc- ture; very hard, firm; few fine roots; few fine pores; thin l patchy clay films; few threads, films, and small concretions of calcium carbonate; calcareous; moderately alkaline; clear smooth boundary. Btk—45 to 70 inches; pink (5YR 8/4) clay loam, pink (5YR 7/4) moist; moderate medium blocky structure; very hard, fri- able; common fine pores; about 46 percent of soil mass consists of soft masses and concretions of calcium carbonate; calcare- ous; moderately alkaline. Site N0. 6 Soil Type: Olton clay loam Location: Castro County, Texas; in a cultivated field 2800 feet north of Farm Road 145, 1.1 miles west of its intersection with Farm Road 168 in Hart. Pedon description: Sample No. S91TX069~2-(l-5) Ap-O to 7 inches; brown (7.5YR 4/3) clay loam, dark brown (7.5YR 3/3) moist; weak me- dium subangular blocky struc- ture; slightly hard, friable; many fine and medium roots; common fine and medium pores; neutral; abrupt smooth boundary. Bt1—7 to 20 inches; dark brown (7.5YR 4/2) clay loam, dark brown (7.5YR 3/2) moist; moderate medium blocky structure; hard, friable; com- mon fine roots; few fine pores; thin patchy clay films; neutral; gradual smooth boundary. Bt2—20 to 38 inches; reddish brown (5YR 5/4) clay loam, reddish brown (5YR 4/4) moist; moderate medium blocky structure; very hard, firm; few fine roots; few fine pores; thin patchy clay films; few threads and films of cal- cium carbonate; calcareous; moderately alkaline; gradual smooth boundary. Bt3—38 to 49 inches; yellowish red (5YR 5/6) clay loam, yel- lowish red (5YR 4/6) moist; weak medium blocky struc- ture; very hard, firm; few fine roots; few fine pores; thin patchy clay films; few threads, films, and small concretions of calcium carbonate; calcareous; moderately alkaline; clear smooth boundary. Btk—49 to 70 inches; pink (5YR 8/4) clay loam, pink (5YR 7/4) moist; moderate medium blocky structure; very hard, fri- able; common fine pores; about 46 percent of soil mass consists of soft masses and concretions of calcium carbonate; calcare- ous; moderately alkaline. Site N0. 7 Soil Type: Olton clay loam Location: Hale County, Texas; in a cultivated field 600 feet north of unpaved county road, 0.35 mile west and 1.0 mile north of U.S. Hwy 70; 3.0 miles west of its in- tersection with Farm Road 2284; 4.0 miles west of Halfway. Pedon description: Sample No. S90TX19l-1-(1-5) Ap-O to 7 inches; dark brown (7.5YR 4/2) clay loam, dark brown (5YR 4/3) moist; weak medium subangular blocky structure; slightly hard, fri- able; many fine and medium roots; common fine and me- dium pores; neutral; abrupt smooth boundary. 12 Bt1—7 to 18 inches; dark brown (7.5YR 4/2) clay loam, dark brown (7.5YR 3/2) moist; moderate medium blocky structure; very hard, very firm; common fine roots; few fine pores; thin continuous clay films; neutral; gradual smooth boundary. Bt2—18 to 32 inches; brown (7.5YR 5/4) clay loam, brown (7.5YR 4/4) moist; moderate medium blocky structure; very hard, very firm; few fine roots; few fine pores; thin con- tinuous clay films; few threads and films of calcium carbon- ate; calcareous; neutral; gradual smooth boundary. Bt3—32 to 42 inches; reddish brown (5YR 5/4) clay loam, reddish brown (5YR 4/4) moist; weak medium blocky structure; very hard, firm; few fine roots; few fine pores; thin patchy clay films; few threads, films, and small concretions of calcium carbonate; calcareous; mildly alkaline; clear smooth boundary. Btk—42 to 72 inches; pink (5YR 8/4) clay loam, pink (5YR 7/4) moist; moderate medium blocky structure; very hard, fri- able; common fine pores; about 35 percent of soil mass consists of soft masses and concretions of calcium carbonate; calcare- ous; moderately alkaline. Site N0. 8 Soil Type: Olton clay loam Location: Hale County, Texas; in a cultivated field 1 200 feet south of Farm Road 37, 4.0 miles west of Farm Road 179 in Cotton Center. Pedon description: Sample No. S9lTXl9l-2-(1-4) Ap-O to 7 inches; reddish brown (5YR 4/3) loam, dark reddish brown (5YR 3/3) moist; weak medium subangular blocky structure; slightly hard, friable; many fine and medium roots; common fine and medium pores; neutral; abrupt smooth boundary. Btl—7 t0 24 inches; dark reddish brown (5YR 4/5) clay loam, dark reddish brown (5YR 5/5) moist; moderate medium blocky structure; very hard, firm; common fine roots; few fine pores; thin continuous clay films; neutral; gradual smooth boundary. Bt2—24 to 58 inches; reddish brown (5YR 4/4) clay loam, dark reddish brown (5YR 5/4) moist; moderate medium blocky structure; very hard, very firm; few fine roots; few fine pores; thin continuous clay films; few threads and films of calcium carbonate; calcareous; mildly alkaline; gradual smooth boundary. Btk—58 to 7O inches; yellowish red (5YR 6/6) clay; yellowish red (5YR 7/6) moist; moderate medium blocky structure; erate medium blocky structure; hard, friable; common fine roots; few fine pores; thin con- tinuous clay films; mildly alka- line; gradual smooth boundary. Bt2—26 to 47 inches; reddish brown (5YR 5/ 5) clay loam, reddish brown (5YR 4/5) moist; weak medium blocky structure; very hard, firm; few fine roots; few fine pores; thin patchy clay films; few threads and films of calcium carbonate; calcareous; mildly alkaline; gradual smooth boundary. Btk—-47 to 80 inches; reddish yel- low (5YR 6/6) clay; reddish yellow (5YR 7/6) moist; mod- erate medium blocky struc- ture; very hard, friable; com- mon fine pores; about 58 per- cent of soil mass consists of soft masses and concretions of calcium carbonate; calcareous; moderately alkaline. very hard, friable; common fine pores; about 58 percent of soil mass consists of soft masses and concretions of cal- cium carbonate; calcareous; moderately alkaline. Site N0. 10 Soil Type: Olton clay loam Location: Lubbock County, Texas; in a cultivated field 150 feet east of unpaved county road, 0.5 mile south of U.S. Highway 82, 4.5 miles east of Farm Road 400 in Site N0. 9 Soil Type: Olton clay loam Location: Lubbock County, Texas; in a cultivated field 2200 feet east of Farm Road 2528, 1.55 miles south of its intersection with Farm Road 1729, 6.0 miles west of In- terstate Highway 27 at New Deal. Pedon description: Sample No. S91TX503-1-(1-4) Ap—0 to 11 inches; dark brown Idalou. Pedon description: Sample No. S90TX305-2-(1-5) Ap—0 to 7 inches; dark brown (7.5YR 4/5) clay loam, dark brown (7.5YR 5/5) moist; weak medium subangular structure; slightly hard, fri- able; many fine and medium roots; common fine and me- dium pores; neutral; abrupt (7.5YR 4/5) clay loam, dark brown (7.5YR 5/5) moist; moderate medium blocky structure; slightly hard, fri- able; many fine and medium roots; common fine and me- dium pores; mildly alkaline; abrupt smooth boundary. Btl-ll to 26 inches; dark brown (7.5YR 4/5) clay loam, dark brown (7 .5YR 3/3) moist; mod- smooth boundary. Btl—7 to 25 inches; brown (7.5YR 4/5) clay loam, dark brown (7.5YR 5/5) moist; weak coarse prismatic struc- ture, parting to weak medium blocky structure; very hard, firm; common fine roots; few fine pores; thin patchy clay films; mildly alkaline; gradual smooth boundary. 13 Bt2—25 to 55 inches; yellowish red (5YR 5/6) clay loam, yel- lowish red (5YR 4/6) moist; weak medium blocky struc- ture; very hard, firm; few fine roots; few fine pores; thin patchy clay films; calcareous; mildly alkaline; gradual smooth boundary. Bt5-53 to 42 inches; yellowish red (5YR 5/8) clay loam, yel- lowish red (5YR 4/6) moist; weak medium blocky struc- ture; very hard, firm; few fine roots; few fine pores; thin patchy clay films; few threads, films, and small concretions of calcium carbonate; calcareous; mildly alkaline; clear smooth boundary. Btk—42 to 70 inches; pink (5YR 8/4) clay 10am, pink (5YR 7/4) moist; moderate medium blocky structure; very hard, fri- able; common fine pores; about 55 percent of soil mass consists of soft masses and concretions of calcium carbonate; calcare- ous; moderately alkaline. Site N0. 1 1 Soil Type: Olton clay loam Location: Crosby County, Texas; in a cultivated field 950 feet south of a field turnrow, 0.85 miles south of curve in Farm Road 651, 1.85 miles south of its junction with Farm Road 1471 at Big Four. Pedon description: Sample No. S91TX107-1-(1-5) Ap—0 to 10 inches; brown (7 .5YR 4/5) clay loam, dark brown (7.5YR 5/5) moist; weak me- dium subangular blocky struc- ture; slightly hard, friable; many fine and medium roots; common fine and medium pores; mildly alkaline; clear smooth boundary. Bt1—10 to 22 inches; brown (7.5YR 4/4) clay loam, dark brown (7 .5YR 5/4) moist; weak coarse prismatic structure, parting to weak medium blocky structure; very hard, very firm; common fine roots; few fine pores; common continuous clay films; mildly alkaline; gradual smooth boundary. Bt2—22 to 33 inches; reddish brown (5YR 5/4) clay loam, reddish brown (5YR 4/4) moist; weak coarse prismatic structure, parting to moderate medium blocky structure; very hard, very firm; few fine roots; few fine pores; common clay films; mildly alkaline; gradual smooth boundary. Bt3—33 to 42 inches; red (2.5YR 4/6) clay loam, dark red (5YR 4/3) moist; weak medium blocky structure; very hard, firm; few fine roots; few fine pores; thin patchy clay films; few threads and films of cal- cium carbonate; calcareous; mildly alkaline; clear smooth boundary. Btk—42 to 72 inches; reddish yel- low (5YR 6/8) silty clay loam, yellowish red (5YR 5/8) moist; moderate medium blocky structure; very hard, friable; common fine pores; about 44 percent of soil mass consists of soft masses and concretions of calcium carbonate; calcare- ous; moderately alkaline. Site N0. I2 Soil Type: Olton sandy clay loam Location: Crosby County, Texas; in a cultivated field 1700 feet west of FM 651, 1.1 miles south of Texas Highway 82 in Crosbyton. Pedon description: Sample No. S91TX107-2-(1-5) Ap-O to 9 inches; brown (7.5YR 4/2) sandy clay loam, dark brown (7.5YR 3/2) moist; weak medium subangular blocky structure; slightly hard, friable; many fine and medium roots; common fine and me- dium pores; neutral; abrupt smooth boundary. Bt1—9 to 22 inches; brown (7.5YR 4/4) clay, dark brown (7.5YR 3/4) moist; weak coarse prismatic structure, parting to weak medium blocky structure; hard, firm; common fine roots; few fine pores; common continuous clay films; mildly alkaline; gradual smooth boundary. Bt2—22 to 29 inches; reddish brown (5YR 5/4) clay, reddish brown (5YR 4/4) moist; mod- erate medium blocky struc- ture; very hard, very firm; few fine roots; few fine pores; common continuous clay films; few threads and films of calcium carbonate; calcareous; mildly alkaline; gradual smooth boundary. Bt3—29 to 35 inches; yellowish red (5YR 5/6) clay loam, yel- lowish red (5YR 4/6) moist; moderate medium blocky structure; very hard, very firm; few fine roots; few fine pores; common clay films; few threads, films, and small con- cretions of calcium carbonate; calcareous; mildly alkaline; clear smooth boundary. Btk-SS to 7O inches; pink (5YR 8/ 4) clay; pink (5YR 7/4) moist; moderate medium blocky struc- ture; very hard, friable; com- mon fine pores; about 47 per- cent of soil mass consists of soft masses and concretions of cal- cium carbonate; calcareous; moderately alkaline. Based on the field descriptions and laboratory determination of particle size distribution, profiles at the various sites differed mainly in horizon thickness, color, and tex- ture, and depth to the calcic hori- zon. Profiles present at the differ- ent sites are indicated in Table 4. The Ap, Btl, Bt2, and Btk horizons were present at all sites, and the Bt3 horizon was present at all sites, ex- cept at Sites 8 and 9. The Ap horizon was generally 6 to 8 inches thick at all sites, but was 9 inches at Site 12, 1O inches at Site 11, and 11 inches at Site 9. The color of dry soil was brown or dark brown at all sites, except Site 8 14 where it was reddish brown. The color of moist soil was dark brown at all sites, except Site 8 where it was dark reddish brown. The Ap horizon textures are clay loam at Sites 1, 4, 7, and 1 1; sandy clay loam at Sites 2, 3, 5, 6, 8, 9, and 12; and sandy loam at Site 10. This