HARDPAN PURMATIUN In Coarse and Medium-textured Soils In the Lower Bio Brande Valle]; of Texas TEXAS A&M UNIVERSITY Texas Agricultural Experiment Station R. E. Patterson, Director, [Iollege Station, Texas Research on hardpans by the Texas Agri Y Experiment Station has contributed to a better" standing of their make-up from the standpoin, physical, chemical and mineralogical charac Hardpans are a function of the interactions factors, such as, (1) rate of moisture loss, ( perature, (3) time or aging, (4) sodium co tion of water and soil, (5)~-,sf';_1nd, silt, clay, s0' gate and organic matter content of the soil tillage. Soil hardpans are found in virgin, as S cultivated areas, because the rates of moist um m- and temperatures are apparently optimum for), fying soil strength. Soil strength is negatively; to the rate of moisture loss. The greatest soil I has been achieved at approximately 27° C. moisture loss and temperature in the top f are often optimum in the Lower Rio Gran g for increasing soil strength in the compact These factors, plus aging, probably are res for the presence of hardpans in virgin soilsf Valley and other areas having similar soils. Factors such as the sodium content of soil and irrigation water, the low percentage stable aggregates due to low contents of g organic matter, the high percentage,of fine I fine sand, and silt make the above proc contribute to soil strength even more effectf Research has established that coarse-text g are extremely susceptible to compaction w are tilled at high moisture content. This is p‘ the most important factor influencing com soils under cultivation, although plant roots compactive forces under certain moisture c Plant roots may contribute to compaction by, compactive forces during the process of y the soil and by setting up tension forces d absorption of water. g Hardpan conditions can be alleviated i mized by (1) periodic subsoiling, (2) discr ,_ intensive farming practices over extended I time (cotton-vegetable rotation), (3) use "i quality water except in emergencies, (4) til ‘g soil moisture content is such that minimum tion takes place, which would occur when three to four inches is fairly dry and (5) use; manure crops in the crop rotation. Y Summary ............................................................. .. Introduction ...................................................... .§ Descri tion of Hard ans ................................. .. E U B n- t s Physical, Chemical alfid Mineralogical Descri of Hardpan and Associated Soil Layers; Investigations of Factors Which Influence I Hardpan Formation .................................. Suggested Management Practices to g Alleviate Hardpan Conditions ................ OPACTED OR INDURATED SOIL LAYERS of reduced "lity in the soil profile are commonly called V. The influence of hardpans on plant growth I recognized and has been discussed by many ' ors. The literature has been comprehen- iewed by Lutz (12) , Raney, et al. (15) , and nd Simonson (I7). Raney et al. (I5) classi- pacted zones into induced and genetic hard- nse layers in soils which were produced by ve forces such as tillage implements were 10 as induced hardpans; whereas genetic hard- f, been used to (lescribe those dense layers ve been produced during the soil weathering development and occurrence of hardpans in “ils have been attributed to many different factors. Some of them which have been y: as contributing to hardpan formation are and aluminum oxides, (2) amount and type (3) dispersed organic matter, (4) soluble y» (5) colloidal silica, (6) cultivation when [is at optimum moisture content for com- I d (7) close-packing of soil particles. rding to a report by the American Society of Engineers in I958, investigators in 21 u several Canadian provinces were actively {in soil compaction research. This report is of the widespread occurrence of soil com- d of the significance attached to its effects itltural production. fanning practices have resulted in the fuse of heavy equipment which is conducive paction. Many of the above investigators ged in field studies aimed at finding ways ize the adverse effects of compaction due to fiplements. This type of research is