13-1725 July 1995 Texas Agricultural Experiment Station Edward A. Hiler, Director The Texas A&M University System College Station, Texas ‘Ea 1\ ~_'~ -<~¢~.-' \ “ ~ -'~ ..., __,:'-,-» H“)? —T v’ Plant Nutrition and Fertilizer Management for Winter Wheat Production in the Blackland Prairie Tim C. Knowles, Billy W. Hipp, David S. Marshall, and Russell L. Sutton Respectively: Research associate, professor, professor, and assistant research scientist. Texas Agricul- tural Experiment Station, Texas A&M University Research and Extension Center at Dallas. Keywords: Nitrogen, phosphorus, soil, plant testing, timing, residue, no-till, grain , .1 . 5. \ ._> .\ 1 a _ x 9'1». *‘~.~.-f.= ‘ 15:1 _ < “L ml. a? ‘ ' _ . n; ~ _ ’ 1 , | /_ . _ \- ' '\ \ . 3 ~"-_ .- ' ' Contents Introduction ....................................................... ....................................................................................................... .. 1 Rainfall and Winter Wheat iWater Use ................................................................................................................. .. 1 Blackland Prairie Soils ............................................................................................................................................ .. 2 Soil Nutrient Status ....................................................................................... .. ......................................................... .. 2 Phosphorus Fertilization ......................................................................................................................................... .. 3 The Nitrogen Cycle .................................. .................................. ............................................. ............................. .. 5 Effects of Cropping System and Previous Residue Management on Nitrogen Fertilization .................... .. '7 Winter Wheat Production Costs in the Texas Blackland Prairie .................................................................. .. 12 Fertilizer Economics .............................................................................................................................................. .. 12 Fertilizer Materials for Winter Wheat Production .......................................................................................... .. 14 Winter Wheat Developmental Growth Stages .................................................................................................. .. 17 Timing and Methods of Fertilization for Blackland Prairie Winter Wheat Production .......................... .. 20 Interactions of Fertilizer With Disease and Insect Pests of Winter Wheat ............................................... .. 22 Interactions of Fertilizer With Winter Wheat Varieties ................................................................................ .0 22 Soil and Plant Tissue Testing for Profitable Blackland Prairie Winter Wheat Production .................... .. 24 References ................................................................................................................................................................. .. 27 i {.4} 2:11;; - i; w-g” '.,- .- ‘s c 13:- 1, 1-3 *' ,*'_i‘§sv_>4-_,=;:A¢%=-- - ' Q? *2»- “Ifii-vx. _ w; r ‘;'"~*re' . s " » ~; ,_ - ~ W»), 4", ‘ \ '. . r'.‘ -_. - ~ . _ , \ ' \. ' \ i . ‘ e ' . , . . \ . ‘ — ' . I ~ ~ .- . L 1 , ‘l — ' . " _ . ,. l _ _ . Introduction Nitrogen (N), and to a lesser extent phosphorus (P), are the two fertilizer nutrients that are often re- quired for maximum grain and forage yield of Black- land Prairie winter wheat crops. Occasional sulfur (S) and zinc (Zn) deficiencies in Blackland Prairie soils have also been identified by soil testing prior to planting winter wheat. Interrelationships between soil moisture availability, soil temperature variation, soil chemistry and nutrient cycling, wheat plant nutrient availability and uptake, and efficient fertil- izer management are complex and dynamic. Exces- sive amounts, poor timing late in the wheat grow- ing season, and ineffective techniques of fertilizer application can lower winter wheat yields, have the potential to create environmental problems such as pollution of ground and surface water with nitrate, and are expensive to growers since very little of the total applied fertilizer can be utilized by the wheat crop. Therefore, understanding dynamic processes occurring in the plant, soil, and climate, and their interrelationships between efficient fertilizer man- agement practices, will enable wheat producers to make informed sound fertilization decisions that increase net profits and reduce the potential for en- vironmental pollution. The Blackland Prairie is one of the major agricul- tural regions of Texas. Approximately 12.6 million acres are included in the Texas Blackland Prairie which is a belt of land extending from the Red River to near San Antonio (Figure 1). About 5.6 million acres of the Blackland Prairie are in the Houston Black-Heiden-Austin soils association. Other major soil associations in the Blackland Prairie include the Wilson-Crockett-Burleson (Greylands) association, Burleson-Heiden-Orockett association and Austin- Stephen-Eddy association. Blackland soils are dark colored, high in montmorillonite clays (smectites), swell when wet, and shrink when dry. Cracks three feet deep and three inches wide during dry periods are common on Blackland soils classified as Vertisols. Primary Blackland crops include wheat, oats, grain sorghum, corn, cotton, forage sorghum, and legumes. About half of the region is rangeland with pasture crops consisting primarily of Coastal bermudagrass, clover, alfalfa, wheat, oats, and na- tive grasses such as bluestem, Indiangrass, switch- grass, and sideoats grama. Erratic rainfall during the summer growing period can limit production of cotton, sorghum, and corn in some years. Winter wheat and oats are more adaptable to the fall and spring peak rainfall patterns that occur in the Texas Blackland. Rainfall and Winter Wheat Water Use Long-term average annual rainfall at Dallas is 35.08 inches with distinct peak rainy periods from April through May (4.5 inch/month) and Septem- ber through October (3.5 inch/month). These pe- riods of peak rainfall are extremely important for winter wheat production in the Blackland. Winter wheat water use (including evaporation) peaks twice at 4.8 inch/month during March, then at 5.6 inch/month during May (Figure 2). In a normal year, water requirements of winter wheat exceed available soil moisture supplies resulting in mod- erate moisture stress from February through June. During years that have less than normal rainfall, severe moisture stress can occur during this pe- riod, drastically reducing wheat grain yields. Topdress applications of mobile fertilizer nutrients such as nitrogen require adequate rainfall for in- corporation into the soil and uptake by wheat plant roots. Thus, mobile fertilizer nutrients should be topdressed not only prior to periods of peak wheat demand for these nutrients, but also in time for rainfall to move these nutrients into the soil-plant root zone where they are available for uptake by wheat plant roots. Furthermore, under limited soil moisture, larger winter wheat plants that are de- veloped due to excessive rates of fertilizer which stimulated early season growth may experience severe moisture stress, resulting in lower grain yield than wheat fertilized at sufficient levels. VEGETATIONAL AREAS OF TEXAS ‘Ialaa w Experiment Idea E "Mwood. TAB-PRIOR Tall-balm ED Gull Pralrlaa Q Croaa Tlrnbara Z South Tana Plalna X Eduardo Plabau ED Rolling Plalna High Plalna ‘Dana-Poona, Iountalna and Baalna Figure 1. Location of Texas A&M Research and Extension Cen- ters in the north Blackland region (Courtesy Dennis W. Walker, TAES-Dallas). BE C) | Calculated Wheat Water Use lllllllll n u n a a I I n I a r a n a a r I Deficit h l Rainfall . . - . - - - . - _ - - < - - . . - . I . . u 1 ID I ---.-¢.