horizon represents mainly the plow layer. The differences in thickness and texture of this horizon could be natural because the surface texture of Olton soil varies considerably. Mixing of the upper layers by plow- ing and winnowing of the fine frac- tion by wind possibly contributed to the differences. The Btl horizon was mainly 11 to 16 inches thick, but it ranged from 10 inches Site 3 to 17 inches at Site 8. The texture generally was clay loam, but was clay at Sites 1, 7, and 12; sandy clay loam at Sites 6 and 8; and sandy clay at Site 9. Col- ors were brown or dark brown for dry and moist soil at all sites, except Site 8 where it was dark reddish brown for both conditions. The Bt2 horizon ranged in thick- ness from 7 inches at Site 12 to 21 inches at Site 9, with most sites hav- ing a different thickness of this ho- rizon. The texture was clay loam at Sites 2, 4, 5, 10, and 11; sandy clay loam at Site 9; and clay at Sites 1, 3, 6, 7, 8, and 12. Colors were mainly reddish brown for dry and moist soil, but were brown (dry) and dark brown (moist) at Site 1, brown for both conditions at Site 7, and yellow- ish red for both conditions at Site 10. The Bt3 horizon was mainly 9 to 12 inches thick, but was 6 inches at Site 12, 7 inches at Site 1, and 16 inches at Site 3. The texture was clay loam at Sites 1, 2, 3, 6, 10, 11, and 12; sandy clay loam at Sites 4 and 5; and clay at Site 7. Color was mainly yellowish red, but was red- dish brown (dry) at Site 1, reddish brown for both conditions at Site 7; and red (dry) and dark red (moist) at Site 11. Thickness of the Btk horizon ranged from 21 to 42 inches, with 9 0f the 12 sites having a thickness between 28 and 55 inches. The Btk horizon started at a depth ranging from 55 inches at Site 12 to 49 inches at Site 6. The bottom of this horizon usually was at 70 or 72 inches, but was at 80 inches at Sites 1 and 9. The texture was clay at all sites, except Sites 1, 9, 10, and 11 where it was clay loam. The color generally was pink for both the dry and moist soil, but yellowish red at Site 8 for both conditions, reddish yellow at Site 9 for both conditions, and reddish yellow (dry) and yellow- ish red (moist) at Site 11. Particle Size Distribution Results of the particle size dis- tribution analyses are included in Table 5. Weighted mean sand con- tents for all horizons at a site, except the Btk, ranged from 51.0 percent at Site 7 to 48.8 percent at Site 9. There appeared to be no definite trend in weighted mean sand con- tents due to location of the sampling sites within the region. The sand content for the Ap horizon was high- est (70.4 percent) at Site 10 and low- est (42.6 percent) at Site 11. The sand content at a given site usually was highest in the Ap horizon, but it was equal or greater for the Bt2 and Bt3 at Site 4, for the Bt3 at Site 5, for the Bt1 and Bt2 at Site 9, and the Btk at Site 11. No consistent trends in sand content with profile depth were found among the differ- ent sites. For horizons above the Btk, the overall weighted mean sand con- tent was 40.0 percent. The sand con- tent averaged 50.0 percent for the Ap horizon, 57.2 for the Bt1, 57.5 for the Bt2, 57.9 for the Bt5 (10 sites), and 51.8 in the Btk. The higher average sand content in the Ap horizon than in the underlying horizons suggests that much of the fine material (silt and clay) has been lost from the Ap horizon due to wind erosion. The low sand content of the Btk horizon reflects the high clay content of that horizon. Had the CaCOg been removed before analy- sis, the clay content would have been lower and sand would have been a greater part of the remain- ing material. Size distributions of the sand frac- tion of soil from the different sites are given in Table 6. Of the total, the percent of coarse sand (0.850-mm sieve) was low, with the highest weighted mean amount (excluding the Btk horizon) being 2.6 percent at Site 6. This site also had 5.5 per- cent in the Bt2 horizon, which was the highest for any horizon above the Btk. For the next smaller sieve (0.425 Table 4. Horizons identified in Olton soil profiles at the various sampling sites. Horizon Site, County, State > ‘U Pi _\ Bt2 Bt3 U3 v-o- X 1, Curry, NM 2, Curry, NM 3, Parmer, TX 4, Parmer, TX 5, Castro, TX 6, Castro, TX 7, Hale, TX 8, Hale, TX 9, Lubbock, TX 10, Lubbock, TX 11, Crosby, TX 12, Crosby, TX ><><><><><><><><><><><>< ><><><><><><><><><><><>< ><><><><><><><><><><><>< ><><><><><><><><><><><>< 15 mm), the highest amount was 5.7 percent at Site 7, with the Bt1 hori- zon at that site having the highest amount (7.6 percent) for an indi- vidual horizon. The highest amount for the 0.250-mm sieve was 9.8 per- cent at Site 9. The Ap horizon at this site also had the highest amount (1 1.0 percent) for an individual horizon. For the 0.150-mm size, the highest amount was 24.4 percent at Site 9, but the highest total for an individual horizon was 56.8 percent for the Ap at Site 10. The weighted mean totals for the above size fractions (0.850, 0.425, 0.250, and 0.150 mm) ranged from 7.4 percent at Site 1 to 55.1 percent at Site 9. At all sites, except 9 and 11, the weighted mean percent of sand on the 0.106- mm sieve was greater than the total amount on the four larger sieves. Also, the amount on the fin- est sieve (0.055 mm) was greater than on the next finest sieve (0.106 mm) at all sites, except 9. This site also had the lowest amount on the finest sieve (29.9 percent). The low- est amount on the 0.106-mm sieve was 16.1 percent at Site 1. Surpris- ingly, the highest amount on the fin- est sieve (76.5 percent) also was at Site 1. For individual horizons, the highest amount on the finest sieve was 78.6 percent for the Bt1 at Site 1. It was 46.2 percent for the Bt2 horizon at Site 4 on the 0.106-mm sieve. Lowest amounts for individual horizons were 14.4 percent in the Bt1 at Site 1 on the 0.106-mm sieve and 25.1 percent in the Ap at Site 10 on the 0055mm sieve. v Of the three soil particle sizes (sand, silt, and clay), the weighted mean percent was lowest for silt at all sites, except Site 10, where it was 0.4 percentage unit greater than for clay. The amount of silt was highest (50.4 percent) at Site 10 and lowest (19.0 percent) at Site 6. For indi- vidual horizons above the Btk, the highest amount (37.4 percent) was in the Bt5 at Site 10 and the lowest amount (15.6 percent) was in the Bt1 at Site 9. Silt contents were vari- able in the profiles, and no horizon Table 5. Characteristics of the Olton soil at the study sites. Particle size Site, county distribution USDA Bulk Organic CaCOS Water content1 and state Horizon Depth Sand Silt Clay texture density matter equiv. pH CEC -0.33 -15.0 Plant-available water inches % % % g/cm3 % % meg/100 g % by volume % in/in in/hor. 48-in. depths 1, Curry, NM 1, Ap 0-8 43.5 26.3 30.2 Clay loam 1.352 2.21 — 7.2 17.2 33.12 21.80 11.32 13 0.90 2, Bt1 8-23 32.5 25.1 42.4 Clay 1.60 0.70 — 7.7 18.3 36.50 27.27 9.23 0.092 1.38 3, Bt2 23-31 38.4 22.0 39.6 Clay loam 1.65 0.40 — 7.9 15.4 33.39 25.11 8.28 0.083 0.66 4, Bt3 31-38 36.0 27.5 38.7 Clay loam 1.50 0.15 — 7.9 15.2 30.20 22.09 8.11 0.081 0.57 5, Btk 38-80 35.8 25.6 38.6 Clay loam 1.52 0.32 57.0 8.1 7.0 — — — — — Weighted mean4 36.7 25.1 38.6 — 1.54 0.85 — 7.7 16.9 33.97 24.71 9.26 0.093 — Profile total-in. — — — — — — — — — — — — — 3.51 4.32 2, Curry, NM 1, Ap 0-7 49.9 24.5 25.6 Sandy clay loam 1.35 1.50 — 6.6 15.9 26.21 16.64 9.57 0.096 0.67 2, Bt1 7-20 31.1 30.8 38.1 Clay loam 1.59 0.60 — 6.7 16.4 32.79 24.16 8.63 0.096 1.12 3, Bt2 20-28 36.9 28.2 34.9 Clay loam 1.50 0.58 — 7.8 18.0 29.60 21.11 8.49 0.095 0.76 4, Bt3 28-39 42.0 26.2 31.8 Clay loam 1.52 0.43 — 7.3 11.7 26.77 18.92 7.85 0.079 0.87 5, Btk 39-70 26.6 27.8 45.6 Clay 1.40 0.29 59.4 7.7 6.8 — — — — — Weighted mean 38.7 27.8 33.4 — 1.51 0.71 — 7.1 15.3 29.26 20.71 8.55 0.086 — Profile total-in. — — — - — — — — — — — — —— 3.42 4.13 3, Parmer, TX 1, Ap 0-6 47.0 26.8 26.2 Sandy clay loam 1.35 1.98 — 7.2 17.8 29.06 18.57 10.49 0.105 0.63 2, Bt1 6-16 29.1 31.9 39.0 Clay loam 1.56 1.15 — 6.9 19.4 35.96 26.17 9.79 0.098 0.98 3, Bt2 16-25 28.1 31.0 40.9 Clay 1.54 1.26 — 7.1 15.2 37.72 27.48 10.24 0.102 0.92 4, Bt3 25-41 31.2 29.1 39.7 Clay loam 1.46 0.90 — 7.5 12.4 34.36 24.69 9.67 0.097 1.55 5, Btk 41-72 22.7 29.1 47.2 Clay 1.31 1.06 47.3 7.8 6.2 — — — — — Weighted mean 32.3 29.9 37.8 — 1.49 1.20 — 7.2 15.5 34.71 24.77 9.94 0.099 — Profile total-in. — — — — — — — — — — — — — 4.08 4.76 4, Parmer, TX 1, Ap 0-8 43.5 29.1 27.4 Clay loam 1.35 1.71 — 7.7 16.5 28.53 18.45 10.08 0.101 0.81 2, Bt1 8-24 35.4 30.0 34.6 Clay loam 1.57 1.16 — 7.8 19.1 32.90 23.57 9.33 0.093 1.49 3, Bt2 24-37 44.4 22.2 33.4 Clay loam 1.54 0.36 — 7.8 14.9 27.75 19.91 7.84 0.078 1.01 4, Bt3 37-48 53.8 14.7 31.5 Sandy clay loam 1.51 0.26 — 8.0 13.7 25.61 18.08 7.53 0.075 0.83 5, Btk 48-72 31.8 23.9 44.3 Clay 1.54 0.24 57.4 7.6 4.1 — — — — - Weighted mean 43.4 24.2 32.4 — 1.51 0.83 — 7.8 16.3 29.11 20.47 8.64 0.086 — Profile total-in. — — — — — — — — — — — — — 4.14 4.14 5, Castro, TX 1, Ap 0-6 45.0 27.3 27.7 Sandy clay loam 1.35 1.96 — 7.8 12.1 30.05 19.44 10.61 0.106 0.64 2, Bt1 6-17 36.8 28.3 34.9 Clay loam 1.50 1.65 — 7.2 19.1 34.99 24.57 10.42 0.104 1.14 3, Bt2 17-33 35.2 27.7 37.1 Clay loam 1.58 1.21 - 7.0 16.3 35.05 25.40 9.65 0.097 1.55 4, Bt3 33-45 45.7 21.9 32.4 Sandy clay loam 1.64 0.78 — 7.1 13.2 29.99 21.75 8.24 0.082 0.98 5, Btk 45-72 31.2 26.7 42.1 Clay 1.31 0.53 58.4 7.3 6.3 — — — — — Weighted mean 39.7 26.2 34.1 — 1.55 1.30 — 7.2 15.6 33.02 23.43 9.59 0.096 — Profile total-in. ,— — — — — — — — — — — — — 4.31 4.56 6, Castro, TX 1, Ap 0-7 57.3 19.3 23.4 Sandy clay loam 1.35 1.05 — 6.7 11.9 22.34 13.83 8.51 0.085 0.60 2, Bt1 7-20 51.2 16.4 32.4 Sandy clay loam 1.45 0.92 — 7.1 17.6 29.08 20.11 8.97 0.090 1.17 3, Bt2 20-38 37.1 20.2 42.7 Clay 1.51 0.69 — 7.3 20.0 35.91 26.43 9.48 0.095 1.71 4, Bt3 38-49 40.4 19.7 39.9 Clay loam 1.67 0.53 — 7.6 13.2 34.43 25.94 8.49 0.085 0.94 5, Btk 49-70 25.9 23.5 50.6 Clay 1.57 0.20 57.2 7.8 12.4 — — — — — Weighted mean 44.5 19.0 36.6 — 1.51 0.77 — 7.2 16.7 31.83 22.84 9.01 0.090 — Profile total—-in. — — — — — — — — — -— — — — 4.42 4.34 16 Table 5. Continued. Particle size Site, county distribution USDA Bulk Organic CaC03 Water content1 and state Horizon Depth Sand Silt Clay texture density matter equiv. pH CEC -0.33 -15.0 Plant-available water inches i % % g/cms % i meg/100 g % by volume i in/in in/hor. 48-in. depths 7, Hale, TX 1, Ap 0-7 41.0 29.4 29.6 Clay loam 1.35 1.29 — 6.8 15.4 28.06 18.46 9.60 0.096 0.67 2, Bt1 7-18 28.0 31.1 40.9 Clay 1.55 0.74 — 7.6 20.1 35.19 25.91 9.28 0.093 1.02 3, Bt2 18-32 34.5 24.9 40.6 Clay 1.57 0.60 — 7.7 17.2 34.44 25.50 8.94 0.089 1.25 4, Bt3 32-42 22.4 36.7 40.9 Clay 1.40 0.43 — 7.8 13.2 32.35 23.26 9.09 0.091 0.91 5, Btk 42-72 25.9 28.6 45.6 Clay 1.34 0.40 61.0 7.9 8.3 — — — — — Weighted mean 31.0 30.1 38.9 — 1.49 0.71 — 7.5 16.7 33.08 23.90 9.18 0.092 — Profile total-in. — — — — — — — — — — — — — 3.85 4.40 8, Hale, TX 1, Ap 0-7 62.8 14.1 23.1 Sandy clay loam 1.35 0.84 — 7.4 15.0 21.07 12.96 8.11 0.081 0.57 2, Bt1 7-24 48.7 17.9 33.4 Sandy clay loam 1.54 0.74 — 7.5 16.9 29.66 21.14 8.52 0.085 1.45 3, Bt2 24-38 39.3 18.9 41.8 Clay 1.56 0.43 — 7.7 15.4 34.37 25.58 8.79 0.088 1.23 4, Btk 38-70 38.3 19.6 42.1 Clay 1.55 0.43 41.0 7.9 8.2 — — — — — Weighted mean 47.8 17.6 34.6 — 1.51 0.55 — 7.6 16.0 29.81 21.27 8.54 0.085 — Profile total-in. — — — — — — — — — — — — — 3.25 4.13 9, Lubbock, TX 1, Ap 0-11 47.7 30.5 26.8 Sandy clay loam 1.35 0.25 — 6.7 14.3 20.79 13.36 7.43 0.074 0.81 2, Bt1 11-26 48.0 13.6 38.4 Sandy clay 1.61 0.90 — 7.3 17.8 34.69 25.54 9.15 0.092 1.38 3, Bt2 26-47 50.0 16.9 33.1 Sandy clay loam 1.56 0.18 — 7.6 12.6 26.79 19.36 7.43 0.074 0.81 4, Btk 47-80 39.0 21.4 39.6 Clay loam 1.48 0.28 36.8 7.8 7.7 — — — — - Weighted mean 48.8 17.9 33.3 — 1.53 0.43 — 7.3 14.7 27.91 19.93 7.98 0.080 — Profile totaI—-in. — — — — — — — — — — — — — — 3.00 3.07 10, Lubbock, TX 1, Ap 0-7 70.4 15.3 14.3 Sandy loam 1.35 0.96 — 7.6 10.9 15.29 7.88 7.41 0.074 0.52 2, Bt1 7-16 34.0 31.7 34.3 Clay loam 1.61 0.85 — 7.3 19.2 31.46 22.82 8.64 0.086 1.38 3, Bt2 16-33 34.0 32.6 33.4 Clay loam 1.44 0.55 — 7.4 16.1 27.85 19.42 8.43 0.084 0.84 4, Bt3 33-42 32.0 37.4 30.6 Clay loam 1.66 0.35 — 7.9 12.4 26.68 19.46 7.22 0.072 0.65 5, Btk 42-70 24.0 37.0 39.0 Clay loam 1.50 0.31 47.3 7.2 9.0 — — — — — Weighted mean 39.6 30.4 30.0 — 1.51 0.69 — 7.5 15.6 26.28 18.23 8.05 0.081 — Profile total-in. — — — — — — — — — — — — — 3.39 3.82 11, Crosby, TX 1, Ap 0-10 42.6 20.3 37.1 Clay loam 1.35 1.59 — 6.6 13.2 35.01 24.09 10.92 0.109 1.09 2, Bt1 10-22 41.8 23.0 35.2 Clay loam 1.58 1.10 — 6.7 18.8 33.12 23.86 9.26 0.093 1.12 3, Bt2 22-33 38.0 22.7 39.3 Clay loam 1.48 0.79 - 7.9 15.3 33.68 24.31 9.37 0.093 1.02 4, Bt3 33-42 34.0 29.8 36.2 Clay loam 1.41 0.42 — 7.3 12.6 28.98 20.41 8.57 0.086 0.77 5, Btk 42-72 43.7 17.0 39.3 Clay loam 1.47 0.28 39.9 7.6 7.5 — — — — — Weighted mean 39.3 23.7 36.9 — 1.46 0.99 — 7.1 15.2 32.83 23.29 9.54 0.095 — Profile total-in. — — -— — — — — — — — — — — 4.00 4.52 12, Crosby, TX 1,Ap 0-9 49.0 22.6 28.4 Sandy clay loam 1.35 1.70 — 7.5 12.8 29.25 19.03 10.22 0.102 0.92 2, Bt1 9-22 29.4 27.9 42.7 Clay 1.51 1.29 — 7.5 19.1 38.93 28.37 10.56 0.106 1.38 3, Bt2 22-29 34.0 25.1 40.9 Clay 1.61 0.95 — 7.9 16.3 36.76 27.25 9.51 0.095 0.67 4, Bt3 29-35 41.6 20.7 37.7 Clay loam 1.52 0.42 — 6.9 13.2 31.00 22.56 8.44 0.084 0.50 5, Btk 35-70 31.7 26.5 41.8 Clay 1.61 0.43 38.8 7.6 9.1 — — — — — Weighted mean 37.5 24.7 37.8 — 1.49 1.18 — 7.5 15.9 34.65 24.75 9.90 0.099 — Profile total-in. -— — — — — — — - — — -— — - 3.47 4.56 1 Water contents at the -1/3 and -15 bar matric potentials were calculated by Equations 1 and 2, respectively, in Table 7, of Unger (1975). 