needed, ‘ually does not contribute to a basic under- “ of compacted layers. This is particularly 4 pacted layers that resemble induced hard- occur under virgin conditions. A basic ding of the factors contributing to hardpan I is essential to the establishment of manage- ctices which Will alleviate or minimize the- affects of genetic or induced hardpans. l- ic understanding of these hardpans can ly, associate soil physicist, associate agronomist and ent, Substation No. 15, Weslaco, Texas; and head fflu Department of Soil and Crop Sciences, College exas. i, PAN FORMATION IN [IOARSE AND MEDIUM-TEXTURED SOILS IN THE LOWER RIO ORANOE VALLEY OF TEXAS ll J. Gerard, O. A. Burleson, W. R. Cowley, M. E. Bluodworth and E. W. Kunze* also contribute to a better understanding of soil properties such as crusting and aggregation. In 1955, research was initiated at the Weslaco station in cooperation with the Department of Soil and Crop Sciences, Texas A8cM University, to obtain a better understanding of hardpans in coarse and medium-textured soils in the Lower Rio Grande Valley. This information would apply not only to the Lower Rio Grande Valley, but also- to similar hardpans in Texas and the United States. This publi- cation is a summary of research conducted during 1955-62. The objectives of this publication are (1) to point out the prevalence of certain hardpans both locally and nationally and to emphasize their influ- ence on plant growth, (2) to present a description of the physical, chemical and mineralogical properties of such hardpans, (S) to discuss some of the factors which influence or contribute to the formation of hardpans and (4) to suggest methods and manage- ment practices for alleviating the unfavorable soil conditions caused by soil hardpans. DESCRIPTION OF HARDPANS Intensively farmed, irrigated, coarse and medium- textured soils, such as the Willacy fine sandy loam, are particularly susceptible to hardpan formation. These soils. develop hardpans under both cultivated and virgin conditions. It is likely, therefore, that factors other than compaction from tillage imple- ments contribute to their formation. The hardpans develop in the first foot of the soil profile and are usually from 3 to 6 inches thick. In the Lower Rio Grande Valley, hardpans occur on most of the coarse and medium-textured soils. The intensity and thickness of the hardpans will vary from location to location, and indications are that hardpans on the coarse-textured soils usually are of greater intensity than those on soils of medium texture. Possible explanations for this occurrence will be presented later in the manuscript. Most of the investigations reported here were conducted on Willacy fine sandy loam and Willacy loam soils. However, related soil types such as Hidalgofine sandy Ioaml and Hidalgo loam soils are known to have hardpans. ‘Subsoiling study (2) reported on page 9 was on a Hidalgo loam soil. Similar hardpans have been reported by investi- gators throughout the United States. Locke et al. (ll) described similar hardpans at Woodward, Okla- homa and Mandan, North Dakota. Taylor and Gardner (16) have reported the occurrence of similar hardpans on the Amarillo fine sandy loam soil in the Southern Great Plains of Texas. In early 1900, Hilgard (9) observed that a sandy loam soil in California would develop hardpans which were im- pervious to water and roots. He attributed the formation of these hardpans to close-packing of soil particles. The close-packing of soil particles mentioned by Hilgard (9) was apparently similar to the hardpans that occur in the Lower Rio Grande Valley, with the exception that those described by Hilgard occurred at soil depths of l8 to 36 inches. It is apparent from a survey of the literature that similar hardpans, as described above and more completely defined in the next section, are fairly widespread. Their importance in soil and crop management has long been recog- nized and is not a problem of recent origin. Soil hardpans may significantly affect the growth of plants and the production of crops. Reduced permeability of the soil to air, water and plant root activity may result in a significant reduction in crop