-‘. . . . . . . _ - . . _ - . _ . - - _ . - - - . - - . . ~ ~ t4 Mont-lily Rain or (imp Water Use (in) l I Planting l NOV DEC JAN FEB 5-6 Leaf Joint I l l MAR APR MAY J UN Boot Heading l l C) Date Figure 2. Normal monthly rainfall and calculated winter wheat crop water use recorded from 1947 to 1983 at TAES-Dallas. Blackland Prairie Soils Blackland soils are high in clay (45 to 60%), and productivity is generally regulated by available soil moisture (Welch et al., 1977). When soils are dry, initial water intake (infiltration) is relatively high. However, swelling of the shrink/swell montmorillo- nitic clay closes pores and cracks upon wetting, thereby substantially reducing further infiltration of water into the soil. This condition increases the potential for runoff on the soil surface from intense rainfall and soil losses by erosion. Erosion control measures that reduce runoff from soils and increase water infiltration are necessary for efficient agricul- tural production. This may be accomplished by main- taining a vegetative or crop residue cover on the soil surface through crop residue management (Hipp and Simpson, 1988). Terraces and diversions that reduce the length of slopes also can help reduce run- off and erosion. The light colored chalk and interbedded marl subsoil from which most Blackland soils formed is primarily calcium carbonate (lime or calcite). The surface soils are high in calcium carbonate (generally ranging from 15 to 45%), consequently these soils are classified as calcareous (Hipp, Knowles, and Simpson, 1992). Most Blackland soils are not exceedingly high in soluble salt. An abundance of calcium carbonate in these soils results in high concentrations of extractable cal- cium (often exceeds 1%), magnesium (often ex- ceeds 0.02%), potassium (often exceeds 0.03%), and sodium (often exceeds 0.002%). Therefore, these mineral elements are rarely deficient in Blackland soils, and fertilizer applications are seldom sug- gested. Organic matter content of the surface soil is somewhat high (ranging from 2 to 5%) which contributes to the dark color of surface soil, thus some Blackland soils are classified as carbonatic. Montmorillonitic clay and organic matter content contribute to high cation exchange capacities (CEC) of these soils (ranges from 25 to 50 meq/ 100 g). Soil pH, a measure of hydrogen ion activ- ity in soil, greatly affects availability of plant nu- trients. Alkaline soil pH values ranging from 7.6 to 8.3 reflect the calcareous nature of Blackland soils. Major effects of alkaline soil pH often re- duces the solubility (and subsequently availabil- ity) of iron and zinc and the availability of soil and fertilizer phosphorus due to formation of insoluble calcium phosphate. Soil Nutrient Status Blackland soils normally test very high in calcium (Ca), magnesium (Mg), and potassium (K) and ad- equate in sulfur (S) due to their clayey, calcareous, carbonatic, and alkaline properties. Soil test concen- trations normally exceed critical levels suggested for winter wheat by the Texas A&M Soil Testing Laboratory which are 180, 50, and 126 parts per million (ppm) for exchangeable Ca, Mg, and K, re- spectively, and 25 ppm for sulfate-S. Fertilizer addi- tions are suggested when soil test values are below these critical levels. Therefore, fertilizer additions of Ca, Mg, K, and S are rarely profitable and seldom suggested in the Blackland. Calcium is abundant due to high levels of calcium carbonate in these soils. Large quantities of exchangeable K are fixed within clay minerals and are slowly available to plants. Magnesium levels also appear adequate for current production levels. Sulfur fertilizer applications are normally not suggested for winter wheat grown on Blackland soils since the S released from organic matter and that inadvertently supplied by nitrogen and phosphorus fertilizer applications provide ad- equate S to prevent widespread S shortages (Welch et al., 1977). Soil micronutrients, including iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), boron (B), mo- lybdenum (Mo), and chlorine (Cl), are required in trace amounts for optimum plant growth plus maximum grain yield, but can be toxic at even slightly excessive levels. Soil levels of these min- eral elements have been adequate for current win- ter wheat production levels. Conditions do exist where iron and/or zinc fertilizer application can increase wheat grain yields, however, yield in- creases from these applications are rarely profit- able. Zinc and iron deficiencies are sometimes as- sociated with alkaline, calcareous soils that are inherently high in soil phosphorus (P). Excessive fertilizer-P applications on Zn deficient Blackland soils can result in severe Zn deficiencies that ap- pear to retard development and maturation of wheat grain heads (Hipp and Hooks, 1978). A pre- plant soil test should be used to confirm micronu- trient deficiency or adequacy. Currently, soil-test critical levels defined by the Texas A&M Univer- sity Soil Testing Laboratory for winter wheat are 4.2 and 0.28 ppm for Fe and Zn, respectively. Nitrogen (N), and to a lesser extent phosphorus (P), are the two essential nutrient elements which most often limit the production of wheat in the Texas Blackland. Cold soils in late winter can decrease P uptake by young rapidly growing winter wheat plant roots and can reduce the availability of soil-P re- serves. Thus, a cool-season crop such as winter wheat often responds to fertilizer P applications when N is applied in adequate amounts. Phospho- rus fertilizers are normally broadcast and worked into the surface 4 to 6 inches of soil prior to plant- ing, or banded below the seed with the grain drill at planting due to the immobility of P in soils. Follow- ing 35 years of non-fertilized continuous wheat, Blackland soils had the nutrient-supplying capac- ity to yield about 20 to 30 bushels of grain per acre (Hipp and Simpson, 1988). When fertilized with suf- ficient N and P, Blackland soils can yield 40 to 70 bushels of wheat per acre, or up to 100 bushels of oats per acre when rainfall is adequate to meet sea- sonal crop moisture requirements. Split applications of N fertilizer are not required for maximum grain yield of winter wheat that is not grazed or planted in dense or no-till crop residues. These soils will min- eralize (make N available to wheat from organic matter) from 40 to 120 pounds N/acre/year, depend- ing on soil moisture, temperature, crop residue man- agement, and soil organic N levels. Wheat in no-till and non-fallow rotations (such as wheat immediately following sorghum, corn, or cotton) requires higher N application rates than if a fallow period has oc- curred between crops (for example, wheat after wheat, or sorghum-fallow-wheat). Again, a preplant soil test will confirm N and P deficiencies. Currently, soil test critical levels defined by the Texas A&M Uni- versity Soil Testing Laboratory for winter wheat at typical Blackland yield potentials are 40 and 26 ppm for nitrate-N and TAEX-phosphorus, respectively, Phosphorus Fertilization A soil sample taken up to one month prior to plant- ing winter wheat, and analyzed for available P con- tent should be used to predict if additional P fertili- zation may increase winter wheat grain yields. Ide- ally, this soil test characterizes the quantity of soil- P reserves that is readily available to, and that can readily be assimilated by a winter wheat crop. The accuracy of a preplant soil test in predicting P fer- tilizer requirements at planting can be affected by stratification of available P in the soil profile and the laboratory chemical solution (the extractant) used to extract available soil P. When stratification of soil-P occurs, available P concentrations in the surface 2 inches of soil can be two to four times the concentration of available P at soil depths ranging from 2 to 8 inches. Sampling this 2 inch band of con- centrated soil P can result in erroneously high con- centrations of available P resulting in insufficient recommendations for fertilizer-P application in the soil test report. Consequently, soil should be sampled from a 2 to 6 (or preferably 8) inch soil depth for available P soil tests. Two chemical solutions are currently used to ex- tract available P from alkaline, calcareous Blackland soils. The Texas A&M University Soil Testing Lab utilizes an ammonium acetate-EDTA extractant for available P soil analyses (TAEX-P). Some soil test- ing laboratories use a sodium bicarbonate solution to extract available P from soil (Olsen-P). For rou- tine determinations of available soil-P in regions where a wide range of acid, neutral, and alkaline soils are found such as in the state of Texas, the acidic TAEX extractant