2 Bulk density of the Ap horizon was based on a value obtained from another study on Olton soil because this horizon was the loosened tillage layer and core sampling was not possible when the samples were obtained for this study. 3 Adjusted to 48-inch depth for all horizons by adding or subtracting plant-available water based on water retention of the horizon above or the horizon occurring at the 48-inch depth. 4The calcic horizon (Btk) was not included in the weighted mean calculations. For water content, weighted means were calculated only to the depth to which data are presented. 17 had the lowest or highest amount in all cases. Also, trends for silt con- tent generally were opposite the trends for sand content. The over- all weighted mean silt content was 24.7 percent. For individual hori- zons, silt content averaged 23.4 per- cent for the Ap, 25.6 for the Bt1, 24.4 for the Bt2, 26.4 for the Bt3 (10 sites), and 25.6 for the Btk. While silt content varied considerably among horizons at a given site, av- erage silt contents for the horizons were remarkably uniform, ranging from 23.4 percent for the Ap to 26.4 percent for the Bt3. Weighted mean clay contents var- ied less among sites than those for sand and silt, ranging from 30.0 percent at Site 10 to 38.9 percent at Site 7. Excluding the Btk hori- zon, clay content usually was high- est in the Btl or Bt2 horizon and lowest in the Ap horizon. For indi- vidual horizons above the Btk, clay content was lowest (14. 3 percent) in the Ap at Site 10 and highest (42.7 percent) in the Bt2 at Site 6 and the Btl at Site 12. The overall weighted mean clay content was 35.4 percent. Clay contents averaged 26.7 percent in the Ap horizon, 37.2 in the Btl, 38.1 in the Bt2, 35.9 in the Bt3 (10 sites), and 43.0 in the Btk. The CaCOg was not removed before de- termining the particle size distribu- tion for the Btk horizon. The trends in average clay content for the dif- ferent horizons are opposite the trends for sand. Bulk Density Bulk density of the Ap horizon was not determined because this horizon was disturbed by tillage and generally remained in a loosened condition when sampling occurred. Other studies, however, have shown that type and recency of tillage greatly affect the density of this ho- rizon. Therefore, for this study, we assumed the bulk density to be 1.35 g/cm” (Table 5), which was the bulk density for the Ap horizon of Olton soil in Lamb County (B. L. Allen, Lubbock, Texas, personal commu- Table 6. Content and size distribution of sand in Olton soil. Site, county, Total 0.850 0.425 0.250 0.150 0.106 0.053 and state Horizon Depth sand (#20) (#40) (#60) (#100) (#140) (#270) in % 1, Curry, NM Ap 0-8 43.5 0.5 1.1 2.3 3.4 17.6 75.1 Bt1 8-23 32.5 1.2 1.3 1.0 3.5 14.4 78.6 Bt2 23-31 38.4 2.5 2.2 1.4 3.0 14.8 76.1 Bt3 31-38 31.0 0.9 1.6 0.9 3.2 19.3 74.1 Btk 38-80 36.0 1.4 1.9 1.7 2.6 15.8 76.6 Weighted mean‘ 35.8 1.3 1.5 1.3 3.3 16.1 76.5 2, Curry, NM Ap 0-7 49.9 2.2 2.3 1.3 2.9 27.7 63.6 Bt1 7-20 31.1 4.8 2.7 2.0 4.3 33.2 53.0 _-» Bt2 20-28 36.8 1.1 3.6 2.7 4.6 35.3 52.7 Bt3 28-39 42.0 1.1 3.0 2.6 4.6 34.2 54.5 Btk 39-80 26.6 1.8 4.2 3.3 5.3 34.4 51.0 Weighted mean 38.7 2.5 2.9 2.2 4.2 32.9 55.3 3, Parmer, TX Ap 0-6 47.0 2.1 0.7 0.8 2.8 27.1 66.6 Bt1 6-16 29.1 0.4 0.8 0.7 2.4 19.5 76.2 Bt2 16-25 29.9 1.3 1.5 1.1 2.3 18.3 75.5 Bt3 25-41 31.7 3.2 2.6 2.9 2.4 20.9 68.0 Btk 41-80 22.7 7.4 4.5 2.3 4.2 23.0 58.6 Weighted mean 32.9 1.9 1.6 1.7 2.4 20.9 71.4 4, Parmer, TX Ap 0-8 43.5 0.4 1.4 1.1 4.0 30.7 62.4 Bt1 8-24 35.4 0.2 1.3 1.2 7.1 43.2 47.0 Bt2 24-37 44.4 2.4 1.5 1.5 7.1 46.2 41.3 Bt3 37-48 53.8 0.3 1.5 1.5 11.8 41.3 43.6 Btk 48-80 31.7 7.9 4.5 3.5 11.4 29.2 43.5 Weighted mean 43.4 0.9 1.4 1.3 7.7 41.5 47.2 5, Castro, TX Ap 0-6 45.0 0.1 0.3 0.4 7.2 26.1 60.6 Bt1 6-17 36.8 0.2 1.1 2.7 16.5 32.5 47.0 Bt2 17-33 35.2 1.0 2.0 2.0 20.6 34.0 40.4 Bt3 33-45 45.7 0.5 0.9 1.8 18.3 33.0 45.5 Btk 45-80 31.2 1.2 1.8 2.3 23.1 32.7 38.9 Weighted mean 39.7 0.6 1.3 1.9 17.3 32.5 46.4 6, Castro, TX Ap 0-7 57.3 1.0 1.1 5.0 10.1 32.7 50.1 Bt1 7-21 51.2 1.7 1.1 4.2 10.3 31.4 51.3 Bt2 21-38 37.1 5.3 1.0 2.2 7.1 22.3 62.1 Bt3 38-49 40.4 0.4 0.8 2.9 6.7 32.4 56.8 Btk 49-72 25.9 3.1 1.2 2.7 7.4 25.5 60.1 Weighted mean 44.8 2.6 1.0 2.7 8.4 28.7 56.1 7, Hale, TX Ap 0-7 41.1 4.1 0.8 2.4 6.3 27.0 59.4 Bt1 7-18 30.5 2.1 7.6 2.0 9.4 32.3 46.6 Bt2 18-32 35.0 1.6 2.1 2.6 6.6 39.7 47.4 Bt3 32-42 22.7 3.6 3.7 4.6 8.3 31.9 47.9 Btk 42-72 25.9 — — — — —- — Weighted mean 31.9 1.8 3.7 2.9 7.7 33.8 4.93 8, Hale, TX Ap 0-7 62.8 0.1 0.8 8.0 20.3 28.3 42.5 Bt1 7-24 48.7 0.1 0.8 5.1 14.5 25.9 53.6 Bt2 24-38 39.3 0.5 1.4 7.8 11.0 21.0 58.3 Btk 38-72 38.3 1.5 2.7 6.8 11.6 20.2 57.2 Weighted mean 47.8 0.6 1.0 6.6 14.3 24.4 53.1 9, Lubbock, TX Ap 0-11 47.7 0.2 0.3 11.0 27.0 32.3 29.2 Bt1 11-26 48.0 0.2 0.4 8.6 22.2 34.6 34.0 Bt2 26-47 50.0 0.5 0.8 10.1 24.5 36.7 27.4 Btk 47-80 39.0 1.4 1.8 9.6 20.3 32.7 34.2 Weighted mean 48.4 0.3 0.6 9.8 24.4 35.0 29.9 10, Lubbock, TX Ap 0-7 70.3 0.0 0.3 10.8 36.8 27.0 25.1 Bt1 7-23 34.0 0.4 0.6 4.2 11.6 33.6 50.6 Bt2 23-33 35.1 0.9 1.0 4.3 10.0 34.6 49.2 Bt3 33-42 32.0 2.4 2.4 7.3 14.9 37.8 35.2 Btk 42-72 24.0 5.3 4.8 7.4 12.0 33.0 37.5 Weighted mean 39.9 0.9 1.0 6.0 16.1 33.6 42.7 Sand retained on sieves with openings of (mm) 18 i Table 6. Continued. Sand retained on sieves with openings oi (mm) Site, county, Total 0.850 0.425 0.250 0.150 0.106 0.053 and state Horizon Depth sand (#20) (#40) (#60) (#100) (#140) (#270) in o/o 11, Crosby, TX Ap 0-10 42.6 0.2 0.5 6.0 23.1 28.0 42.2 Bt1 10-22 41.8 0.2 0.1 6.2 22.9 29.4 41.2 B12 22-33 38.0 0.5 0.8 5.7 20.3 27.3 45.4 Bt3 33-42 34.0 3.9 6.6 8.1 18.3 26.4 35.3 Btk 42-72 42.6 0.6 1.1 5.8 20.4 28.8 43.3 Weighted mean 39.4 1.1 1.8 6.4 21.3 27.9 41.3 12, Crosby, TX Ap 0-9 49.0 0.4 0.9 3.3 13.1 22.2 60.1 Bt1 9-22 29.4 0.3 0.6 4.4 14.5 28.5 51.7 Bt2 22-29 34.0 0.7 1.0 4.2 14.1 26.2 53.8 Bt3 29-35 41.6 0.6 1.0 4.3 14.8 26.2 54.0 Btk 35-72 31.7 1.9 2.8 5.7 15.2 26.4 48.0 Weighted mean 37.5 0.5 0.8 4.1 14.1 26.0 54.7 ‘The calcic (Btk) horizon was not included in calculation o1 the weighted means. nication, 1997). A value was needed to calculate the available water con- tent 0f the Ap horizon in a later sec- tion. Weighted mean bulk densities for horizons above the Btk were rela- tively constant and ranged from 1.46 g/cm‘ at Site 11 to 1.55 g/cm?‘ at Site 5. The bulk density for the Bt1 ho- rizon was lowest (1.45 g/cm‘) at Site 6 and highest (1.61 g/cm”) at Sites 9 and 10. The density was lowest (1.44 g/cmi”) at Site 10 and highest (1.65 g/cml”) at Site 1 for the Bt2 ho- rizon. For the Bt3 horizon, densi- ties ranged from 1.40 g/cm‘ at Site 7 to 1.67 g/cmi‘ at Site 6. The larg- est difference between the lowest and highest densities was in the Btk horizon, for which the lowest (1.3 1 g/cm‘) was at Sites 3 and 5 and the highest (1.61 g/cm3) was at Site 12. At most sites, bulk density was highest in the Bt1 or Bt2 horizon, indicating that some soil compac- tion had occurred. Based on stud- ies conducted by Taylor and Gardner (1963), however, the den- sities were not high enough to pre- vent plant root penetration if the soil contains adequate water, but some reduction in root penetration could occur. Root penetration is af- fected by soil strength, which is a function of soil bulk density and water content (Taylor and Gardner, 1963). Their study with Amarillo fine sandy loam, for example, showed that some roots penetrated the soil when the bulk density was 1.75 g/cm” if the soil matric poten- tial was -1/2 bar or higher (wetter). At a drier soil condition (-2/3 bar matric potential), some root pen- etration occurred when the bulk density was 1.65 g/cm?’ or less. The maximum density at the Olton soil sites sampled was 1.67 g/cm?’ (Bt3 horizon, Site 6). Such density could reduce root penetration as the soil becomes drier. However, resistance to root penetration at similar soil water contents and bulk densities may not be the same in Olton soil as in the Amarillo soil. Also, the method of obtaining samples for bulk density determination may in- fluence the results obtained. For this study, core sampling was used for which the bulk density repre- sents an average density of the sampled volume, including the soil and the shrinkage cracks that de- velop as the soil dries. When clod sampling is used, shrinkage cracks are excluded and the measured bulk density generally is higher than with core sampling. If shrinkage cracks develop as the soil dries, roots may grow through those cracks, even if the soil density is high. Therefore, no definite conclusions regarding effects of measured bulk densities on root penetration are warranted. 19 Organic Matter At all sites, except Site 9, the or- ganic matter content was highest in the Ap horizon (Table 5). The ex- tremely low content at Site 9 (0.25 percent) possibly resulted from till- age that inverted the surface layer of soil. For the remaining sites, the organic matter content in the Ap ranged from 0.84 percent at Site 8 to 2.21 percent at Site 1. For the Bt1 horizon, organic matter contents ranged from 0.60 percent at Site 2 to 1.65 percent at Site 5. It ranged from 0.18 percent at Site 9 to 1.26 percent at Site 3 for the Bt2 hori- zon and from 0.15 percent at Site 1 to 0.90 percent at Site 3 for the Bt3 horizon. The Btk horizon contained less than 0.50 percent organic mat- ter at all sites, except at Site 3 where it was 1.06 percent and Site 5 where it was 0.53 percent. The weighted mean organic mat- ter content, which does not include the Btk horizon, was lowest (0.43 percent) at Site 9. This site had the lowest content in the Ap and Bt2 ho- rizons, but its content in the Bt2 ho- rizon was not greatly different from that of the Bt1 horizon at most other sites. The weighted mean was high- est (1.30 percent) at Site 5 and next highest (1.20 percent) at Site 3. At both sites, the three upper horizons contained more than 1.00 percent organic matter. Calcium Carbonate ((121005) Equivalent The CaCOa equivalent was deter- mined for the calcic (Btk) horizon of the profiles. The values ranged from 36.8 percent at Site 9 to 61.0 percent at Site 7 (Table 5). High percentages occurred also at Sites 1 (57.0), 2 (59.4), 4 (57.4), 5 (58.4), and 6 (57.2). The CaCO3 equivalent refers to the neutralizing power of the material. However, although the CaCQ, equivalent was above 50 per- cent in the Btk horizon at some lo- cations, the material is considered low in value for liming purposes (Lawton and Kurtz, 1957). pH Soil pH (Table 5) was lowest in the Ap horizon in