yield, making the problem one of economic import- ance to the farmer. PHYSICAL, CHEMICAL AND MINERALUBIEAL DESCRIPTION [IF HARDPAN AND ASSOCIATED SUIL LAYERS A summary of the physical and chemical proper- ties o-f T/Villacy fine sandy loam from five locations is indicated in Tables 1 and 2 (l3, l4). The hard- pan layer can be identified by lower water perme- ability as indicated by hydraulic conductiv although the bulk density values were n? different from the layers below the hardp‘ existence of the hardpan is easily distinguis penetrometer analyses as indicated in Figu senluewnzssunz m POUNDS The coarse-textured Willacy fine sandy l is characterized further by a low percenta_* and high percentage of fine and very fine s According to Milford (l4) , the content of c‘ agents essential for aggregate formation is indicated by low percentages of clay, organi and extractable STO2, Fe2O3 and A1203, Water-stable aggregates were found to o y) 5 l0 as 2o son. DEPTH-INCHES Figure 1. Typical field hardpan in Willacy fine sandy I acterized by two penetrometer curves (7). of the variability in the intensity and thickness of the ha 1 I . 4.3 The two curves ‘g TABLE 1. PHYSICAL PROPERTIES OF THE WILLACY FINE SANDY LOAM SOIL (13). Depm of Bu|k Hydruunc Particle size distribution, percent Sand separates, percent” cl Samplesl sample, density, conductivity, _ 5 inches g_/¢m_ inches/ht Sand Silt Clay o 5mm 0.5- O.25- 0.10- 2_° f‘ 2-0.05mm. 50-2,u. <2,u. ' ' 0.25mm. 0.10mm. 0.05mm. '. 1-T 1-4 1.34 2.7 71.4 17.7 10.9 0.2 1.6 57.7 11.9 1-H 16-12 1.38 0.8 68.9 18.4 12.7 0.2 1.7 55.4 11.6 1-U 13-19 1.40 4 9 65.9 18.9 15.2 0.2 1.7 53.4 10.6 2-T 1-4 1.36 2.0 ' 77.7 14.5 7.8 0.2 1.9 62.1 13.5 2-H 51/2-10 1.60 1.1 71.2 16.8 12.0 0.2 1.3 55.9 13.8 2-U 12-16 1.60 1.4 70.4 15.3 14.3 0.2 1.4 56.3 12.5 3-T 1-4 1.44 0.8 80.0 11.6 8.4 0.2 1.8 66.7 11.3 3-H 7-12 1.56 0.3 73.9 12.7 13.4 0.2 1.5 62.1 10.0 3-U 14-19 1.39 5.2 68.0 14.7 17.3 0.2 1.2 55.5 11.1 4-T 1-4 1.41 7.5 84.6 8.8 6.6 0.1 0.4 75.6 8.5 4-H 5-9 1.53 1.5 82.8 8.5 8.7 0.1 0.4 73.7 8.6 4-U 9-14 1.51 2.9 78.8 9 1 12.1 0.1 0.3 69.4 9.0 5-T 1-31/1 1.44 4.5 64.3 21.3 14.4 0.1 0.4, 49.9 13.9 5-H 31/1-71/1 1.62 1.9 65.5 18.4 16.1 0.1 0.4 50.6 14.4 5-U 8-12 1.55 3.7 61.2 18.4 20.4 0.1 0.4 47.5 13.2 ‘Samples 1, 2 and 3 are from cultivated sites, while 4 and 5 are from virgin sites. the hardpan sample; and the letter U, the sample subiacent to the hardpan. 2Sand separates are reported as percentage of the soil, while clay separates are reported as percentage of the clay fraction. 4 The Letter T denotes the surface sample; if» and often less than 2 percent Such l properties indicate that this soil is essen- ‘lngle-grained in structure. The 10w content Le aggregates makes the soil extremely suscep- g close-packing but not susceptible to the de- int of planes of weakness in the profile" such r in the finer-textured soils. The amount of iing agents and aggregation have been found ‘greater in the medium than in the coarse- c soils. This might help to explain the greater ngth of hardpans in coarse than in medium- 1v soils. e exchangeable sodium percentages reported 0rd (l4) (Table 2) are rather low except in of site 3. However, Gerard et al. (6) have in higher concentrations of exchangeable i, in similar soils, Table 3. This factor would i; - soils more susceptible to hardpan formation. fium ion disperses the soil particles and, i , makes the soil more susceptible to com- .3 h et al. (l3) reported quartz to be the major lent of the sand and silt fractions of the I fine sandy loam, although feldspars and h ere present. They also found the clay frac- “ be composed predominantly of illite and a “ crystallized, weathered product of illite with (Amounts of kaolinite and quartz. dies by Milford et al. (l3) indicated no dif- in the physical, chemical and mineralogical fies between the hardpan and adjacent layers. hydraulic conductivity data of undisturbed these layers indicated the existence of a In. Pe-netrometer analyses of these soils (7) also 1*: the presence of a hardpan. letrometer analyses have indicated that soil _. in the hardpan was a function of moisture A‘ Taylor and Gardner (16) also have pointed fact by stating that plant root development TABLE 3. EXCHANGEABLE SODIUM PERCENTAGES OF WILLACY FINE SANDY LOAM IN 1957 AND 1958 AS INFLUENCED BY SOIL AND WATER TREATMENTS (6) Year— 1957 Year-— 1958 treasthilents Deplh Depth 0-6 6-12 0-6 6-12 No treatment Canall 13.7 5.4 4.0 5.8 well’ 25.5 16.9 10.7 14.1 Krilium Canal 13.2 7.4 3.7 5.0 Well 25.5 21.3 11.1 12.8 Gypsum Canal 15.6 6.8 3.5 4.8 Well 20.5 20.1 9.1 12.9 Sulfur Canal 10.1 4.0 3.4 4.3 Well 18.3 11.0 10.2 12.7 lCanal water (good quality water) contained about 800 ppm total salt. The cation concentration was about 50 percent sodium and 50 percent calcium plus magnesium. 