is preferred if a laboratory is lim- ited to the use of only one extractant. However, in alkaline, calcareous soils of the Blackland, the weak acid in the TAEX extractant is rapidly neutralized by soil carbonate rather than utilized in extraction of calcium phosphates. Additionally, the EDTA com- ponent of the extracting solution can dissolve cal- cium carbonate in calcareous soils and the associ- ated soil calcium-P that may be unavailable to wheat plants, resulting in overestimation of available P in Blackland soil. When calcareous Blackland soil is extracted with the alkaline Olsen’s sodium bicar- bonate solution, the calcium concentration in the ex- traction solution is decreased by precipitation as calcium carbonate, and as a result, the concentra- tion of P from calcium phosphate in the extracting solution increases. Often, the Olsen soil test gives a better index of plant available or soluble soil-P for alkaline, calcareous soils such as those found in the Texas Blackland. Various factors affect the availability of fertilizer phosphorus (P) to winter wheat, including fertilizer rate, placement of P (Hipp and Hooks, 1978), soil test level P, soil temperature or planting date (Hipp, 1987a), and available soil moisture. Phosphorus is immobile, in that it does not move very much in the soil profile. Therefore, fertilizer P must be either incorporated into surface soil or banded with seed at planting in order to be positionally available to wheat plant roots. Phosphorus fertilizer formulations consist primarily of two chemical forms, orthophos- phate (ordinary superphosphate, triple superphos- phate, ammonium phosphate, and potassium phos- phate fertilizers) and polyphosphate (urea ammo- nium phosphate, ammonium polyphosphate, and potassium polyphosphate fertilizers). Plant roots take up the orthophosphate form, and polyphos- phates are readily converted to orthophosphate in the soil. Thus, both fertilizer forms give essentially equal results as P fertilizer sources. Only a small percentage of applied P fertilizer is utilized for crop growth, and the remaining fertilizer-P will eventu- ally enrich various pools of soil P. This residual soil- P can make significant contributions to the P nutri- tion of succeeding crops. Winter wheat often responds to P fertilizer ap- plications due to reduced availability of soil-P re- sulting from cold winter soil temperatures. Cold soil temperatures reduce microbial decomposition of organic matter and subsequent release of avail- able soil-P (Spence and Welch, 1977). Conse- quently, less available P is liberated from soil or- ganic matter during the growing season of a cool season crop (winter or fall) than during growth of a warm season crop (spring or summer). Further- more, the absorption and utilization of soil P re- serves by winter wheat plants are decreased by cold winter soil temperatures. Therefore, applica- tions of fertilizer P with or near the wheat seed often increase grain yield of winter wheat, even in fields that normally do not require P applica- tions for maximum yield of warm-season spring crops such as corn, sorghum, and cotton. Field studies were conducted to determine the influence of rate and placement of fertilizer-P at three planting dates or soil temperatures (Hipp and Hooks, 1978, Figure 3). Planting dates were September 15, November 15, and December 15 at average daily air temperatures of 67.8, 46.7, and 36.5 degrees Fahrenheit, respectively. The Septem- ber 15 planting date is typical of winter wheat grown for forage production (pasture), while the November and December planting dates are more typical of winter wheat grown for grain produc- tion. Wheat forage and grain yields were not in- creased by P fertilizer applied to wheat planted at the relatively warm September 15 planting date, but P application increased forage and grain yields of wheat planted later at colder soil temperatures oc- curring in November and December (Figure 3a, 3b). Furthermore, incorporation of fertilizer P into sur- face soil was necessary for optimum positional ava.il- ability to plant roots and maximum winter wheat forage and yields at the November and Decem- ber planting dates (Figure 3c). Broadcast fertilizer P without soil incorporation did not result in higher wheat forage and grain yields compared to unfertil- ized wheat. Field studies over a 4-year period indi- cated an interaction between soil-P level required for winter wheat production and planting date or temperature (I-Iipp, 1987a). Early planting (Septem- L-No P Applied Use n» nos/a. 1_5_. . . . . . . . . . . . . = . . . . . - - - - - - - - - - - - - - - ‘i a e . DrylletteronAprlH E 3 " Q ' ' ' ’ ‘ “ ' T ‘ ' ‘ ' G Q " a O U 2 205-» - ~ - - - - ‘ = ~ - '- g cd ; d °_ Z/ September 15 November 15 December 15 DeteotPlentlng 5°? "',' ' ‘ ‘ ' ' ' * ' ° "fIna¢A5pi|a¢'Qsa|ai=§dsi-a' - e i f i b 34°1 ‘ Q : 3 l ~ - ;’°- b > -: ,5 1 83°- " " ' 0 i g,“- . . . ,3 September 15 November 15 December 15 DateotPlentlng ,,I II"°'°*PP"“Q§°.'?!’?'Q".'f--E3I°9P?Q‘Z'F- ‘i I i“; .3. 1 I I zw- <= i 0”" ‘i Phoephorue Application Method Figure 3. Influence of phosphorus (P) rate, date of planting, and P application method on Sturdy winter wheat grain and forage yield. ber-October) resulted in warm temperatures during the five-week period of early growth that is critical for uptake of soil P by winter wheat. Later plantings (NovembenDecember) resulted in much colder air temperatures during this critical five-week period. Therefore, grain and forage yield increases with P fertilization at later planting dates indicated a need for higher amounts of available soil- or fertilizer-P for early growth of winter wheat on cold soils that have marginally adequate reserves of soil P. Regard- less of planting date, P fertilizer applications were not required for maximum winter wheat forage and grain yields when preplant Olsen available P soil levels exceeded the critical level of 19 ppm. Frequently, wheat forage yield increases due to P fertilization are 100 to 200% higher than unfertilized wheat, while grain yield increases may be only 10 to 50% higher than unfertilized wheat. Four hard red winter wheat varieties (‘TAM 201, 2180, Siouxland 89, and TAM 300) and two soft red winter wheat va- rieties (Pioneer 2548 and Coker 9543) were grown with and without P fertilizer at Prosper (Figure 4). Phosphorus, as triple superphosphate (0-4 6-0), banded with the seed during planting at a rate of 50 lb P205 increased grain yields of all six varieties by 10 to 12% compared to unfertilized plots (Figure 4b). At the same fertilizer-P rates, all six varieties had similar grain yields, but Siouxland 89 hard red win- ter wheat had the highest forage yield compared to the other five varieties (data not shown). Nonethe- less, P fertilizer applications to winter wheat did not result in higher net income from wheat grain sell- ing for $2.60/bushel at harvest in 1993 (Figure 4c). ‘Typically, Blackland winter wheat grown for pasture will be grazed out beyond the normal animal removal date of February 15 (jointing) sacrificing grain pro- duction for livestock production when gra.in prices are this low. Therefore, when winter wheat grain prices are low, P fertilization may not be profitable for grain production, but can be profitable due to increased forage yields of pasture wheat that may be planted late on cold, P-deficient soils. When available P is deficient in Blackland soils, applications of 40 to 60 lb P2O5/acre are customarily suggested. When P is band-applied utilizing a grain drill at planting, 40 lb PzOs/acre should be adequate for winter wheat grown on P deficient soils. This is the least expensive application method for P fertili- zation. However, when P is broadcast preplant and incorporated into the surface 4 to 6 inches of soil, the 50 to 60 lb PzOs/acre P application rate is neces- sary. Depending upon the rate, application method and formulation of P fertilizer, 1994 cost per acre of P fertilization ranged from $12 to $15/acre. Grain yield increases due to P fertilization of 3.4 to 6 bushel/ acre higher than unfertilized wheat are the break- even points for the cost of P fertilization when wheat grain is sold for between $2.50 and $3.50/bushel. Many Blackland wheat producers band 100 lb 18-46- 0/acre with the grain drill at planting to supply 18 lb N and 46 lb P205 per acre. This application is ad- visable for P deficient soils and results in a suffi- ‘ ITAI :01 B2100 Elsiunriana 00 i” ' ’ ' ' ' ‘ m»ia..u'a¢t:i1msa' ' mesa-rat.