most cases and generally increased with soil depth. The reason for the exceptions is not known. Although some different trends occurred, differences among horizons at a given site were rela- tively small. Differences ranged from 0.4 pH unit at Site 4 to 1.3 pH units at Site 11. The weighted means ranged from 7.1 at Sites 2 and 11 to 7.8 at Site 4. The soil was near neutral or mod- erately alkaline (pH 6.6 to 8.0) in ho- rizons above the Btk and mildly to moderately alkaline (pH 7.2 to 8.1) in the Btk at all sites, as indicated by the profile descriptions. In most cases, the pH was near or above 7.0. The lowest pH was 6.6 in the Ap horizon at Sites 2 and 11. In no hori- zon above the Btk was the pH at a level that appears detrimental to growth of field crops. As previously mentioned, however, the Btk hori- zon apparently restricts root growth of some crops. Also, sensitive plants could be affected by the alkaline conditions throughout the profile, which may require special treatment of the soil for good plant growth. Cation Exchange Capacity (CEC) The CEC of a soil refers to the sum of exchangeable cations that a soil can adsorb at a specific soil pH (Soil Science Society of America, 1987), and is important with regard to retaining nutrients in soil for later uptake by plants. The weighted mean CEC was relatively constant, ranging from 14.7 meq/ 100 g of soil at Site 9 to 16.9 meq/ 100 g at Site 1 (Table 5). For individual horizons above the Btk, low values were 10.9 for the Ap at Site 10, 16.4 for the Bt1 at Site 2, 12.6 for the Bt2 at Site 9, and 11.7 for the Bt3 at Site 2. High values were 17.8 for the Ap at Site 3, 20.1 for the Bt1 at Site 7, 20.0 for the Bt2 at Site 6, and 15.2 for the Bt3 at Site 1. The higher CECs generally were associated with the higher clay contents, both for the entire profiles (weighted mean val- ues) and for individual horizons at a given site. The CECs were low for the Btk horizon, although the indi- cated clay contents generally were high. Included in the reported clay percentages for the Btk horizon, however, were the clay-sized car- bonate particles that do not contrib- ute to the CEC. Water Retention Water contents at matric poten- tials of -1/3 and -15 bars (Table 5) were calculated by equations devel- oped by Unger (1975). The equa- tions are based on the bulk density, organic matter content, and clay content of the individual horizons. These equations were selected be- cause they are considered appropri- ate for the entire range of organic matter contents involved. Also con- sidered were equations developed by Otto Baumer (unpublished re- port) at the Natural Resources Con- servation Service (formerly, Soil Conservation Service) Laboratory at Lincoln, Nebraska. Those equations, however, are reported to be appro- priate only for soils with less than 1.0 percent organic carbon (about 1.72 percent organic matter), which is less than the amount in some ho- rizons at some sites of this study. The calculated values should be valid because correlation coefficients as- sociated with the equations used were significant at the 0.1 percent level (P = 0.001 level). Water contents in Table 5 are given on a volume basis. The plant-available water (PAW) contents are differences between the -1/3 and -15 bar values and are also given on a volume basis. These values represent the amount of water that can be stored in the soil for use by plants, and are equivalent to the PAW storage capacities of the soils. The -1/3 bar value is the upper limit of water retained in a soil that drains freely. In practice, however, some water above the -1/3 bar value can be used by plants if they are ac- tively growing when water is added by rain or irrigation. Values of PAW in Table 5 for individual horizons 2O were obtained by multiplying the ho- rizon thickness by the determined water content for the given horizon (inches/inch). Totals for a given site are summations of the values for the individual horizons. Plant-available water storage ca- pacities were calculated only for ho- rizons above the calcic (Btk) hori- zon because roots of most crops do not grow into nor extract water from the calcic horizon. At 8 of the 12 sites, the total PAW storage ca!’ pacity was 4.00 inches or less. The low storage capacities were associ- ated mainly with shallow soil depths above the calcic horizon. Low wa- ter retention for individual horizons was a contributing factor. Water storage capacity was low- est (3.25 inches) at Site 8 and high- est (4.42 inches) at Site 6. Depth to the Btk horizon was 38 inches at Site 8 and 49 inches at Site 6. In addi- tion, the weighted mean water stor- age capacity was 0.090 inch/inch of profile depth at Site 6, but only 0.080 inch/inch at Site 8. The weigh- ted mean was the lowest at Site 8 and equally high (0.099 inch/inch) at Sites 3 and 12. Water storage capacity differ- ences among sites were greatest for the Ap horizon, ranging from 0.074 inch/inch at Sites 9 and 10 to 0.113 inch/inch at Site 1. The capacities ranged from 0.085 inch/inch at Site 8 to 0.106 at Site 1 2 for the Btl hori- zon, from 0.074 at Site 9 to 0.102 at Site 3 for the Bt2, and from 0.072 at Site 10 to 0.097 at Site 3 for the Bt3. To obtain a better comparison among sites, the water holding ca- pacity of all profiles was adjusted to a 48-inch depth with the assump- tion that the calcic horizon held as much water (inch/inch) as the ho- rizon immediately above it. On this basis, water storage capacity was lowest at Site 9 (3.07 inches). At all other sites, except at Site 10 where it was 3.82 inches, total water stor- age capacity of the profile was above 4.00 inches, with the highest (4.76 inches) being at Site 3. s.) Water storage capacities differed among sites, with the lowest (3.00 inches) being at Site 9. For all other sites, the maximum difference was 1.03 inches. These results suggest that no major differences in manage- ment are needed in most cases to use the soil as a reservoir to store water for later extraction by crops. How- ever, when managing these soils, several points should be taken into consideration. First, the values given should serve only as a guide because the actual amount of water stored and later extracted by plants is in- fluenced by many soil, crop, and climatic conditions. As a result, field values of soil water storage and use seldom correspond with values ob- tained under laboratory conditions. Second, some crops with well-devel- oped root systems often extract soil water to values below the reported -15 bar value, thus obtaining more water from the soil than the amount indicated by the water storage capac- ity. Third, an undetermined amount of water is made available to crops due to upward capillary movement of water from underlying horizons as plants use water from (dry out) the overlying horizons. Also, as pre- viously mentioned, some water ini- tially held above the -1/3 bar value may be used if plants are actively growing when rain occurs or an ir- rigation is applied. Probably the most important factor regarding a soil’s water holding capacity is that the soil be managed in a way that results in refilling the storage reser- voir with water whenever precipi- tation occurs or irrigation is applied. Such management requires main- taining conditions that result in ef- fective water infiltration into the soil. Soil management is further dis- cussed in a subsequent section. Water Infiltration» Measurements of cumulative wa- ter infiltration at 10 minutes and 20 hours and of infiltration rates at times ranging from 10 minutes to 2O hours were made under varying conditions at the different sites as indicated by the remarks included with the data in Table 7. The differ- ent conditions resulted in major cu- mulative infiltration and infiltration rate differences, both among sites and at a given site. Data for a range- land site are included also, but will not be included in the general dis- cussion of water infiltration. At 10 minutes, no measurable in- filtration had occurred during one test at Sites 6 and 1 1 and the highest amount at one test was 2.64 inches at Site 3. At Site 6, the surface was compacted following corn harvest. Before measuring infiltration at Site 11, the soil was wetted and stirred to disperse the surface aggregates, which effectively sealed the soil sur- face. Such drastic treatment as at Site 1 1 is not natural, but intense rain on a soil with low aggregate stability can quickly cause aggregate disper- sion and surface sealing. As a result, runoff can begin before 0.10 inch of rain has fallen. Other low cumu- lative infiltration amounts at 10 min- utes were 0.12 inch at Site 4, where the measurement was made in a “packed furrow” after corn, and 0.24 inch at Site 1, where a tillage pan was present. At Site 3, the soil was deeply ripped with a blade plow, which resulted in the highest cumulative infiltration at 10 minutes, even though the soil was near field capac- ity due to 9.5 inches of rain about 2 weeks before making the infiltra- tion measurements. The next high- est cumulative infiltration at 10 min- utes (2.24 inches) occurred at Site 11 where the soil was loosened by plowing. Soil conditions such as at Sites 3 and 11 should be highly ef- fective to store water in soil from an intense rain, especially if the sur- face is protected (for example, by residues) to prevent aggregate dis- persion and surface sealing. Cumulative infiltration at 20 hours ranged from 0.38 inch at Site 6 to 13.22 inches at Site 4. Low in- filtration at Site 6 resulted from the compacted surface soil following corn. Infiltration also was low at Site 21 11 (1.00 inch) where the soil was wetted and stirred and at Site 9 (1.32 and 1.43 inches) where grain sor- ghum stubble was disked and the land was seeded to wheat. The soil was soft at Site 4 where infiltration was highest. Infiltration at 20 hours also was high (over 10.00 inches) at Site 3 (12.48 inches) where the soil was deeply ripped with a blade plow, and at Site 9 (10.73 inches) where grain sorghum stubble was disked and the land was seeded to wheat. The reason why some of the lowest and highest infiltration amounts occurred at Site 9 is not known, but these results indicate the large variation in infiltration that can occur, even within a given field. Because of the poor soil condi- tions at Sites 6 and 11, infiltration rates were low initially (no measur- able infiltration) and reached maxi- mums of only 0.04 inch/hour at 10 hours at Site 6 and 0.10 inch/hour at 2 hours at Site 1 1. These low rates throughout the 20-hour period of measurement resulted in the low cumulative amounts at these sites. For other sites, the lowest rate at 10 minutes was 0.53 inch/hour at Site 1 2 where the crop was cotton. Other low rates were 0.56 inch/hour at Site 1 where a tillage pan was present, 0.59 and 0.62 at Site 9 where grain sorghum stubble was disked and the land was seeded to wheat, and 0.69 and 0.71 at Site 12 where the crop was cotton. The measured infiltration rate was extremely high (72.00 inches/ hour) at 10 minutes at Site 3,- the reason for which is not known. It may have been due, however, to an unnatural soil condition such as a filled animal burrow or root chan- nels. Other relatively high rates at 10 minutes were 3.79 inches/hour at Site 9 where grain sorghum stubble was disked and the land was seeded to wheat, 3.27 at Site 2 in a clean tilled furrow, 3.06 at Site 8 where the crop was cotton with a tillage pan present, and 3.06 at Site 10 where the soil was loose with- out a tillage pan. Table 7. Cumulative amount and rate of water infiltration and related data for Olton soils. Cumulative Site’ County! Bulk density infiltration at Infiltration rate at and state Ap‘ B11 10 min 2O hr 10 min 30 min 1 hr 2 hr 5 hr 10 hr 20 hr Remarks g/cma inches inches/hr 1, Curry, NM 1.46 1.41 0.34 4.10 1.80 1.54 0.55 0.24 0.12 0.12 0.07 Wheat stubble; undercut with sweeps at 4-inch depth 1.61 1.56 0.24 1.68 0.56 0.05 0.04 0.04 0.08 0.08 0.03 Tillage pan present at 4-inch depth 1.61 1.56 0.55 5.002 1.29 0.35 0.32 0.51 0.18 — — Equipment failure at 6 hours; tillage pan present 2, Curry, NM 1.41 1.48 0.79 7.39 2.15 1.37 0.96 0.45 0.29 0.22 0.21 Clean tilled; in furrows of row-cropped grain sorghum; weak crust in place 1.61 1.46 1.44 4.68 2.15 0.82 0.55 0.30 0.19 0.13 0.10 Clean tilled; in wheel track furrow or grain sorghum; weak crust in place 1.25 1.46 0.67 5.86 3.27 2.28 0.90 0.63 0.40 0.27 0.21 Clean tilled; in furrow for grain sorghum; weak crust in place A 3, Parmer, TX 1.32 1.44 0.94 5.78 2.25 0.74 0.73 0.50 0.33 0.21 0.19 Wheat-fallow; deep ripped with blade plow; 9.5-inch rain 2 weeks before test; soil at about 90% of field capacity; weak crust in place 1.30 1.44 2.64 12.48 72.00 2.00 1.43 0.63 0.48 0.28 0.25 High initial infiltration, cause unknown 1. 