2Well water (poor quality water) contained about 2,400 ppm total salts. The cation concentration was about 75 percent sodium and 25 percent calcium plus magnesium. is dependent not only upon the occurrence of a soil hardpan, but also on the moisture content of the compacted layer. INVESTIGATIONS OF FACTORS WHICH INFLUENCE HARDPAN FORMATION Research concerning hardpans and their forma- tion was initiated at the Lower Rio Grande Valley Experiment Station and has been directed toward obtaining data that would lead to a better under- standing of the interrelated factors contributing to the fonnation o-f such hard layers in the soil. It was generally agreed that soils in which. the hardpans occurred were susceptible to compaction by tillage implements, but the presence of such compacted layers under virgin conditions has indicated that factors other than forces exerted by tillage implements were instrumental in their formation. Studies were initi- ated and research techniques were developed to evalu- ate the influence of such factors as moisture level treatments, rate of moisture loss, temperature, relative TABLE 2. CHEMICAL PROPERTIES OF THE WILLACY FINE SANDY LOAM SOIL (13, 14) pH Qrganic Cation exchange Exchangeable cation Base _ 1:1 soil paste mane, capacities, me/100g. percentages suiurafion 5'02’ F2031 A1203! ’ ' percent Percent percent 1 hr. 5 hr. Pflcem Soil 2-0.2,u. <0.2,u. Ca Mg K Na Permm 7.6 7.6 1.3 11.1 46 83 70 19 10.2 1.8 101 0.18 0.29 0.34 7.7 7.8 0.9 12.7 47 89 66 16 7.0 1.5 91 0.14 0.28 0.35 7.9 8.0 0.8 14.7 52 93 64 16 5.9 2.8 89 0.13 0.32 0.40 7.3 7.4 1.1 8.7 41 78 65 16 9.9 2.1 93 0.13 0.25 0.31 7.5 7.6 0.9 11.8 44 98 61 17 6.8 2.3 87 0.12 0.30 0.36 7.7 7.8 0.7 13.4 50 90 60 16 4.9 4.4 85 0.09 0.35 0.50 7.3 7.3 1.0 8.9 44 88 59 17 8.2 3.7 88 0.10 0.28 0.16 7.9 7.8 '3‘, 1.0.9 14.3 47 94 51 15 7.4 8.6 '82 0.12 0.28 0.15 8.0 8.2 ‘1 0.8 15.5 54 87 55 16 7.4 13.4 92 0.13 0.36 0.10 6.5 6.6 0.6 6.2 37 76 53 14 6.3 1.4 75 0.12 0.22 0.11 6.3 6.4 0.7 7.4 37 81 51 16 6.3 1.2 75 0.11 0.22 0.35 6.4 6.5 0.6 10.3 41 85 50 16 5.6 1.1 73 0.10 0.23 0.36 7.2 7.3 1.6 13.6 45 77 58 15 10.0 3.7 87 0.20 0.31 0.32 7.3 7.4 1.5 14.7 47 80 63 13 10.0 1.6 88 0.18 0.32 0.32 7.3 7.4 1.2 16.7 45 86 66 15 7.5 1.7 90 0.16 0.41 0.51 IS-Oi TREATMENT N0. l,‘ i ‘as. 2, 'l.llll 3. i j 10.0‘ SPRING PRESSURE IN POUNDS Ga o 5 i sou. DEPTHq-INICHES Figure 2. The influence of treatments 1, 2 and 3 on soil compaction at various depths as evaluated with a self-recording soil penetrometer (7). Description of soil moisture treatments are indicated in Table 4. humidity, and wetting and drying cycles on soil com- paction) or strength. The compactibility of the prob- lem soils at different soil moisture contents has been studied. The influence of different proportions of sand and silt-clay fractions in the soils as related to hardpan formation has also been evaluated. In 1959, a laboratory investigation was initiated to evaluate the influence of certain factors on close- packing of soil particles in the Willacy fine sandy loam soil. Columns 3% inches in diameter and 12 inches high were filled with air-dry soil to a depth of 9 inches. These columns were divided into six TABLE 4. DESCRIPTION OF DIFFERENT SOIL MOISTURE TREATMENTS AND NUMBER OF IRRIGATIONS PRIOR TO ANALYSES WITH SOIL PENETROMETER. COLUMNS WERE DRIED THE INDICATED WETTING AND DRYING CYCLES IN A FORCE-DRAFT OVEN AT 50° C. (7)1 Number of irrigationsl l. Subirrigated when the average soil moisture was 12.2 per cent (a I/3 atm. percentage) 25 2. Subirrigated when the average soil moisture was 10.6 percent (a 3/4 atm. percentage) 23 3. Subirrigatedwhen the average soil moisture was 9.0 percent (is 2 atm. percentage) ' l7 4. Subirrigated when the average soil moisture was 9.0 percent (a: 2 atrn. percentage) ‘l8 The soil surface was mulched with a spatula to a depth of 2 inches when the moisture content of the soil in the columns was approximately I3 percent. 