‘ " §‘°j "mama. ' ' ‘ ' ' ' ' ' ‘ “ ' ' " ' " glfl- - - - - * - - - - ' ' - ' - = ' '__',' a “ F i-I-I -=~~ 1w 6 a _ g ‘i d -:~:~:\:~:~:~ z a 10 - :-:~;-\~:-:-: -I-I-I\I-I-I- o _ f 4O Nitrogen Rate (lb/acre) ITAI 201 E2100 Elsieuxiana 00 5”" “'mpi.a.a"=aai:i1ms@a' " unease" ' ' ' " i s0 ~ é . I 4" ‘i s’- ~ b .5 3° '" 5 i” _ 3 10 -~ o _ I .... I O 50 Phosphorus Rate (lb P205/acre) INOPAppIIQI BUOUPIOI/eml a _ _ , _ _ _ _ _ _ _ _ _ _ _ _ _ . . . . . _ . . . 0 - - ' ‘i i _ . . - . . - . _ » - - - - - ~ , - ~ - - - 8 g C E z Nitrogen Rate (lb/acre) Figure 4. Effect of preplant nitrogen (N) fertilizer and phosphorus applications on winter wheat grain yield in a wheat-cotton rota- tion at Prosper. income was $2.65/bu hard and $2.55/bu soft red wheats; expenses were $0.25/lb N, $0.26IIb P205, and $0.14/bu hauling; production costs were $69.25/acre. cient quantity of P for the winter wheat growing sea- son plus enough N for adequate utilization of P fer- tilizer by wheat plants through the winter months. Excessive rates of P fertilization can result in se- vere winter wheat zinc deficiencies on already zinc deficient Blackland soils. Occasional zinc deficien- cies occur in Blackland soils when soil P and/or P fertilization is excessive; however, at an application cost of $6 to $12/acre for 4 to 6 lb zinc/acre, growers in the Texas Blackland Prairie would seldom break- even on the cost of zinc fertilization since grain yield increases of less than 5 to 10 bushel/acre can nor- mally be expected. Therefore, Blackland wheat pro- ducers should determine the sufficiency of soil-P and zinc from a preplant soil test to ascertain whether P or zinc fertilization at planting would result in prof- itable grain yield increases. The Nitrogen Cycle Nitrogen (N) is the key fertilizer nutrient for prof- itable winter wheat crop production in the Texas Blackland. More than any other plant nutrient, N in the soil is subject to a complex system of gains, losses, and interrelated reactions (Donahue, Miller, and Shickluna, 1983). Intelligent crop management demands a working knowledge of these relationships depicted in Figure 5. Soil-N gains and transforma- tions include fertilizer or manure applications, fixa- tion of atmospheric N2 by legumes (clover, alfalfa, and soybean crops), decomposition of crop residues, mineralization of ammonium (N H 4*) from organic matter, and nitrification transforming ammonium to more readily available nitrate (N 02). Available N losses from the system consist of leaching of soil N as N02, gaseous losses of N by denitrification in water-logged soils and ammonia volatilization of sur- face applied ammonium or urea fertilizers on cal- careous soils. Immobilization of applied N fertilizer by decomposing crop residues and changes in avail- ability such as fixation of ammonium by clay miner- als can result in a temporary loss of N from the sys- tem. Large quantities of soil N exist in the organic form in soil organic matter and N2 gas in the soil atmosphere, but these forms of N cannot be utilized directly by crop plants. Leguminous crops such as alfalfa, clover, and soybean can utilize atmospheric- N, however most non-leguminous crop plants take up soil-N primarily in the nitrate (N 02) form, and to a lesser extent, the ammonium (N H 4*) form. 0nly a small part of soil N pool exists in these forms. Ni- trate-N is easily leached from soil and both nitrate- and ammonium-N can be lost to the atmosphere as N20 and NH2 gases, respectively. The primary source of indigenous soil-N in Blackland soils is derived from mineralization of organic N from soil organic matter. Mineraliza- tion is the conversion of organic N from soil or- ganic matter to ammonium-N mediated by soil T__——> [Ttrnoepheric Nitrogen Gae 1E2] i-larveet Nitrogen Fertilizer / L’..'. mum. 2 \ "Pm" Fixation $1; Plant l Uptglrg Temporarily tied mun.“ b, ir Changed by (|:,:¥,;ma:) Organic Nitrogen :'°"?"'I‘:_N_ (°"9""° "m", (mineralization) f bacteria te um mm u. a----==-»--»-» r\ ‘ Water- v M00“ Changed by bacteria Soil “" 2/ :::..".::::::'.~., ¢l<-----» Clay Lmmm‘ llineraie Leeeee Figure 5. The nitrogen (N) cycle showing N additions, transfor- mations, and losses. bacteria. Principal sources of soil organic matter include decomposing crop, plant and animal resi- dues, and animal manure. Soil ammonium-N ions are short lived in alkaline, calcareous soils. Am- monium-N can become unavailable by temporary fixation within the structure of clay minerals, can be absorbed directly by plant roots or microbes, and/or predominately is oxidized by soil bacteria to nitrite (N02), and then to nitrate-N. Nitrite-N is toxic to plants, but rarely accumulates in large quantities in soil. The oxidation of ammonium-N to nitrate-N by soil bacteria is called “nitrifica- tion”. This process is rapid, normally occurring within hours to 1 or 2 days, and is an important step in soil acidification. Mineralization of N from soil organic matter occurs at a higher rate in non- cropped, fallow soils than it does in soils currently under crop production. Mineralized nitrate-N can accumulate in fallow soils since no crop is present to remove it. Therefore, preplant soil nitrate-N lev- els are normally much higher for winter wheat fol- lowing a 6 to 12 month fallow period than preplant soil nitrate-N levels immediately following spring crops such as sorghum and cotton. Another significant source of soil-N comes from nitrogen fixation. Bacteria of the genera Rhizo- bium and Bradyrhizobium (and a few others) in- duce formation of nodules on roots of legume plants and have the ability to convert atmospheric N 2 into a form of N usable by the host legume plant. Leguminous crop plants commonly grown in the Blackland Prairie include clovers, vetch, soybean, peanut, cowpea, edible beans, and garden peas. The amount of N supplied by symbiotic fixation varies from 30 to 80% of the total N requirement of leguminous crops. Low numbers of Rhizobia gen- erally occur in soils not previously cropped to le- gumes, resulting in poor nodulation of roots. Thus legume seed is often inoculated with the proper bacteria just prior to planting. Active nodules con- taining Rhizobia Iiormally form on roots of le- gumes three to four weeks after planting and leave a red-colored stain when crushed. Applications of fertilizer N to leguminous crops will reduce the number of nodules formed by Rhizobia on roots and can greatly reduce the amount of N fixed by the host plant-bacteria relationship. Significant losses of soil N can occur via leach- ing of soluble N ions (N 03) below plant roots, deni- trification, and volatilization. Nitrate is the most readily leached N form. Since nitrate-N is mobile, heavy amounts of rainfall can temporarily leach ni- trate below the roots of wheat. This leached soil ni- trate-N can become positionally available to wheat plant roots later as the soil profile dries or when wheat roots grow deeper into the soil. Ammonium- N resists leaching because the positively charged ion is held tightly by soil cation exchange sites. How- ever, ammonium is short lived in our alkaline soils, where it is rapidly converted to nitrate-N via nitrifi- cation. Urea-N is intermediate between nitrate-N and ammonium-N in its leachability from surface soil. Leaching losses can be avoided by proper timing and placement of N fertilizer applications. Denitrification primarily occurs in water-logged soils temporarily deficient in oxygen. In this process nitrate-N is converted to gaseous N20 and N 2 which is lost to the atmosphere. Denitrification is rapid, occurring when conditions are right, within several days, or less. The most common condition favoring denitrification in the Blackland Prairie is heavy amounts of rainfall resulting in water-logged soil following N fertilizer applications. Ammonium and anhydrous ammonia (N H3) fertilizers rapidly un- dergo nitrification in our alkaline soils. If oxygen is depleted from water-logged clay soils following pe- riods of heavy rainfall, then appreciable losses of nitrate-N from fertilizer-N via denitrification can oc- cur. Alkaline ammonium or urea N fertilizers ap- plied to calcareous Blackland surface soil without incorporation can also be