65 1. 45 1.52 3. 7D 7. 75 0.58 0.32 0.27 O. 70 0. 06 0. 06‘ Whee/ track furrow; weak crust in p/ace 4 Parmer, TX 1.45 1.45 0.12 5.46 0.72 0.65 0.56 0.42 0.37 0.24 0.14 Corn; packed furrow l X83 ‘f AB N 2B B32 2.72 1.47 1.00 0.42 0.17 0.11 0.10 Packed furrow 1.28 1.47 0.72 13.22 1.89 1.57 1.38 1.24 1.09 0.43 0.20 Soft, unpacked furrow 5, Castro, TX 1.51 1.45 1.30 7.16 2.06 1.09 0.80 0.67 0.44 0.20 0.10 Wheat stubble, underut; no crust present; initial stages of tillage plan development present 1.32 1.45 1.22 8.50 1.87 0.78 0.56 0.41 0.56 0.34 0.25 No tillage pan present, no crust present 1.66 1.55 1.24 3.72 1.16 0.53 0.33 0.26 0.10 0.14 0.07 Tillage pan present; no crust present 6, Castro, TX 1.82 1.68 0.00 0.38 0.00 0.00 0.00 0.00 0.00 0.04 0.01 Corn field following harvest; severe compaction caused by harvesting equipment operations 1.45 1.47 0.28 9.94 1.16 0.89 0.57 0.57 0.39 0.25 0.14 Corn stubble, weak crust present 1.33 1.47 0.50 9.94 1.22 0.77 0.68 0.61 0.60 0.51 0.21 No tillage pan; weak crust present 7, Hale, TX 1.58 1.53 1.59 6.19 2.40 1.33 0.71 0.56 0.29 0.14 0.11 Corn; tillage pan present; weak crust in place 1.64 1.53 1.46 5.46 2.12 0.97 0.64 0.55 0.17 0.13 0.10 Tillage pan present; weak crust in place 1.64 1.53 1.45 4.70 2.36 0.75 0.50 0.32 0.17 0.12 0.09 Tillage pan present; weak crust in place 8, Hale, TX 1.73 1.65 1.32 2.40 2.06 0.18 0.05 0.05 0.07 0.04 0.03 Cotton; dense tillage pan at 5-inch depth; moderate crust in place; severe compaction 1.74 1.58 1.44 4.30 3.06 0.55 0.38 0.21 0.19 0.08 0.05 Cotton; dense tillage pan at 5-inch depth; moderate crust in place; severe compaction 1.65 1.56 1.38 5.92 2.88 1.29 0.97 0.40 0.27 0.18 0.09 Cotton; dense tillage pan at 5-inch depth; moderate crust in place; severe compaction 9, Lubbock, TX 1.73 1.62 0.42 1.32 0.59 0.19 0.10 0.05 0.05 0.06 0.04 Grain sorghum stubble; disked, seeded to wheat; tillage pan present; severe compaction 1.27 1.46 0.96 10.62 3.79 1.50 0.97 0.93 0.59 0.33 0.24 No tillage pan present 1.76 1.66 0.34 1.43 0.62 0.34 0.23 0.14 0.05 0.05 0.04 Severe compaction at 4- to 6-inch depth 10, Lubbock, TX 1.28 1.45 1.50 10.73 3.06 1.45 1.20 0.84 0.47 0.35 0.27 Bulked surface; no tillage pan prsent; no crust prsent 1.46 1.54 1.44 5.76 1.71 0.69 0.58 0.44 0.28 0.13 0.12 Bulked surface; thin tillage pan above Bt horizon; no crust present 1.79 1.64 1.56 2.16 1.44 0.16 0.06 0.03 0.03 0.03 0.03 1.75-inch thick tillage pan above Bt horizon at 4- to 7-inch depth; no crust present 11 , Crosby, TX 1.65 1.55 0.00 1.00 0.00 0.00 0.00 0.10 0.09 0.08 0.07 Soil wetted and stirred to disperse aggregates; test suggests wet crusted surface restricts infiltration and increases runoff 1.15 1.60 -1.50 6.20 2.29 0.85 0.47 0.35 0.33 0.20 0.14 Cotton; soil disturbed by plowing; site not pre-wetted 1.15 1.67 2.26 4.68 1.62 0.48 0.42 0.28 0.11 0.04 _ 0.08 Cotton; soil disturbed by plowing; site not pre-wetted 12, Crosby, TX 1.73 1.64 1.24 2.75 0.71 0.25 0.16 0.14 0.10 0.05 0.04 Cotton; preharvest; severe compaction; weak crust in place 1.73 1.64 0.71 2.40 0.53 0.16 0.11 0.10 0.16 0.07 0.02 Cotton; preharvest; severe compaction; weak crust in place 1.74 1.62 0.72 2.58 0.69 0.25 0.16 0.15 0.14 0.12 0.04 Cotton; preharvest; severe compaction; weak crust in place Flangeland, 1.40 1.46 0.00 9.34 0.00 1.78 1.29 0.52 0.42 0.40 0.38 Initial water repellency caused by surface crust; about 80% grass cover Oldham, TX 3 1.35 1.46 0.00 9.46 0.00 0.00 0.00 0.43 0.50 0.56 0.45 Initial water repellency caused by surface crust; about 80% grass cover 1 Density determined at the 4- to 7-inch depth. 2 Estimated value so that relationships with other variables could be calculated. 3 N.S.S.L. Fiangeland infiltration/runoff research project site. 22 Infiltration rates dropped rapidly after 10 minutes in most cases. At 30 minutes, the greatest decline compared with that at 1O minutes was 97 percent at Site 5 where the rate was exceptionally high at 10 minutes. The two next greatest de- clines were 91 percent where a dense tillage pan was present at the 5-inch depth and 89 percent at Site 10 Where a 1 .75-inch thick pan was present. The least change (10 per- cent) occurred at Site 4 where the furrow was packed and the initial infiltration rate was among the low- est. Other low changes were 14 per- cent at Site 1 and 16 percent at Site 4. At these sites, the rates were among the highest at 50 minutes. Further decreases in infiltration rates occurred throughout the re- mainder of the measurement peri- ods, except where no measured in- filtration had occurred in the early stages at Sites 6 and 11 (discussed above). The highest rate at 2O hours was 0.27 inch/hour at Site 10 where the soil was loose without a tillage pan present. Final rates of less than 0.05 inch/hour usually were associ- ated with a tillage pan or other com- pacted soil condition. High infiltration rates and amounts at 10 minutes and 2O hours as measured at some sites could cause excessive infiltration at the point of water application and, therefore, problems in obtaining uniform soil-water storage with fur- row irrigation. Controlled smooth- ing and compacting the furrow can reduce this problem. Surge irriga- tion also helps to obtain more uni- form water application under such conditions. The high infiltration rates should cause no problems with a well-managed sprinkler irrigation system. Based on multiple linear regres- sion analyses, infiltration rates and amounts at 1O minutes and 2O hours (the dependent variables) were re- lated to few characteristics of the Ap, Bt1, and Bt2 horizons (Table 8) (only significant relationships are included in the table). Soil organic matter content, although included in the regression analyses, was not related to any dependent variable and is not included in Table 8. Infil- tration amount and rate at 10 min- utes were not related to any charac- teristic of the Ap horizon. Bulk den- sity and clay content of the Ap, how- ever, were related to infiltration amount and rate at 2O hours. Bulk density resulted in the highest rank- ing in both cases. In addition, sand content of the Ap horizon was re- lated to the infiltration rate at 2O hours. Both relationships were highly significant (P = 0.001 level). Failure of infiltration amount and rate at 1O minutes to be related to any Ap horizon characteristics is at- tributed to the high variability of those characteristics and of mea- sured infiltration at the sites. Thickness of the Bt1 horizon was related (P = 0.05 level) to total infil- tration at 10 minutes (Table 8). At 20 hours, total infiltration was re- lated (P = 0.001 level) to Bt1 hori- zon bulk density. Infiltration rate at 10 minutes was not related to any Bt1 horizon characteristic, whereas infiltration rate at 20 hours was re- lated (P = 0.001 level) only to bulk density of that horizon. The only significant relationship involving Bt2 horizon characteris- tics and infiltration was between silt content and infiltration amount at 10 minutes. This relationship was significant at the P = 0.05 level. For analyses involving bulk den- sities of the different horizons (Ap and Bt1 determined when in- filtration was measured and Bt2 de- termined when initial sampling was done), infiltration amount and rate at 1O minutes were not related to bulk density (relationships not shown). At 20 hours, both the amount and rate of infiltration were related (P = 0.001 level) to bulk density of Ap and Bt1 horizons (Table 9). Bulk density of the Bt2 horizon had no effect. Lack of effect of most soil hori- zon characteristics other than bulk density of Ap and Bt1 horizons on 25 water infiltration variables suggests that management practices that change the density of those horizons will have a major impact on soil water relations of Olton soil. Be- cause only infiltration amount and rate at 20 hours were affected, the effect of bulk density on soil water storage seems greatest with pro- longed water additions, as with irri- gation. The effect could be particu- larly critical with respect to furrow irrigation. High-density soil could re- strict infiltration so that the profile is not fully recharged with water during a ‘normal’ period of water application. Under such conditions, mechanically loosening the soil may be needed to obtain greater infiltra- tion. In contrast, infiltration may be excessive where the density is low. Such conditions may result in un- even distribution of water in the field, especially with furrow irriga- tion. It also can cause excessive wa- ter movement in soil beyond the rooting depth of plants, thus result- ing in inefficient use of available water resources. Such excessive in- filtration can be reduced by con- trolled smoothing and compacting of furrows to which irrigation wa- ter is applied, using surge irrigation, or using a sprinkler irrigation sys- tem. Because the water holding ca- pacity of soil was around 4.00 inches at most sites, greater appli- cations of water should be avoided in most case in order to make effi- cient use of water. The measured water infiltration rates and amounts may not be rep- resentative of infiltration in all fields in the vicinity of the various sites. The major variability in infiltration may be due to past and current man- agement on the fields such as tillage methods, crops grown, and residue management practices. As a result, producers should evaluate soil con- ditions on their farms and adjust their management practices accord- ingly. For example, if infiltration is low due to a tillage pan or other com- pacted soil condition, an operation such as chiseling, paratilling, deep Table 8. Summary of multiple linear regression analyses associating total infiltration and infiltration rates at 10 minutes and 20 hours with Ap, Bt1", and Bt2 horizon characteristics of Olton soil obtained at 12 sites in Texas and New Mexico. Ranking are based on the standardized partial regression coefficients‘ and levels of significance based on the T-value follow the partial regression coefficient’. Soil horizon and Independent variable“ dependent variable Intercept Sand Clay BD HT R“ Partial regression coefficients AP Total infilitration 30.478 -0.189(2)** -13.104(1)** 0639*“ in 20 hr - in Infiltration rate 0.976 0.003(3)‘ -0.009(2)** -0.309(1)*** 0746*" at 20 hr - in/hr Bt1 Total infiltration 2.549 -0.116(1)* 0.169’ in 10 min - in Total infiltration 46.641 -26.893(1)*** 0452*“ in 20 hr - in infiltration rate 1.039 -0.606(1)*** 0455*" at 20 hr - in/hr Bt2 Total infiltration -0.460 0.061(1)” 0.217* at 10 min - in ‘ Rankings are shown in parantheses immediately after the partial regression coefficients. Rankings in order from 1 (highest) to 3 (lowest). 2 Levels of significance of partial regression coefficients are *(0.05), **(0.01), and ***(0.001). independent variables included in the analyses for which the partial reression coefficients were not significant are identified by double dashes (--). 