5. Surface irrigated (500 cc. of water) when the average soil moisture was 9.0 percent (a 2 atm. percentage). The surface was mulched with a spatula to a depth of 2 inches when the mois- ture content of the soil was approximately I3 percent. ‘I7 6. Subirrigated when the average soil moisture was approximately 1.0 percent or air-dry. 9 ‘Number of irrigations could be called wetting and drying cycles. 6 l5.0 w fi ‘o I I25" IO-O- - \‘ 15-. t: I f 5g. l TREATMENT NO. 4I— SPRING PRESSURE IN POUNDS i3€€ 5-11130‘ _v; 6. i i i 0 - I 0 2 4 sort DEPTH_—_INC_IHES Figure 3. The influence of treatments 4, 5 and 6 on soil c g at various depths as evaluated with a self-recording soil pen - _ (7). Description of soil moisture treatments are indicated in T duplicated treatments as described in Table 4. 4-" saturation the columns were weighed and pla‘ a force-draft oven at 50° C. Each column was " daily until the moisture losses indicated that ‘I time to saturate it again. Each treatment the number of irrigations listed in Table 4. y‘ the indicated number of irrigations, the soil c were evaluated as to soil consistency or comp with a soil penetrometer. During this investigation, Gerard et al. (7)3 able to develop- hardpans under laboratory con, and thus achieve a better understanding of the ‘ affecting their formation. Hardpans produced‘ laboratory were not of the intensity found '_ field but were characteristically similar to th Results of penetrometer analyses as sh‘ Figures 2 and 3, demonstrate that treatmen and 6 produced greater soil strength than trea 1, 2 and 3. The mulching operation may have V‘ differences in soil strength in the case of 4 but this does not explain the greater soil n, with depth under treatment 6, Figure 3. The gators (7) have postulated that a slow rate of i, loss contributed to differential soil strength as ' by the penetrometer measurements under trea-A and possibly treatments 4 and 5. " As a result of the initial findings, further was undertaken to determine if a relationship . between the rate of moisture loss and soil 3 or compaction. Results of this investigatio nitely showed a negative correlation between " of moisture loss and soil strength, Figure 4 andi 5. Briquets, which were imbedded in air-dry") cause slow drying, were 25 to 3O percent stro surface-dried briquets. Lemos and Lutz (l0; reported that the rate of drying on briquet i; was important. Gill (8) postulated also if goof the soil moisture films during drying was itremely important factor in effecting the in- i of soil strength of a clay. V indicated in Figure 4, maximum soil strength _ I ieved at 27° C. Briquets dried at 32° C. ilightly weaker than briquets dried at 27° C; iv- dried at 2l° C. and 75 percent relative ity were markedly lower in strength than dried at 27° C. and 32° C. and 75 percent e humidity. This might suggest that climatic 'ons in the Lower Rio Grande Valley are often f, m for the development of hardpans. Results Q may help» explain the greater soil strength ; by Locke et al. (ll) in Oklahoma than in k Dakota. a: 1960, a laboratory experiment was conducted luate the influence of moisture level, compac- rce and drying cycles on soil compaction. 2-gallon pots were filled to a depth of imately 9 inches. The experiment consisted soil moisture and compactive force treatments j are described in Table 6. The amount of llapplied, mulching and irrigations were con- according to the treatment schedule. Pots laced in a constant temperature room at 32° C. percent relative humidity and were weighed y; every 2 days in order to determine the time plying the scheduled treatment. g e research, to date, has established that i) ature greatly influences soil strength? The "of packing or soil hardness attained at 32° C. ipproximately twice the degree of hardness at 50° C. (4) , Figures 2 and 5. It