lost to the atmosphere as gaseous ammonia (N H3) via ammonia volatilization. Incorporation of ammonium-N fertilizers into the surface 4 to 6 inches of soil, adequate rainfall to wash N into the soil immediately following N applications, and/or injection of anhydrous ammonia at greater depths minimize ammonia volatilization losses in the Texas Blackland Prairie. Soil microorganisms (bacteria and fungi) can tie up (immobilize) both soil- and fertilizer-N during the decomposition of previous crop residues due to in- corporation of N into their cell constituents. Under favorable conditions, crop residues are rapidly bro- ken down by soil microorganisms to dark-colored soil organic matter called soil humus. Some humus has undergone mineralization liberating ammo- nium-N from organic matter. This ammonium-N is rapidly transformed into nitrate-N via nitrification. During decomposition, the massive increase in popu- lations of soil microbes, nitrate and ammonium-N may be consumed directly by heterotrophic (derive energr and carbon from the decay of organic mat- ter) microorganisms as they decompose crop resi- dues that have a high C:N ratio instead of being re- leased to the soil solution. This process is called immobilization. For a short period of time, immobi- lization exceeds mineralization, resulting in lower soil nitrate-N levels than it was before residue was added. This period of nitrate (available N) depres- sion depicted in Figure 6 can be severe enough to cause a crop plant N deficiency and should be al- lowed to occur before such stress can affect current crop growth (Foth et al., 1982). The length of this available N depression period and the final concen- tration of nitrate-N in the soil solution is determined by the carbon to nitrogen (CzN) ratio of the previ- ous crop residue. The narrower the C:N ratio of the residue the shorter the depression period for available N, and the higher the new N concentra- tion in the soil solution (Figure 6). For example, residues of a previous alfalfa crop incorporated into soil would return more nitrogen to the soil in a shorter period of time than incorporating the same amount of wheat straw. Microbial Decomposition of Crop Residues lMlnel-alization > Immobilimtiom Immobilization > Mineralization ¢—-————->Period of maximum | microbial activity | ‘ I Q | | '5' Nitrate | | New t; i levels .... Original , “u m levels in | w g I i-J Mkmbe‘ : Nitrate depression i : x m ' : Residue added‘ TIIVIE D Flatiron-Nitrogen Ratios of Some Typical Soil Residues T Residue C:N Ratio Soil llamas l0 : l (aarrow) Young Legumes 12-1) : 1 4 ‘ Youag Grasses 20-40 : l Maaare 10-50 = l Sorghum or Cara Stalks 40-50 : l Wheat or 0st Straw 60-80 : l n» Leaves 60-100: l Piae Needles 200-250 : l Sawdust 400 : l (wide) Figure 6. When crop residue containing a wide C:N ratio is added to soils, microbial activity increases resulting in a decrease in nitrate-nitrogen availability. During the initial nitrate depression period, all free soil nitrates are being used by microbes and are not available for crop growth. The length of the depression pe- riod is determined by the C:N ratio of added residue. Effects of Cropping System and Previous Residue Management on Nitrogen Fertilization Reduced and no-till crop production systems are becoming common practices in the Texas Black- land (Pigg, 1994). Advantages of tillage systems that leave crop residues at or near the soil surface include reduced soil erosion, improved soil physi- cal condition, less farm fuel use, and improved soil water conservation (Box, 1973). Reduced tillage systems include no-till, mulch-till, ridge-till, and strip-till. For no-till production, soil is left undis- turbed following harvest and prior to planting. The crop is planted directly into undisturbed residues of the previous crop. Weed control is primarily with herbicides. In mulch-till systems previous crop residues are tilled leaving sufficient residue (30 to 60% remaining) on the soil surface to re- duce erosion prior to planting. Only a small por- tion of the previous crop residue is incorporated into the top few inches of soil using tools such as chisels, field cultivators, disks, sweeps, or blades. Weed control is with herbicide and/or cultivation. Ridge-till and strip-till systems are predominately used for row-crops. They involve normal tillage of a narrow band in the seed row prior to planting, leaving previous crop residues between seed row strips and on the soil surface. Residue management begins at harvest since the sequence and number of tillage and planting operations affect the amount of previous crop resi- due remaining on the soil surface. The type of point or blade used with tillage implements has a great impact on the amount of residue remaining on the soil surface (‘Table 1). A shallow chisel plow with sweeps can leave up to 85% of the existing crop residue on the soil surface after one pass while a deep disk chiseling with 4-inch twisted points could leave as little as 30% of the previous crop residue on the surface (John Deere, 1991). V- shaped blades that are 30 or more inches wide undercut and disturb very little of the existing crop residue on the soil surface. These blades can leave up to 85 to 95% of the wheat residue that existed before one tillage pass. Straight chisel points turn and mix the soil less than twisted chisel points. Thus, in wheat residue, straight points can leave 6O to 80% of the residue that existed before a pass, while twisted points leave 50 to 70% of the exist- ing wheat residue. Sweeps with low crowns frac- ture and loosen the soil, but incorporate less crop residue than medium crown sweeps. In wheat resi- due, a chisel plow with 12 inch small crown sweeps can leave 70 to 85% of the residue that ex- isted before the pass. Soybean, cotton, peanut, and low-yielding Wheat (under 45 bu/acre) are fragile crop residues in that considerably less residue remains on the soil surface after tillage, compared to non-fragile corn, sorghum and 45 bu/acre or higher yielding wheat crop residues. Pictorial guides and beaded string-line kits are available from offices of the USDA Natural Resource Con- servation Service (former Soil Conservation Ser- vice) to help growers estimate crop residue levels on the soil surface in fields under conservation plans (Soil Conservation Service, 1992). When crop residues are added to the soil, mi- croorganisms use the carbon (C) for energy and nitrogen (N) for building body tissues. Soil organ- isms assimilate about 30% of the C in crop resi- dues, with the balance given off and lost from the soil as carbon dioxide (CO2). Most crop residues contain about 50% C with varying amounts of N. For instance, wheat straw could contain 50% C and 0.7% N for a C:N ratio of about 70:1, while clover residue could contain 50% C and 4% N for a C:N ratio of about 13:1. When crop residues with a nar- row C:N ratio (lower than 17:1) are added to soil, N will be mineralized and released in plant avail- able inorganic N forms until an equilibrium C:N ratio of about 10:1 is obtained (Figure 6). However, incorporation of crop residues with wide C:N ra- tios exceeding 17:1 (e.g. wheat, sorghum, or corn) into Blackland Prairie soil three to six months prior to planting can result in a N deficiency in the following crop. As a general rule, an additional 15 to 30 lb of N fertilizer per ton residue (dry weight basis) will be adequate to offset any tem- Table 1. Effect of tillage tools on crop residue levels remaining on the soil surface after one pass as a percentage of existing residue. For percentage of existing residue remaining, fragile residues include previous soybean, peanut, cotton, and wheat (under 45 bu grain) crops, while non-fragile residues refer to previous corn, sorghum, and wheat (over 45 bu grain) crops. Residue Remaining After One Pass Residue Remaining After One Pass Tool/Configuration Fragile Non-fragile Tool/Configuration Fragile Non-fragile % % % % PRIMARY TILLAGE TOOLS SECONDARY TILLAGE TOOLS MOLDBOARD PLOW 0 to 5 0 to 10 FIELD CULTIVATOR DISK PLOW 5 to 15 10 to 2O Duckfoot points 35 to 5O 60 to 70 V-RIPPER/SUBSOILER 60 to 80 70 to 90 Sweeps/Shovels 6-12" 50 to 60 7O to 8O SUBSOILER + CHISEL 4O to 50 50 to 7O Sweeps 12-20" 6O to 75 8O to 9O DISK + SUBSOILER 10 to 2O 30 to 5O COMBINATION FINISHING TOOLS CHISEL PLOW Disks, shanks, Ieveler 3O to 5O 