3 independent variables are % sand, % silt, % clay, BD (bulk density), and HT (horizon thickness). “ Coefficient of correlation. Levels of significance are *(0.05), and ***(0.001). sweep plowing, or moldboard plow- ing may be needed to correct the problem. Where infiltration is exces- sive, corrective actions as discussed above may be needed to make more efficient use of available water re- sources and thereby achieve more economical crop production. Implications for Management Plant-Available Water (PAW) The total amount of PAW retained in a profile was influenced by depth to the calcic horizon and by the water holding capacity of soil in dif- ferent horizons. Total amounts ranged from 5.00 inches at Site 8 to 4.42 inches at Site 6 (Table 5). Therefore, a crop could extract about 32 percent more water from the soil above the calcic horizon at Site 6 than at Site 8. This is based on the assumption that both profiles were initially filled to capacity with water and that roots had explored and extracted water from the entire soil volume to the depth indicated (Table 5). Both conditions, however, often are not fulfilled under field conditions at all locations. The above comparison is for profiles with the lowest and highest PAW holding capacities. The differences in water holding capacities were less at other sites. When adjusted to a 48-inch profile depth, the PAW holding capacity ranged from 5.07 inches at Site 9 to 4.76 inches at Site 3, a difference of about 36 percent. Again, differences were less at other sites. Except for Site 9, these rela- tively small differences among sites suggest that no major differences in management are required at the dif- ferent sites when only the soil wa- ter holding capacity is considered. However, factors other than water holding capacity influence the ef- fectiveness with which a soil can be managed for crops to efficiently and effectively use available water re- sources to achieve optimum yields. Based on PAW holding capacities (Table 5) and the measured infiltra- tion rates (Table 7), soil above the calcic (Btk) horizon and to the 48; inch depth at most sites could be completely refilled with water (for example, by irrigation) in less than 20 hours, except where plowpans were present or other forms of com- paction had occurred. If a plowpan is present or severe compaction has occurred, profiles would not be re- filled even at 20 hrs. Excluding the extremely low amounts of infiltra- tion at individual measurements at Sites 6 and 11 (Table 7), less than half of the required amount of water needed to refill the profile infiltrated the soil in 20 hours in some cases at Sites 1 and 9. In several other cases (at Sites 3, 5, 8, 10, and 12), infiltra- tion at 20 hours was considerably less than the amount needed to re- fill the profile. Based on the infiltra- tion rates prevailing at 20 hrs, addi- tional time required to refill the pro- files above the calcic horizon at Sites 1 and 9 would be about 62 hours. Less time would be needed where the deficit was less and the infiltra- tion rates at 20 hours were greater. Usually, prolonging the time of irri- Table 9. Summary of multiple linear regression analyses associating total infiltration and infiltration rates at 10 minutes and 20 hours with bulk densities of Ap, Bt1, and Bt2 horizons of Olton soil obtained at 12 sites in Texas and New Mexico. Rankings‘ are based on standardized partial regression coefficients and levels of significance’ of the coefficients follow the rankings. independent variables“ Dependent variable intercept BD of Ap BD of Bt1 BD of Bt2 R” Total infiltration 42.205 -0.874(2)*** -15.152(1)**’ 0651"‘ in 20 hr - in Infiltration rate at 0.925 -0.228(2)*** -0.304(1)*' 0722*“ at 20 hr - in/hr l Rankings are shown in parantheses immediately after the partial regression coefficients. Rankings in order from 1 (high- est to 2 (lowest). 2 Levels of significance of partial regression coefficients are **(0.01) and **’(0.001). Independent variables included in the analyses for which the partial regression coefficients were not significant are identified by double dashes (--). 3 Independent variables are bulk density (g/cma) of the indicated horizons. 4 Coefficient of correlation. Levels of significance are ***(0.001). 24 gation to fill the profile with water is not practical under prevailing con- ditions, and profiles at some sites normally would not be refilled with water, except during prolonged Wet periods, or occasionally with re- peated irrigations. Profiles usually would contain around 4 inches of PAW when irrigated for 20 hours (less time in some cases) and would, therefore, provide considerable wa- ter for plant use, although some pro- files may not be filled to capacity. Root penetration into soil varies with plant species and soil-water sta- tus. Sunflower roots grew into and used water from the calcic horizon on Pullman clay loam at Bushland. In contrast, sorghum generally uses water only from horizons overlying the calcic layer. Pullman and Olton soils are similar morphologically, and root proliferation would prob- ably be similar in both soils. There- fore, although there are differences in water holding capacity and soil depth at the different Olton soil sites, management required (for ex- ample, irrigation frequency) to ob- tain similar yields with a given amount of water may be nearly iden- tical at all sites, at least for crops that do not root deeply. The water appli- cation rate, however, may need to be varied because of infiltration rate differences. Crops that root deeply, tolerate stress, and deplete soil wa- ter to low levels would probably perform well on dryland and, if irri- gated, would require less frequent irrigation than crops that root less deeply, are sensitive to stress, and fail to extract all PAW. Marked differ- ences in water extraction by sun- flower and grain sorghum have oc- "curred on the Pullman soil at Bush- land. When grown on adjacent fal- lowed plots, sunflower extracted more water from the soil at all depths than sorghum (Unger and Pringle, 1981). Similar differences probably would occur on the Olton soil. Water Application Furrow irrigation (Figures 1O and 11) is widely practiced on Olton soils, commonly with furrow lengths of one-quarter mile. Because of the generally low infiltration rates asso- ciated with high soil bulk densities commonly observed in clean tillage systems, it is believed that deep per- colation of water can be controlled on this soil. Infiltration measure- ments at the various sites (Table 7), however, suggest that considerable deep percolation may be occurring at some sites. Consequently, to use irrigation water resources effi- ciently, a knowledge of the delivery capacity of the irrigation system and of the soil water storage capacity is required. Because of water quality concerns coupled with energy costs, deep percolation should be avoided. To evaluate irrigation practices, as- sistance is available through the wa- ter conservation districts and the Natural Resources Conservation Ser- vice. Where excessive deep perco- lation is a problem under furrow ir- rigation, set times may need to be shortened. Other alternatives are to use higher flow rates per furrow with shorter set times, smooth fur- row bottoms for more rapid water advance, use the surge-irrigation sys- tem, or irrigate alternate packed fur- rows (Musick and Pringle, 1986). Ad- ditional water savings can be achiev- ed by using underground pipelines rather than open ditches to convey water to irrigation furrows. On sites where infiltration is low, tailwater runoff may be high from furrow irrigation unless cutback flow rates are used. Runoff may also occur where sprinkler irrigation is used. Some runoff water can be re- cycled through recovery systems, but building the systems and repumping the water increases pro- duction costs. Pumping costs are higher for sprinkler systems than for furrow irrigation because of the extra head required to pressurize the system. Labor requirements for sprinkler systems such as center-pivot sys- tems, however, are lower than for furrow systems. With sprinklers, wa- ter can also be applied at rates com- 25 patible with infiltration rates. In an ideally designed sprinkler system, water should be applied at a rate slightly less than the soil’s infiltra- tion rate. This reduces the potential for water collecting on the surface, which could result in runoff losses. High pressure sprinkler systems apply water over a relatively large area, thus reducing runoff prob- lems. High pressure systems, how- ever, are energy intensive and may result in high water losses due to evaporation from falling droplets or fine spray or due to excessive drift on windy days. Low pressure sprin- klers require less energy, but apply water over a smaller area (Figure l5). Evaporative losses should be lower, but runoff losses could be higher unless special provisions are made to reduce runoff. Lyle (1979) controlled runoff and used water efficiently with a low-pressure, pre- cision application system that in- cluded furrow dikes (Figure 15). Another possibility would be to add booms with attached nozzles at right angles to the main frame of the sprinkler, thus applying water to a larger area at the same time. Water Infiltration Variation Data in Table 7 show up to a 22- fold variation among observations in cumulative infiltration at 10 min- utes and even greater differences in infiltration rates at most times of determination. These variations seemed most closely related to bulk density of Ap horizons determined in the field when infiltration was determined, but cumulative infiltra- tion and infiltration rates also var- ied considerably among measure- ments at a given site in some cases. Such variation resulted from local conditions such as surface crusting and soil compaction, which sug- gests that soil behavior on a given field near the sampling sites may differ considerably from that indi- cated by the data (Table 7). Where infiltration is much lower than expected, a compacted zone such as a plowpan may be present. Deeper-than-normal plowing 0r chiseling While the soil is relatively dry is a possible remedy for over- coming infiltration problems asso- ciated with plowpans or other dense soil layers. Another possible remedy is the use of reduced- or no-tillage cropping systems, which reduce soil compaction because of less traf- fic across the field. The adverse soil condition (plowpan or other dense layer), however, should be corrected before a reduced- or no-tillage sys- tem is initiated because those sys- tems are not highly effective for correcting degraded soil conditions. Those systems protect the soil sur- face, decrease rates of runoff, and reduce surface evaporation losses because of crop residues retained on the soil surface (Figure 16). As a result, they help maintain a favor- able soil condition once it has been established. However, when resi- dues are limited and soil crusts are left undisturbed, the crusts can be- come stronger and thicker with time and infiltration rates may not be improved and may even be reduced. Based on the measurements, large variations in infiltration are possible at all sites on Olton soils. Where problems are suspected, appropri- ate corrective measures should be taken to increase infiltration where it is too low, or decrease it where excessive deep percolation occurs. Crop Sequences Wheat, cotton, grain sorghum, corn, sunflowers, sugarbeets, al- falfa, and vegetable crops such as potatoes (Solcmum tuberosum) and onions (Allium cepa) are adaptable and grown in some part or through- out the area of Olton soils. Much of the grain produced in the region is stored in elevators, then transport- ed to area feedlots or to seaports for export to foreign countries. Whether crops are grown continu- ously or in rotations depends on fac- tors such as crop prices; water avail- ability; fertilizer cost and availabil- ity; weed, insect, and disease prob- lems; and producers’ preferences. When irrigated crops that do not root deeply are grown continuously, some water generally moves beyond the plant’s rooting depth and, there- fore, reduces water use efficiency for crop production. Unless a deep- rooted crop is later grown, such water is lost for crop production purposes unless it eventually reaches the aquifer from where it could be pumped again. Recharge (return flow) to the aquifer, how- ever, is limited in the region. Water losses from deep percola- tion can be reduced by growing deep-rooted crops in rotation with shallower-rooted crops. The effec- tiveness of deep-rooted