was also ted from these data that soil hardness or I was proportional to the degree of packing particles and /or inversely proportional to the l ment of m.inute planes of weakness in the soil g In a previous paper the authors (7) have i out that penetrometer analyses are generally ‘red indices of soil consistency, compaction or acking of soil particles. However, the pene- measurement may be an indication of rela- i‘; mbers of planes of weakness occurring within il mass. These two conditions would not be .ngth refers to the ability of the soil to resist force or ‘tion. THE RELATIONSHIP BETWEEN BREAKING STRENGTH OF (MILLIBARS) AND RATE OF MOISTURE LOSS (G./HR.) (4) . . . mg —,_ mime Equation R3 0c p‘ 21 y1= ~4o.2 x2 + 234.3 +0165 27 y= ~—a2.a x + 345.1 —o.99a 32 y= —49.1 x + 331.5 —o.996 fulus of rupture in millibars. of moisture loss in g./hr. coefficient. 400- gaoo- “,4 9 _1 v / ._I _»2OQ - m i » 2| "c. -- 27°C.wIIII4I 32°c..._ _ Ioo- " O - w u 0 25 so 75 RELATIVE HUMIDITY; % Figure 4. The influence of temperature and relative humidity on briquet strength expressed in millibars (4). synonymous necessarily since a close-packed soil could either have few or numerous planes of weakness. Further evidence concerning the influence of moisture loss rates on soil strength is apparent from a comparison of Figures 5, 6 and 7, (treatments 1, 4 and 7) . The increase in soil strength shown in these figures with successive wetting and drying cycles would indicate relatively rapid rate of particle rearrange- ment. This also would suggest that the beneficial effect of subsoiling in irrigated soils of the Valley may be short lived. Other data (7) indicate that these soils are ex- tremely susceptible to compaction by tillage imple- TABLE 6. DESCRIPTION OF SOIL MOISTURE AND COMPACTIVE FORCE TREATMENTS. TREATMENTS WERE DRIED AT 32° C. AND 25 PERCENT RELATIVE HUMIDITY (4)1 Percent moisture Percent at time of moisture compaction and when mulching‘ irrigated Force applied lb./sq. in.3 Treatment numberz 'l . ‘I . '| . I ‘OQQQUI-hwhi-I O 5 O O 5 O O 5 O t"."'l"i°f°i°””” Ooocnuicnuuuuui PPP9PK°I°Z°I° uruiuicnuiuuuicnuu ‘Penetrometer analyses of treatments 1, 3, 4 and 7 will be presented as Figures 5, 6, 7 and 8; penetrometer analyses of treatments 2, 5, 6, 8 and 9 will not be presented in this manuscript but are presented in (4). 2Each treatment was duplicated. 3The force was applied by using a hydraulic iack and platform scale. 412.5 percent a 1/; atmosphere percentage; 9.5 percent 2 7 atmos- phere percentage; 6.5 percent ‘I5 atmosphere percentage. 7 DRYING CYCLES 3i N O 6 it'll; Qii SPRING PRESSURE IN POUNDS 6 é 4 s son. DEPTH-INCHES Figure 5. The influence of treatment l (0 force, mulching at 12.5 percent moisture and dried at 32° C. and 25 percent relative humid- ity) and numbers of drying cycles on compaction as evaluated with a soil penetrometer (4). ments. The coarse-textured soils are often cultivated after surface drying when subsurface moisture is optimum for compaction. Evidence of the suscepti- bility of these soils to compaction is indicated in Table 7 and Figure 8. Cultivation of these soils when subsurface mois- ture is at a high level is probably conducive to com- paction because of the compactive force of the tillage implements and the behavior of the soil moisture films within the compacted layer. Compaction due to tillage implements often improves the capillary conductivity of soils and, therefore, increases the prob- ability of replacing the water films which are evapo- rated from the dense layer. Evaporation of the water films from. the compacted layer usually occurs by vapor movement through the cultivated or mulched surface soil. The evaporation of the water films and subsequent capillary conductivity results in the action of repeated co-hesive forces on the soil. particles in the affected zone. The cohesive forces exerted by 20f (D D Z 3 E Ii film-i", Z u m. I D (D 3 DRYING CYCLES & TY§III— g 9_ _ E O- (D o I I ' O 2 4 6 SOIL DEPTH-INCHES Figure 6. The influence of treatment 4 (0 force, mulching_ at 9.5 percent moisture and dried at 32° C. and 25 percent relative humid- ity) and numbers of drying cycles on compaction as evaluated with a soil penetrometer (4). 