5O to 7O Sweeps 5O to 6O 7O to 85 Rollers and spring teeth 5O to 7O 7O to 90 Straight points 4O to 6O 6O to 8O Spring tooth harrow 5O to 7O 6O to 8O Twisted points 30 to 4O 5O to 7O Spike tooth harrow 6O to 8O 7O to 9O COMBINATION CHISEL PLOW Flex-tine tooth 7O to 85 75 to 9O Coulter + sweeps 40 to 5O 6O to 8O CULTIPACKER ROLLER 5O to 7O 6O to 8O Coulter + straight chisel points 3O to 4O 5O to 7O PACKER ROLLER 9O to 95 9O to 95 Coulter + twisted chisel points 2O to 3O 4O to 6O ROTARY TILLER Disk + sweeps 3O to 5O 6O to 7O 3" deep 2O to 4O 4O to 60 Disk + straight chisel points 3O to 4O 5O to 6O 6" deep 5 to 15 15 to 35 Disk + twisted chisel points 20 to 3O 3O to 5O STUBBLE-MULCH PLOWS OFFSET/TANDEM DISK HARROWS V-blades 7O to 8O 85 to 95 Plowing >10" spacing 10 to 25 25 to 5O Sweeps 1 65 to 75 8O to 9O Primary >9" spacing 2O to 40 3O to 60 ROTARY RODWEEDER 5O to 6O 8O to 90 Finishing 7-9" spacing 25 to 4O 4O to 70 PLANTERS/DRILLS 9O to 95 90 to 95 One way w/12-16" blades 2O to 4O 4O to 5O Overwinter/seasonal soil surface residue losses from weathering, wind, etc. can range from 10 to 30%. Generally, shallower depths and lower operating speeds leave more residue on the soil surface. Source: John Deere, 1991. porary immobilization of soil-N by the decomposi- tion processes (Box, 1973). It is estimated that about 2 tons of residue/acre are required per year for most Blackland Prairie soils to maintain the status quo of soil organic matter (Box, 1973). Typi- cal Blackland Prairie wheat and sorghum crops yield from 2 to four tons of dry straw (residue)/ acre. If this straw contains 0.7% N, 2 to 4 tons/acre of residue would supply 28 to 56 lb N/acre to soil microorganisms. About 30 lb soil- and fertilizer-N per ton of residue is necessary for optimum resi- due decomposition in Blackland soils, therefore an additional 32 to 64 lb fertilizer-N/acre would be required to offset losses of soil-N immobilized by microbes during decomposition of crop residue when a short fallow period occurs between crops. Nitrogen requirements of Blackland winter wheat in conventional and no-till grain sorghum (maize) and winter wheat residues were deter- mined on 4 different sites at Dallas (Knowles et al., 1993). Winter wheat was grown in conventional till (shred, disk, plow), no-till (standing), and re- moved (burned or baled) grain sorghum and win- ter wheat residues. Preplant soil testing by the Texas A&M University Soil Testing Laboratory in- dicated that fertilizer-N applications were required for optimum winter wheat production following sorghum and wheat in no-till and conventional till- age systems (Figure 7). Grain sorghum residues reduced soil-N levels in the top 6-inch of soil to a greater extent than winter wheat residues. At low rates of broadcast N fertilizer (40 lb N/acre), win- ter wheat was; (1) more N deficient in no-till resi- due management systems compared to conven- tional tillage systems; (2) more N deficient under conventional tillage systems than when sorghum and wheat residues were removed; and (3) more N deficient when winter wheat followed grain sor- ghum compared to continuous wheat (Figure 8). At the highest N fertilizer application rate (120 lb N /acre) grain yields of winter wheat did not differ between either no-till and conventional tillage sys- tems, nor previous sorghum and wheat rotations. The N deficiency of winter wheat growing in no- till systems and sorghum-wheat rotations was at- tributed primarily to a reduced mineralization rate of soil-N in sorghum-wheat rotations, and immo- bilization of fertilizer-N by decomposing no-till crop residues. Although sorghum straw has a nar- rower C:N ratio (ranges 20:1 to 30:1) compared to wheat (ranges 50:1 to 60:1), winter wheat grain yields following sorghum were less at low N rates compared to continuous wheat. Sorghum was chemically terminated with herbicide applications ” E _ _ _ _ _ FollovvingWinterWheat i EInuUsunzEIuune Q on $3.“; ' ' ' ' ' ' ' ' ‘ ' ' ' ' ' " Z : Q25: - ~ ' - ' ' ' ' ‘ ' ‘ "j ' ' ' ' ' ' ' ' " éggl > . . . . . - . _ - . - - - - - - - - - - » a i: I 8 151* a! - ‘ ' - ‘ '" *4 " ‘a’ 1 a a i"? 222s; 5 =- 2222; ._ 1 ::::: I/ No-Tlll Conventional ‘llll Remove llaairlneManagementPl-actice E s," 1),,“ (m) Following Grain Sorghum A35". ' ' ‘ ' ' ' ' ' ' ' ' ' ' ‘ ' ' ' ' ' ' ' ' ” ‘ ' ' ‘ ‘ ' ' ' ' g ;IewcElau1zE]1zu1a @305 » - - - - - - - - - - - - - - - - - - - - - - - - - ~ = - - ~ = = - z E fl I fl q = - z”? ‘ ' ' ‘ ' ' " ' " ‘ “ ‘ ' ' ' ' ' ' ' ' ' ' " b $155 » - - - ~ » - ~54 - ~ - ~ - - - - - - - - ~ - -~=a--¢4-~=~- u j Cd g _ i"? §§§§§ E =-: iiiii .5 g::::: T No-Till Conventionalllll ReeiclneManagementPl-actice ,, ss-*I.1!~e".-<.'-.-> . . _ . , , -_1:"~!1e-.*-.=-ss»:~.~-.- - -- i ;IeusE]amuE]1zm1a Q99‘) $305 - - - - - - - - - - - - - ~ - - - - - - - - - - - - ~ - ~ ~ - - - ¥ I .- iu_g____i__,,,,l,g _ _ _ . . . . _ _ _ t G ¢ , , ~ = -- "ilvi - - - - ~ * ~ » ~ ~ ~ * - - - - ~ - - - - - = " ‘ K = ‘ " ” " '" 5 . - _ Q ~ - - _ - , ~ K = - @ - - - ~ -- _, . I 3 I g a": ~~h~ a ~ E55- bc-k-M-bcbec bcbe-c--~- '5 tin‘ fi] I No-‘llll Conventional ’l'lll Remove Fallow Residue Management Practice Figure 7. Effect of previous crop residue management prac- tices on preplant soil nitrate-N (NOa-N) for winter wheat follow- ing wheat (a) and sorghum (b) in 1990, and following soybean (c) in 1994 at TAES-Dallas. following grain harvest in August or September, which was about three months later than senes- cence (death) of winter wheat harvested for grain in May or June. Thus, sorghum was removing N from the soil while fallow soil previously under wheat was mineralizing soil-N from residue addi- tions and soil organic matter. Previous wheat resi- due soils were fallow about five months allowing time for initial immobilization of and final miner- alization of soil-N from crop residue and soil or- ganic matter while sorghum was terminated only two months prior to planting winter wheat. There- fore, sorghum residues from sorghum-wheat rota- tions removed soil-N during the summer months which decreased the amount of time available for Q70 - Nitrogen'Ap|Ilie1l'(l'bIJAc-'):' i101 I! 349.519-81.19 . . '5- - 1 350- - ~ - - - - - - - - - - - - §40---~~§§§ ‘c a g U 20— 5 . 10- + 0d I .. No-Till Conventional ‘llll Residne Management Practice ,3 7° - urmpi-Appirerrohuacz): % ' Io U40 Else B120 g 60- > - ~ - - - - - - - - - - ~ ' §5U~ = ‘ ‘ ‘ = ‘ - -. * ' ' ' ‘ ‘ ' ‘ 5.0- . , , , , 1» _ » - - - - -~ b éao: c ‘i 55 8 i 20- " in a £10- o_ No-Tlll Conventional ‘llll Remove Resillne Mangement Practice i, “ '5 fleet"! ';"."_"Y”?‘?‘?‘ . . - - F°!"."'.".= “Y”?! - c . g a ‘None B“ U80 E110 (1990 g - . - 5 \‘ j ii ¢ *3 s; E if \ -; Remove Fallow Soybean Residue Management Practice Figure 8. Effect of previous crop residue management prac- tices and preplant nitrogen (N) fertilizer application rate on win- ter wheat grain yields following wheat (a) and sorghum (b) from 1988 to 1991, and following soybean (c) in 1994 at TAES-Dallas. mineralization of previous crop residue and soil organic matter prior to planting winter wheat. In contrast, decomposing no-till wheat residues and, to a lesser extent, decomposing sorghum residues immobilized surface applied N fertilizer. Greater quantities of surface-applied N fertilizer were required for optimum grain yield of winter wheat grown two consecutive seasons in decompos- ing wheat residue and immediately following grain sorghum compared to residue removal. Decompos- ing wheat residues under conventional tillage and no-till management systems required an additional 25 and 50 lb N/acre, respectively, above that required for optimum grain yield when wheat residue was re- moved (burned, cut and baled for hay, or tilled deep). When wheat followed sorghum, a reduction in time available for mineralization of soil-N was responsible for a 50 lb N/acre increase in Qvinter wheat N fertil- izer requirements for optimum grain yield above that needed for continuous wheat. Decomposing conven- tional and no-till sorghum residues immobilized enough surface-applied N fertilizer to require an ad- ditional 15 lb N/acre above that required for opti- mum grain yield when sorghum residues were re- moved (burned, cut and baled for hay, or plowed deep). The N contribution from decomposing crop residues to winter wheat grown in no-till production systems was not determined beyond the two