crops for ex- tracting water from deep in the pro- file is improved when they are grown without irrigation or with limited irrigation. In either case, adequate water must be available throughout the profile so that root growth is not restricted by a dry zone in the soil. Limited irrigation is effective for crops such as wheat and grain sorghum that can tolerate some water stress, and especially when the soil profile is filled with water at the start of the growing season (Musick and Dusek, 1971; Schneider et al., 1969). Tillage and Cropping Practices Concern about the steady decline of water in the Ogallala Aquifer, which supplies the water used to ir- rigate Olton soils, and rising produc- tion costs have triggered an inter- est in conservation of irrigation water. As a result, more emphasis is being placed on conservation and use of precipitation for crop pro- duction. Studies conducted on Pull- man soil, which is morphologically similar to Olton soil, can aid in un- derstanding the effects of conserva- tion practices on Olton soils. Under dryland conditions, more water from precipitation was stored in soil and grain yields were higher where stubble mulch tillage instead of one-way disk tillage was used in continuous wheat or wheat-fallow cropping systems (Johnson and Davis, 1972). Other practices that conserved water and increased crop yields on dryland are conservation bench terraces (Figure 17) and level bench terraces (Jones, 1975; Jones and Hauser, 1975); narrow benches, narrow conservation benches, and large contour furrows (Jones, 1981); and furrow blocking (Clark and Hudspeth, 1976; Clark and Jones, 1981) (Figure 18). These practices retained potential runoff water Figure 15. The LEPA (Low Energy Precision Application) irrigation system applies water to furrow-diked land at a low pressure, thus resulting in energy savings, precision water application, and reduced evaporation. 26 where it fell or retained it on part 0f the field, thus increasing the amount 0f water available for crop use. Little benefit was obtained with respect t0 reduced evaporation be- cause residues produced by dryland crops (Figures 19 and 20) generally were not adequate to greatly reduce evaporation, even when all residues were maintained on the surface in no-tillage systems (Wiese et al., 1960, 1967). Use of no-tillage on dryland, however, results in water conservation equal to or greater than that obtained with stubble mulch tillage (Jones et al., 1994). This is because the thorough soil drying that occurs when tillage loos- ens and/or inverts the plow layer is avoided. As a result, less water must infiltrate a no-tillage soil than a tilled soil to provide an equal amount of stored soil water. In contrast to the limited re- sponse to surface residues for in- creasing water storage from precipi- tation in no-tillage systems on dry- land, major increases in water stor- age occurred when residues from ir- rigated wheat (Figure 21) were re- tained on the surface with no-tillage systems compared to working them into the soil with tillage (Musick et al., 1977; Unger, 1984; Unger et al., 1971; Unger and Wiese, 1979). The additional stored water decreased the amount of irrigation water needed for grain sorghum (Musick et al., 1977) and resulted in good growth (Figure 22) and yields of dry- land grain sorghum (Unger, 1984; Unger and Wiese, 1979). In a study for which wheat residues (straw) were placed on the soil surface, rwater storage during fallow (from the time of wheat harvest until sor- ghum planting) and subsequent grain sorghum yields increased as surface residues increased from 0 to about 11,000 pounds per acre (Unger, 1978b). Dryland wheat in the region often yields only about 1,500 to 2,500 pounds of residue per acre at Bushland. In contrast, irri- gated wheat often yields 4,000 to 6,000 pounds of residue per acre, and amounts of 10,000 or more pounds per acre have been obtained in some years (Unger, 1977; Unger et al., 1975). The residue amounts produced by irrigated wheat are in the range that substantially increased water storage and grain sorghum yields (Unger, 1978b). When maintained on the soil surface, residues of irrigated wheat increased water use effi- ciency for crop production on Pull- man soils. By inference, that should also occur on Olton soils. Subse- quent crops (grain sorghum, corn, and sunflower) have been success- fully planted into the residues of ir- . j Figure 16. Corn planted by the no-tillage method in wheat residues. The residues re- rigated wheat crops (Musick et al., 1977; Unger, 1981, 1984, 1986; Unger and Wiese, 1979). While large amounts of residues may cause plant- ing problems, very little trouble was encountered when crops were no- tillage planted on the undisturbed ridges between drill rows of the pre- vious irrigated wheat crop that was planted on ridges and in furrows. When wheat is flat planted, as with sprinkler irrigation, more problems may be encountered in planting the next crop by the no-till- age method, especially if definite row patterns are not maintained and the interval between crops is not tained on the surface reduce runoff, provide for rapid water infiltration, reduce evapora- tion, and reduce the chance for plant injury caused by wind-blown soil. Figure 17. Conservation bench terraces are designed to keep runoff water on the land. Runoff from the natural sloping area is trapped on the leveled area, thus providing more water for more intensive cropping on the leveled area. 27 adequate to allow major decompo- sition 0f residues. Weed and volun- teer crop plant control may also be more difficult. However, even with relatively large amounts of residues on the surface from flat-planted wheat, grain sorghum was success- fully planted in residues by planting it in a row direction at right angles to the direction of the wheat rows (Unger, personal experience). Plant- ing in a row direction other than that of the previous crop should re- sult in satisfactory planting in most cases. Such planting avoids placing most seed on crowns of the previ- ous crop, thus resulting in generally better seed-soil contact for im- proved germination and seedling emergence. Benefits from surface residues re- sult from greater total infiltration and less evaporation of water. Be- cause of their generally greater wa- ter storage capacity, profiles at Sites 5, 4, 5, 6, and 11 may derive more benefit from surface residues than those at sites having less storage capacity. Soils with less capacity are more readily filled with water be- Figure 18. Blocked furrows (with furrow dikes) retain water on the land, thus improving water conservation. Runoff occurs from open furrows (photo provided by O. R. Jones, USDA-ARS, Bushland, Texas). Figure 19. The amount of residue produced by dryland grain sorghum generally is low and provides little protection of the soil surface in the region where Olton soils occur. 28 cause less water is required, pro- vided water infiltration rates are suf- ficiently high. The greater response to surface residues on Pullman soils at a deep site at Bushland compared with a shallower site near Lubbock was verified by Baumhardt (1980), who compared effects of disk till- age and no-tillagei-fafter wheat on water storage during fallow and sub- sequent growth and yield of grain sorghum. Because rainfall essen- tially filled the low-capacity profile .- with water with both tillage meth- ods near Lubbock, sorghum yields were not different due to tillage. At Bushland, where the storage capac- ity was greater, grain yields of sor- ghum were greater with no-tillage than with disk tillage when the sor- ghum was not irrigated. With irri- gation, sorghum yields were similar with both treatments. A benefit from lower evaporation with surface residues is the longer time the surface layer remains wet enough to beneficially influence seed germination. Whereas rapid decreases in surface soil water con- tent due to evaporation may cause poor germination on a relatively smooth bare soil, slower evapora- tion on mulched soils may result in favorable germination Surface resi- dues also reduce warm-season soil temperatures, which can be an ad- ditional benefit with respect to crop establishment and early seedling growth (Allen et al., 1975). Ranching and Livestock Production Ranching and livestock produc- tion are important agricultural en- terprises on the High Plains. Native grasses cover about 228,700 acres, or 18 percent of the total land area of Qlton soils. Most ranches are cow- calf operations, although stocker steers make up a large percentage of many herds. Usually, these stocker cattle are placed in nearby feedlots for finishing. In many cases, forage produced on rangeland is supplemented by crop stubble and small grain. In win- ter, the native forage is often supple- Figure 20. The amount of residue produced by dryland winter wheat generally is low in the region where Olton soils occur. Figure 21. An irrigated wheat crop generally produces a large amount of residues (USDA- NRCS photo). mented with protein concentrate. Creep feeding 0f calves and year- lings to increase market weight is practiced on some ranches. The native vegetation in many parts of the area has been greatly de- pleted by continued excessive use. Forage production now may be less than half of the original production (Natural Resources Conservation Service, 1991). Range productivity can be increased by using manage- ment practices that are effective for specific soils and range sites. Where climate and topography are similar, differences in type of cli- max vegetation grown and amount of dry matter produced on range- land are related closely to the soil type present. For effective manage- ment of rangelands, relationships among soils, vegetation, and water supplies must be considered. The typical climax plant commu- nity and the expected percentage of each species on a typical deep hardland range site are given in Table 10. The potential total annual dry matter production in favorable, normal, and unfavorable years is about 2,000, 1,500, and 1,000 or less pounds per acre, respectively. Besides knowing soil properties and climax plant community, range 29 management requires evaluating the present condition of the range veg- etation in relation to its production potential. Range condition on a par- ticular site is determined by compar- ing the present plant community with the climax plant community for the site. The more closely the exist- ing community resembles the cli- max community, the better the range condition (Figure 23). The objective in range management gen- erally is to control grazing so that plants at a given site are similar in type and percentage composition to the climax plant community for that site. Such management generally results in maximum dry matter pro- duction, water conservation, and erosion control. Sometimes, how- ever, a range condition somewhat below the climax meets grazing needs, provides desirable wildlife habitat, and protects soil and water resources. The major objective of manage- ment on most rangelands is to con- trol grazing so that the types and f" .: " Figure 22. An excellent dryland grain sor- ghum crop on land where residues from the previous irrigated winter wheat crop were managed on the soil surface by us- ing no-tillage methods. Table 10. Typical vegetation on Olton soils (deep hardland range site). Plant name Percentage of annual Common Scientific production of dry matter Blue grama Bouteloua graci/is 40 Buitalograss Buchloe dactyloides 24 Sideoats grama Boute/oua curtipendula 5 Western wheatgrass Agropyron smith/i 5 Vine-mesquite Panicum obtusum 5 Silver bluestem Andropogon saccharoides 5 Tobosa Hi/aria mutica 5 Other perennial grasses — 5 Other perennial forbs — 5 Figure 23. Range grasses near climax condition provide excellent grazing for cattle (USDA- NRCS photo). percentages of plants that make up the climax plant community can be- come reestablished. Controlling brush and minimizing soil erosion by wind and water also are impor- tant objectives. Aids to good range management include adequate fenc- ing so that different tracts can be grazed on a rotation basis and stra- tegic positioning of water and min- eral supplements so that livestock will visit different parts of the tracts during their daily quest for forage, water, and minerals (Merrill, 1985). If sound range management based on soil information and rangeland inventories is applied, the potential is good for increasing the produc- tivity of rangelands. Summary With a land area of 1.26 million acres, Olton soils are among the major arable soils in Texas. A small area of Olton soils also occurs in eastern New Mexico. Olton soils occupy parts of fifteen counties in the Southern High Plains of Texas and two counties in eastern New Mexico. The area of Olton soils is bounded by the area of Pullman soils on the north and east, Yellowhouse Creek on the south, the Caprock escarpment at the High Plains-Roll- ing Plains boundary on the south- east, and Blackwater Draw extend- ing from near Earth, Texas, to Portales, New Mexico, on the south- west. Olton soils occupy about 19 percent of the land within the coun- ties of occurrence. Much of the re- maining land is occupied by soils having similar morphology and oc- cupying similar landscape positions as the Olton soils. About 79 percent of the Olton soil area is cropland, 18 percent is rangeland, and the remainder is in 3O towns, roads, and other nonagricul- tural uses. Irrigation is used on about 45 percent of the cropland. Major crops are cotton, wheat, grain sorghum, and corn. To determine the variability of soil characteristics, Olton soils were sampled at 12 widely separated lo- cations. The profiles were described in the field at sampling time, and samples were analyzed in the labo- ratory for sand, silt, and clay con- tent; bulk density; organic matter .