8 N O 1' I I f”: o" ’ f DRYING CYCLES l 3i v 6 jloloi , SPRING PRESSURE IN POUNDS a O ' ' O 2 4 SOIL DEPTH-INCHES Figure 7. The influence of treatment 7 (O force, mulching percent moisture and dried at 32° C. and 75 percent relativ ity) and numbers of drying cycles on compaction as evalu a soil penetrometer (4). the moisture films are apparently a function or the rate of evaporation and of temperature. the evaporative process, repeated cohesive ac y the moisture films on the soil particles is co w: to further strengthening of the already co n; layer. if There is indirect evidence that consil force is probably exerted by plant roots. of plant roots and their effect on soil com. have not been comprehensively evaluated. i fluence of plant roots on soil structure has ge been considered completely beneficial roots return organic matter to the soil and ca _ of weakness in the soil mass. In spite of su If ficial effects of plant roots, it is important t’ nize that plant roots can and probably d‘ excessively high compactive forces which are o; functions of both soil moisture content and plant. Furthermore, stresses developed by pla' during the absorption of water may contri nificantly to soil compaction. More research is‘) to evaluate these effects. Bauer (1) has submitted different soil hi: of sand and silt-clay fractions of Willacy fi z. TABLE 7. THE INFLUENCE OF SOIL MOISTURE ON THE BlLlTY OF WILLACY FINE SANDY LOAM (_7‘) Soil moisture Bulk do percentage g./cc ‘ A Standard Proctor Apparatus was used to apply the _ force. The compaction force was a 5-pound hammer and fall, 3 layers-—25 blows per layer. Compaction force =1 pounds per cubic inch. i \ ‘ll ~IIII\ 90‘. . . §§_ \ l DRYING CYCLES sjlalli Sii . c‘ " . Qgmlflu‘ " ‘WiBLh saflwwmemi. ‘w. massheuuwn» - . é i é SOIL DEPTH-INCHES The influence of treatment 3 (l0 Ib./in.2 of force, mulch- 1.5 percent moisture and dried at 32° C. and 25 percent midity) and number of drying cycles on compaction as with a soil penetrometer (4). different soil moisture .treatments. He re- that the wetting and drying of the different 2' tures did not yield any evidence which would the formation of compacted layers. The _ data suggest the need for further investiga- '1 modification of these treatments before final tation of the role of particle size and moisture tment and their interaction on soil strength igmade. Compactive curves of these mixtures ted in Figures 9, l0, 11 and 12. The curves maximum compaction for mixtures II and 80 r 519° y as PERCENT SAND l- IIJ E 5.1.60 - Ill U 2 g . Q l.7O P o: O E o g o <1 .60 - I ‘? >' . '1. g |.so - o IJJ Q d 3 l l l l I s |o as 2o 2s so SOIL MOISTURE-PERCENT Figure 10. Compactibility of 85 percent sand and ‘l5 percent silt and clay at different moisture contents (Mixture ll) ('l). III was approximately 13.5 percent moisture and fo-r mixture IV was 15.0 percent moisture. SUGGESTED MANAGEMENT PRACTICES Tl] ALLEVIATE HARDPAN [IUNDITIUNS The practice most generally recommended for alleviating the undesirable effects of hardpan as out- lined in this publication, is subsoiling. Burleson et al. (2) have reported substantial increases in cotton yield /O0 PERCENT SAND - O A O O p.60 — O w O O 9 (.50 - | _ | I | | 1 O 2.5 5.0 7.5 l0.0 l2.5 |5.0 l7.5 SOIL MOISTURE-PERCENT Figure 9. Compactibility of ‘I00 percent sand at different moisture contents, (Mixture I) (l). 1.90 - 70 PERCENT SAND 1.70 1.50 1.50 . SOIL DENSITY-GRAMS PER CUBIC CENTIMETER I l I I I 5 IO l5 _ 2O 25 3O SOIL MOISTURE-PERCENT Figure 11. Compactibility of 70 percent sand and 3O percent silt and clay at different moisture contents (Mixture III) (1). from subsoiling. These data are shown in Table 8. Increased growth of cotton due to subsoiling was especially marked in 1956 but only minor differences in height were note-d in 1957, Figure l3 (2). Root distribution as influenced by subsoiling are indicated in Table 9. The increase in the concentration of cotton roots in the 6 to 12-inch zone could have been a result of subsoiling. A comparison of a hardpan condition before and after thorough cross-chiseling3 is presented in Figure 14. Obviously, the chiseling operation was effective in breaking up the hardpan. The subsoiling operation often does. not eliminate the hardpan entirely but “Cross-chiseling is a term used to described two chiseling opera- tions at right angles to each other. TABLE 8. SUMMARY OF SUBSOILING AND FERTILIZER PLACEMENT TREATMENTS IN 1956 AND 1957 (2) Average pounds of lint Tnrneeil; Description of treatment "ml" P" "F"? l 956 l 957 A‘ Subsoiled to 18 inches and conventional method of fertilizer application with 6O pounds of‘ N per acre applied as a’ side- dressing at squaring. 