year rotation. However, we expect that following five to ten years of no-till crop production, the need for in- creased rates of N fertilization relative to conven- tional tillage would be reduced somewhat. Microbial immobilization of surface-applied N fertilizer occurs predominantly at the surface of re- duced and no-till soils in the residue layer of the pre- vious crop. Therefore, a longer fallow period follow- ing grain sorghum and/ or application of N fertilizer below this sorghum residue straw-duff layer at the soil surface could alleviate N deficiencies observed with surface-applied N to winter wheat following sorghum or in reduced or no-till residue manage- ment systems. Banding granular fertilizer-N with a grain drill at planting or band injection of anhydrous ammonia or UAN-32 would apply N fertilizer below this zone where microbes are most actively decom- posing crop residues and should reduce immobili- zation losses of N fertilizer. Therefore, winter wheat was grown at Dallas in no-till forage sorghum (haygrazer) residue with either surface-applied N fertilizer (34-0-0 at 50 and 100 lb N/acre), N fertilizer banded with the seed at planting (16-20-0 at 50 lb N/ acre), and unfertilized (no N applied). Winter wheat also received 60 lb PzOs/acre as either 16-20-0 or O- 46-0 at planting (Knowles and Hipp, unpublished data). These N fertilizer treatments were applied to wheat planted in 4 previously established no-till for- age sorghum residue rates ranging from three to six tons dry straw/acre remaining on the soil sur- face. The tonnage of forage sorghum residue remain- ing on the soil surface had no effect on N fertilizer requirements for optimum winter wheat grain yield. However, economically optimum winter wheat grain yields resulted from either 100 lb N broadcast as 34- 0-0 to surface soil or 50 lb N/acre from 16-20-0 banded with a grain drill at planting (Figure 9). Conse- quently, band application of N fertilizer below no-till forage sorghum residue at planting reduced N fer- tilizer requirements for optimum winter wheat grain yield about 50 lb N/acre and had a higher N fertil- izer use efficiency compared to surface-applied 34- 0-0. Split N fertilization, application of 1/3 to 1/2 of the total N required at planting, then one or two ad- ditional applications prior to spring to meet the to- tal N requirement, could also benefit winter wheat grown in no-till residue and/or for grazing plus grain production. 1O Legume residues such as those from clover, al- falfa, and cowpea can provide substantial amounts of soil-N for optimum grain yield of subsequent winter wheat crops. Studies were conducted on two N deficient Blackland Prairie soils at Dallas to determine the influence of conventionally tilled red clover residue and mineralization of soil N dur- ing fallow on N nutrition of subsequent “Sturdy” winter wheat crops (Hipp, 1987b). Application of N did not increase yield of the first crop of winter wheat after clover since incorporated clover resi- due provided sufficient available N for maximum grain yield (Figure 10). Wheat following clover pro- duced maximum grain yield (5 1 bu/acre) while un- fertilized winter wheat following 17 months of fal- g2: .......... . ...... __/ - -/- gIli“f"_nf'_ff'f".'.s."'.°'."flilii%T>/%i” §IIiII1IT/IL'W%I% b /;%/ iiii;;iiii smi/ » _ _ _ _ §..:% ~° Figure 9. Effect of no-till forage sorghum residue and nitrogen (N) fertilizer applications on Coker 9803 winter wheat grain yield and N fertilizer use efficiency at Dallas. Income was $2.60/bu and expenses were $0.25! lb N, $0.14/bu hauling, with produc- tion costs of $76.05lacre. 370- NIIruQQWAppIlQGKIIN/bav) E,O_',.I!IZ.=§.EI$N.I=- - - 359i O {>1 i40- -30- 8 . i.» gm: Q_. Alter Clover After Fallow PreviousPmductionPractice A?" ‘iNltnI¢¢l'Ap|fll6d'(lb mm) ~ * " * ' ‘ ' ' " ' ' ' ' ' " ' " " '5 . swo-awg, §,,__,l9 121116595115 , , , . , . . . . . . - f‘? 9Y1’- . . . é - ‘B ___________‘_=____=G _ , _ _ _ _ _ _ _ _ _ _ __ a”- a b . 54°“ ' ° K “ ” ° ' ' ' ‘h :31: fi ' a - - ~ ~ ' IIIIII\ IIIII\ i=1» E223z2\- ~ - - -- 2225\- h _ :::::: §§§§§\ After Clover After Fallow PrevioIsPloductionPractice Figure 10. Influence of N fertilizer applications on Sturdy win- ter wheat grain yield grown after either clover or 17 months fallow. low yielded about 14 bu/acre less than unfertil- ized wheat after clover. Thus, winter wheat planted as the first crop following clover required 50 lb less N/acre than wheat grown following 17 months fallow. Winter wheat grown the second year after clover produced 38 bu/acre without N fertilization, and 45 bu/acre with 5O lb N/acre applied. Maxi- mum grain yield was obtained with 25 lb N/acre less for the second winter wheat crop after clover compared with winter wheat following fallow. Therefore, decomposed clover residue provided sufficient residual available N to supply 5O lb N/acre to the first subsequent winter wheat crop, then 25 lb N/acre to the second winter wheat crop. Ben- efits from increased residual soil-N levels result- ing from decomposed clover residue would prob- ably be minimal for the third subsequent winter wheat crop. Other field studies on two different sites at Pros- per examined the effects of decomposing conven- tional and no-till soybean residue on winter wheat N fertilizer requirements (Knowles and Hipp, un- published data). Winter wheat was grown in a two- year rotation with soybeans. Northrup King S-4884 soybean residue strips and unplanted fallow strips were established one year prior to planting win- ter wheat. The soybean crop had a bean yield of 11 25 bu/acre, post-harvest straw dry weight of 5700 lb/acre, and total above ground plant N content of 46 lb/acre. Approximately two months prior to planting winter wheat, soybean residue manage- ment systems including conventional tillage, no- till, and residue removal (bale or burn) were initi- ated. Approximately one week prior to planting winter wheat, soil samples were taken to an 18- inch depth in 6-inch increments for laboratory analysis of preplant nitrate-N (NOa-N). Preplant soil analysis indicated severely N deficient condi- tions existed within all residue strips previously cropped to soybeans (Figure 7c). Preplant soil profiles sampled for winter wheat following 15 months fallow had higher preplant soil nitrate-N levels than preplant soil samples for winter wheat following soybeans. Immediately after planting Coker 9803 soft red winter wheat, all residue and fallow plots received four rates of N (0, 40, 80, and 120 lb/acre) broadcast by hand as 34-0-0. Unfertilized winter wheat following soybeans was severely N deficient. Nitrogen deficiency symptoms, including stunting and chlorosis, were observed in unfertilized wheat, and at the 40 lb N / acre rate as early as March 1. Broadcast N fertil- izer applications of 120 lb N/acre were required for maximum grain yield of winter wheat (55-65 bu/acre) grown in conventional-till, no-till, and re- moved soybean residue management systems (Fig- ure 8c). However, economic analysis of grain yield data revealed that 80 lb N/acre applied to winter wheat in rotation with soybeans resulted in the most profitable grain yield, with increases rang- ing from 11 to 25 bu/acre compared to unfertil- ized wheat plots. Winter Wheat planted after a 15 month fallow period did not require application of N fertilizer to achieve economically optimum (and maximum) grain yield. Averaged across N application rates, winter wheat grain yields within fallow and no- till soybean residue management systems were higher than wheat grain yields within conven- tional till and soybean residue removal systems. Soybean residue-derived N was not mineral- ized in a timely manner to become a readily available N source for a succeeding winter wheat crop. Approximately 80 lb N/acre supplemental fertilizer N was required for optimum grain yield when winter wheat followed soybeans com- pared to fallowed land. This was due in part to the short fallow period of two to three months between soybean harvest and winter wheat planting that reduced the amount of time avail- able for decomposition of soybean crop residues and mineralization of soybean derived residue- N and organic matter derived soil-N. Higher grain yield in no-till systems compared to con- ventional tillage systems was attributed prima- rily to increased plant available water