- content; CaCO5 equivalent; pH; and cation exchange capacity. Soil wa- ter retention was calculated with previously established equations. Plant-available water was calculated from soil horizon thicknesses and water retention values. Water infil- tration was measured at the sam- pling sites. The thickness of profiles to the calcic horizon was variable. It ranged from 55 inches at Site 12 in Crosby County, Texas, to 48 inches at Site 4 in Parmer County, Texas. Cumulative water infiltration amounts and infiltration rates at 1O minutes were highly variable and seemed more closely related to bulk density of the Ap horizon when measurements were made than to any other determined profile char- acteristic. Excluding the two deter- minations for which infiltration was extremely low due to soil puddling or compaction (at Sites 6 and 11), cumulative infiltration at 2O hours ranged from 1. 52 inches at Site 9 in Lubbock County, Texas, to 13.22 inches at Site 4 in Parmer County, Texas. The low cumulative infiltra- tion in 2O hours resulted from gen- erally low infiltration rates from 1 to 2O hours after water application, probably due to past and present management on the field. Low infil- tration for some individual determi- nations at some other sites generally was associated with a plowpan or other compacted soil condition. Various measurements indicated that about 2O or fewer hours of wa- ter application would fill the profile with water (about 4 inches) at most sites; up to 82 hours would be needed at others. Applying irriga- tion water for more than about 20 hours is not practical because tailwater runoff and/or deep perco- lation losses become excessive. Because of declining supplies of water for irrigation, water conser- vation has received considerable at- tention in recent years. Practices such as conservation-bench and level-bench terraces, contours, blocked furrows, and reduced- and no-tillage systems that conserve water from rainfall are applicable to Olton soils. These practices con- serve water by reducing runoff, in- creasing infiltration, or reducing evaporation. Crop yields have been increased where these practices were used on Pullman soils and should give similar results on Olton soils because the Olton and Pullman soils are morphologically similar. Practices for conserving irrigation water include use of improved wa- ter application techniques, tailwater recovery systems, and reduced- or no-tillage farming. Acknowledgment The assistance of LarryJ. Fulton, Biological Technician, in conduct- ing this study and statistically ana- lyzing the data is gratefully acknowl- edged. Literature Cited Allen, R.R., Musick, J.T., Wood, F.O., and Dusek, D.A. 1975. No-till seed- ing of irrigated sorghum double- cropped after wheat. Trans. Am. Soc. Agric. Eng. 18:1109~1113. Baumhardt, R.L. 1980. Influence of till- " age and irrigation on grain sorghum production. M.S. Thesis, Texas Tech Univ., Lubbock. Clark, R.N., and Hudspeth, E.B. 1976. Runoff control for summer crop pro- duction in the Southern Plains. Trans. Am. Soc. Agric. Eng., St. Jo- seph, MI. Paper No. 76-2008. \ Clark, R.N., and Jones, O.R. 1981. Fur- row dams for conserving rainwater in a semiarid climate. p. 198-206. In Proc. Crop Production with Con- servation in the 80’s, Chicago, IL, De- cember 1980. Am. Soc. Agric. Eng., St. Joseph, MI. Day, P.R. 1965. Particle fractionation and particle-size analysis. In C.A. Black (ed.) Methods of Soil Analysis, Part I. Agronomy 91545-567. Ezekial, M., and Fox, K.A. 1959. Meth- ods of correlation and regression analysis, 3rd ed. John Wiley & Sons, Inc., New York. Haise, H.R., Donnan, W.W., Phelan,J.T., Lawhon, L.F., and Shockley, D.G. 1956. The use of cylinder infiltration to determine intake characteristics of irrigated soils. U.S. Dept. Agric., ARS and SCS, ARS-41-7. U.S. Gov. Print. Off., Washington, DC. Jackson, M.L. 1958. Organic matter de- termination for soils. p. 205-226. In Soil Chemical Analysis. Prentice-Hall Inc., Englewood Cliffs, NJ. Johnson, W.C., and Davis, R.G. 1972. Research on stubble-mulch farming of winter wheat. U.S. Dept Agric., Agric. Res. Serv. Conserv. Res. Rpt. 16. U.S. Gov. Print. Off., Washing- ton, DC. 32 p. Jones, O.R. 1975. Yields and water-use efficiencies of dryland winter wheat and grain sorghum production sys- tems in the Southern High Plains. Soil Sci. Soc. Am. Proc. 39:98-103. Jones, O.R. 1981. Land forming effects of dryland sorghum production in the Southern High Plains. Soil Sci. Soc. Am. J. 45:606-611. Jones, O.R., and Hauser, V.L. 1975. Run- off utilization for grain production. p. 277-283. In Proc. Water Harv. Symp., Phoenix, AZ, March 1974. U.S. Dept. Agric., Agric. Res. Serv. W-22. Jones, O.R., Hauser, V.L., and Popham, T.W. 1994. No-tillage effects on in- filtration, runoff and water conser- vation on dryland. Trans. Am. Soc. Agric. Eng. 37:475- 479. Lawton, K., and Kurtz, L.T. 1957. Soil reaction and liming. p. 184-193. In Alfred Stefferud (ed.) Soil, the Year- book of Agriculture. U.S. Gov. Print. Off., Washington, DC. 31 Lyle, W.M. 1979. Low energy precision water application system. p. F1-5. In Proc. Crop Prod. and Util. Symp., Amarillo, TX, February 1979. Merrill, J. 1983. The XXX Ranch: Man- aging range for ecology and economy. p. 86-95. In Jack Hayes (ed.) Using Our Natural Resources, the Yearbook of Agriculture. U.S. Gov. Print. Off., Washington, DC. Musick, J.T., and Dusek, D.A. 1971. Grain sorghum response to number, timing, and size of irrigations in the Southern High Plains. Trans. Am. Soc. Agric. Eng. 141401-404, 410. Musick, J.T., and Pringle, F.B. 1986. Tractor wheel compaction of wide- spaced irrigated furrows for reduc- ing water application. Appl. Eng. Agric. 2:123-128. Musick,J.T., Wiese, A.F., and Allen, R.R. 1977. Management of bed-furrow ir- rigated soil with limited- and no-till- age systems. Trans. Am. Soc. Agric. Eng. 20:666-672. Natural Resources Conservation Ser- vice. 1991. USDA-NRCS Field Office Technical Guide, Section 2E, Range Site N. 077CY022TX. U.S. Dept. Agric., Washington, DC. Schneider, A.D., Musick, J.T., and Dusek, D.A. 1969. Efficient wheat irrigation with limited water. Trans. Am. Soc. Agric. Eng. 12:23-26. Soil Conservation Service (now Natu- ral Resources Conservation Service). 1962. Soil Survey of Lamb County, Texas. U.S. Dept. Agric., Soil Conserv. Serv. U.S. Gov. Print. Off., Washington, DC. Soil Conservation Service (now Natu- ral Resources Conservation Service). 1974. Soil Survey of Hale County, Texas. U.S. Dept. Agric., Soil Conserv. Serv. U.S. Gov. Print. Off., Washington, DC. Soil Conservation Service (now Natu- ral Resources Conservation Service). 1979. Soil Survey of Lubbock County, Texas. U.S. Dept. Agric., Soil Conserv. Serv. U.S. Gov. Print. Off., Washington, DC. Soil Conservation Service (now Natu- ral Resources Conservation Service). 1984. p. 29-32. In Procedures for col- lecting soil samples and methods of analysis for soil survey. Soil Survey Investigations Report N0. 1, U.S. Dept. Agric., Soil Conserv. Serv., Lin- coln, NE. Soil Science Society of America. 1987. Glossary of soil science terms. Soil Sci. Soc. Am., Madison, WI. Steel, R.G.D., and Torrie, J.H. 1960. Principles and Procedures of Statis- tics with Special Reference to the Biological Sciences. McGraw-Hill Book Co., New York. Taylor, H.M., and Gardner, H.R. 1963. Penetration of cotton seedling tap roots as influenced by bulk density, moisture content, and strength of soil. Soil Sci. 96:153-156. Texas Department of Agriculture. 1995. Texas Agricultural Statistics. Bull No. 253[2], Texas Dept. Agric., Austin. Unger, P.W. 1975. Relationships be- tween water retention, texture, den- sity and organic matter content of West and South Central Texas soils. Misc. Publ. MP-1192C, Texas Agric. Exp. Stn., College Station. 20 p. Unger, P.W. 1977. Tillage effects of win- ter wheat production where the ir- rigated and dryland crops are alter- nated. Agron. J. 69:944-950. Unger, P.W. 1978a. Effect of irrigation frequency and timing on sunflower growth and yield. p. 117-129. In Proc. 8th Int. Sunflower Conf., July 1978, Minneapolis, MN. Unger, P.W. 1978b. Straw-mulch rate effect on soil water storage and sor- ghum yield. Soil Sci. Soc. Am. J. 42:486-491. Unger, P.W. 1981. Tillage effects on wheat and sunflower grown in rota- tion. Soil Sci. Soc. Am.J. 45:941-945. Unger, P.W. 1984. Tillage and residue effects on wheat, sorghum, and sun- flower grown in rotation. Soil Sci. Soc. Am. J. 48:885-891. Unger, P.W. 1986. Wheat residue man- agement effects on soil water stor- age and corn production. Soil Sci. Soc. Am. J. 50:765-770. Unger, P.W., Allen, R.R., and Parker, J.J. 1973. Cultural practices for irrigated winter wheat production. Soil Sci. Soc. Am. Proc. 30:437-442. Unger, P.W., Allen, R.R., and Wiese, A.F. 1971. Tillage and herbicides for sur- face residue maintenance, weed con- trol, and water conservation. J. Soil Water Conserv. 26: 147-150. 32 Unger, P.W., and Pringle, F.B. 1981. Pullman soils: Distribution, impor- tance, variability, and management. Bull. B-1372, Texas Agric. Exp. Stn., College Station. 24 p. Unger, P.W., and Wiese, A.F. 1979. Managing irrigated winter wheat residues for water storage and sub- sequent dryland grain sorghum pro- duction. Soil Sci. Soc. Am.J. 43:582- 588. U.S. Dept.Agric. 1955. Typical water- holding capacities of different-tex- _,. tured soils. p. 120. In Alfred Stefferud (ed.) Water, the Yearbook of Agriculture. U.S. Gov. Print. Off., Washington, DC. Wiese, A.F., Bond, J.J., and Army, T.J. 1960. Chemical fallow in dryland cropping sequences. Weeds 8:284- 290. Wiese, A.F., Burnett, E., and Box, J.E., Jr. 1967. Chemical fallow in dry- land cropping sequences. Agron. J. 59:175-177. ‘<- [Blank Page in Original Bulletin] ' ,1 "-4 Texas Agricultural Experiment Station The Texas A&M University System Produced by Agricultural Communications, The Texas A&M University System Mention of a trademark or a proprietary product does not constitute aguarantee or a warranty of the product by the Texas Agricultural Experiment Station and does not imply its approval to the exclusion of other products that also may be suitable. All programs and information of the Texas Agricultural Experiment Station are available to everyone without regard to race, color, religion, sex, age, handicap, or national origin. fl“