1156 689 B Non-subsoiled and conventional method for fertilizer application. Sidedressed as in A. I094 570 C Subsoiled and deep placement of ferti- lizer at 6 to 18 inches deep. Side- dressed as in A. 1187 609 D Non-subsoiled with deep placement as in C. Sidedressed as in A. 1087 538 L.S.D. (0.05) N. S. 103 ‘In 1956, conventional method of fertilizer application refers to 60 pounds of N and 60 pounds of P205 placed in the soil approximately 3 inches below the seed zone before planting. The P205 was in- creased to 120 pounds in 1957. l0 r a: l.90- m t- m E I- 55 PERCENT SAND Z L80" LIJ . o o o 2 ° o 3 O ' __o 2110- . m “ O 0- o m O 3 n: l.60— ‘I’ >- l: Qmo- _ o h-l o o d O <0 I | I 1 5 IO I5 2O 25 SOIL MOISTURE-PERCENT Figure 12. Compactibility of 55 percent sand and 45 and clay at different moisture contents (Mixture IV) (1) does create some planes of weakness in the s’ Research is needed to evaluate further the _ ness of the different subsoiling procedures a " residual influence on crop growth and yield. Gerard et al. (6) and others have report _; correlation between exchangeable sodium strength as evaluated by modulus of rupture i‘ The use of “poor” quality water (high sodium i‘ can definitely intensify soil strength and fonnation in these soils. In these investigati “poor” quality water contained about 2,400 million of total salt and 75 percent of the “5 cations were sodium. The sodium ion dis particles and intensifies close-packing. St Weslaco have shown the existence of a high; tration of exchangeable sodium in the top.‘ soil, Table 3. Dispersion of soil particles‘ sodium ion probably accelerates the close-pal soil particles due to tillage or cohesive acti moisture films.‘ For this reason, the use quality irrigation water may have a marked i, on the subsoiling requirement of these s0ils._'._ TABLE 9. TOTAL WEIGHT AND DISTRIBUTION OF CO i‘ AS INFLUENCED BY SUBSOILING AND DEEP FERTILIZA Depth, Percent of total weight by In C Y1 inches A B 0-6 5.26 33.33 27.01 6-12 82.41 46.75 51.00 12-18 4.04 4.11 3.84 18-24 3.32 5.40 5.02 24-36 3.47 6.88 9.83 36-48 1.25 2.79 2.67 48-60 .24 .71 1.13 60-72 .06 .03 .49 Total weight (g.) 4.5 5.3 5.9 GROWTH RATE OF OOTTON- I958 O 2 3 e SI 5' E 1 I I l1 I l I n | l I l IO 2O 3O IO 2O 3| IO 2O 3O IO 2O 3| IPRL HAY all‘ ul-LY GROWTH RATE OF OOTTON-IQFI 3 3 ‘i’. - ,1 o "i ‘l: -' I C Z n n n n l-l n n n [T1 a I | | D 2O 3O IO 2O 3| IO 2O 3O |O 2O 5| QPRL HAY all‘ Jill-Y ‘ "p13. The effect of subsoiling and fertilizer placement on the of cotton (2). Treatments are described in Table 8. Results from cropping system studies at Substation 715 (5) have demonstrated that rotations, which fie vegetable production generally accelerate the ption and intensity of soil hardpans. Manage- ’ practices in the production of vegetables almost l‘ s necessitates cultivation when subsurface mois- ‘is optimum for compaction, as illustrated in , s 10, ll and 12. Vegetable production is often ucive to accumulations of exchangeable sodium pe soil because of greater frequency of irrigation periods (especially in the fall) when the irri- j. water supply is of poorer quality. Cropping u consisting of cotton followed immediately by egetables should not be used over an extended ‘do of years because this particular practice causes ‘ed deterioration soil structure. Soils which are allowed to approach air dryness depth of 12 inches or more will develop greater trength, which subsequently will impede air and 1 movement as well as root development. For reason, subsoiling will probably be desirable a ing several seasons of drouth. Laboratory Ul O b O or Q IIiJJiJJl m 9 l! l ' NON-SUBSOILED i -”- SUBSOILED 1//\\‘