through- out the soil profile at planting due to a reduc- tion in soil disturbance resulting from reduced tillage. However, in a previous study, clover crop resi- due-derrived N was mineralized in a timely man- ner to be a readily available source of N for a suc- ceeding winter wheat crop. Other research at Texas Agricultural Experiment Station at Corpus Christi showed that a four year cotton-soybean rotation was necessary before soybeans contrib- uted enough available soil-N so that cotton re- quired minimal N fertilizer inputs for maximum lint yield (Matocha, 1995). Although both soybean and clover are legume crops, clover provided N to a succeeding winter wheat crop in a more timely fashion than did soybeans. We expect a longer fal- low period between clover and winter wheat com- pared to wheat after soybeans contributed to in- creased N availability in the clover-winter wheat rotation compared to the soybean-winter wheat rotation. Furthermore, clover produced five to seven times more tonnage of crop residue com- pared to soybean residue. Thus, we suspect that more residue-derrived N was available from the immense volume of clover residue incorporated into the soil compared to the relatively small amount of soybean residue incorporated into soil prior to planting winter wheat. The contribution of soil N from decomposition of previous crop residues and fallow periods to N requirements of the first subsequent winter wheat crop is described in Table 2. If winter wheat grown in the Blackland Prairie normally requires 1.5 lb N/bushel for grain production only, or 2.0 lb N/ bushel for livestock grazing plus grain production, and typical Blackland Prairie winter wheat grain yields range from 40 to 60 bu/acre, then wheat would have soil- plus fertilizer-N requirements ranging from 60 to 90 lb N/acre for grain produc- tion only, or 80 to 120 lb N/acre for grazing plus grain production. Suggested fertilizer N applica- tion rates for an optimum grain yield of 60 bu/acre ungrazed winter wheat following soybean, clover, wheat, oat, sorghum (grain or forage), corn, or cot- ton crops are also listed in Table 2. Extra fertil- izer N requirements of winter wheat due to losses via immobilization of soil- and fertilizer-N by de- composing previous crop residues are included in these suggested estimates of N application rate. Moreover, these rates of N fertilization could be reduced by 30% for a subsequent winter oat grain crop, or up to 50% if the preplant N application is either banded (granular-N) or injected (fluid-N) below the soil surface beneath decomposing crop residues, or N is foliar applied just after winter recovery and prior to vigorous spring growth. Winter Wheat Production Costs in the Texas Blackland Prairie Production costs for winter wheat grown in the Texas Blackland Prairie include seed, fertilizer, op- erator labor, fuel, machinery repair, harvesting, and hauling grain to the mill (Fable 3). Most years, her- bicide applications are not required since early sea- 12 Table 2. Mineralization of nitrogen (N) from soil and previous crop residues and suggested N fertilization for the subsequent winter wheat growing season. N Mineralized Previous Management for Winter Suggested N Crop Practice Wheat (60 bu Grain) lb N/acre lb N/acre Alfalfa or Clover Fallow 15-16 Months 50 to 100 0 to 40 Remove (Bale) 40 to 80 0 to 80 Conventional ‘lillage 40 to 100 0 to 80 No-Till 40 to 80 0 to 60 Wheat or Oat Fallow 17-18 Months 60 to 140 0 to 30 Remove (Bale) 60 to 80 10 to 30 Conventional Tillage 55 to 75 35 to 6O No-Till 30 to 60 80 to 110 Sorghum, Corn, Fallow 14-15 Months 40 to 100 0 to 50 or Soybean Remove (Bale) 30 to 60 30 to 60 Conventional 'Fll 30 to 55 6O to 110 No-Till 25 to 30 80 to 130 Cotton Fallow 10-11 Months 60 to 120 0 to 40 Conventional Till 30 to 60 30 to 80 son growth of winter wheat rapidly covers drill rows, outcompeting weed growth. However, when a good, vigorous stand of winter wheat is not obtained within two to four weeks after planting, a $10 to $20 per acre herbicide application may be necessary. Typi- cally, one year in four, an insecticide application costing $10 to $15 per acre may be necessary for control of greenbug populations. At the yield levels outlined in Table 3, grain prices of $2.27 and $1.63 per bushel would be required to recover production costs of hard and soft red winter wheats, respectively. Currently, commercial soft red winter wheat variet- ies are more tolerant of wheat diseases common in the Blackland Prairie. Therefore, soft red winter wheat normally outyields hard red winter wheat, and is more profitable to Blackland producers. Of all the expenses summarized in Table 3, fertilizer costs would probably be the most flexible expense that is under the control of a winter wheat producer by sim- ply reducing fertilizer rates for optimum efficiency. A preplant soil test to determine optimum but not excessive fertilizer requirements, more efficient ap- plication methods, and fertilization at optimum stages of growth can all reduce fertilizer applica- tion rates required for optimum grain yield of win- ter wheat. Fertilizer Economics The true cost of fertilization is based on the ac- tual cost of the fertilizer material, the nutrient composition (analysis) of the fertilizer, and the cost of the most reasonable application methods for the fertilizer. Fertilizer labels show the minimum per- centages by weight of nitrogen (N), phosphorus (P205), and potassium (K20) which is the fertilizer guarantee or grade (%N-%P2O5-%K2O). Although fertilizer materials may cost the same, the actual cost of a particular nutrient may be different. For instance, assuming equal effectiveness, if urea (4 6- 0-0) and ammonium nitrate (34-0-0) both are the Table 3. Typical Blackland winter wheat production costs at Dallas in 1994 (after Williams, 1992). Unit Price Quantity Amount Item $/unit $/acre DIRECT EXPENSES FERTILIZER Ammonium Phosphate (18-46-0) lb .13 100 13.00 Ammonium Nitrate (34-0-0) lb N .25 80 20.00 HAUL Haul Grain (Soft Red) bu .14 60 8.40 Haul Grain (Hard Red) bu .14 40 5.60 SEED Soft Red Wheat Seed lb .15 80 12.00 Hard Red Wheat Seed lb .10 80 8.00 OPERATOR LABOR Tractors hr 5.63 .32 1 .80 Self-propelled Equipment hr 5.63 .18 1.01 HAND LABOR Labor (Flagman) hr 5.63 .02 .11 DIESEL FUEL Tractors gal .83 1 .89 1 .57 Self-propelled Equipment gal .83 .86 .72 REPAIR & MAINTENANCE Tractors acre 1 .01 1 1 .01 Self-propelled Equipment acre 7.81 1 7.81 Implements acre 2.55 1 2.55 UNALLOCATED LABOR hr 5.63 .41 2.31 INTEREST ON OP CAP acre 2.30 1 2.30 SOFT RED WINTER WHEAT TOTAL DIRECT EXPENSES 74.59 HARD RED WINTER WHEAT TOTAL DIRECT EXPENSES 67.79 FIXED EXPENSES I Tractors acre 3.28 1 3.28 Self-propelled Equipment acre 14.19 1 14.19 Implements acre 5.44 1 5.44 TOTAL FIXED EXPENSES 22.91 SOFT RED WINTER WHEAT TOTAL SPECIFIED EXPENSES 97.50 HARD RED WINTER WHEAT TOTAL SPECIFIED EXPE_NSES 90.70 INCOME Soft Red Wheat Grain bu 3.00 60 180.00 Hard Red Wheat Grain bu 3.50 40 140.00 SOFT RED WINTER WHEAT RETURNS ABOVE DIRECT EXPENSES acre 105.41 RETURNS ABOVE SPECIFIED EXPENSES acre 82.50 HARD RED WINTER WHEAT RETURNS ABOVE DIRECT EXPENSES acre 72.21 RETURNS ABOVE SPECIFIED EXPENSES acre 49.30 same price per ton, urea is the better buy since it contains 920 lb N/ton compared with 670 lb N/ton of ammonium nitrate. If a producer was to apply 80 lb N/acre, only 174 lb urea per acre is required to equal the N applied in 235 lb ammonium ni- trate per acre. Phosphorus and N granular fertil- izers banded with the seed using a grain drill at planting is cheaper and more efficient requiring lower application rates than broadcast/incorpo- rate application since nutrients are applied below the soil surface directly to plant roots. Addition- ally, broadcast applications often require higher 13 N and P fertilizer rates compared to band applica- tions. Often, foliar fertilization is an inexpensive application method if fluid fertilizers are tank mixed with compatible pesticides. Figure 1 1 shows break-even winter wheat grain yield increases required to recover the cost of N fertilization at 3 N rates depending on the market value of wheat grain. Three values for fertilizer N expense are represented; $0.20/lb N which is typi- cal for urea and aqua ammonia (Figure 1 1a), $0.25/ lb N which is typical for ammonium nitrate and UAN 32 (Figure 1 1b), and $0.30/lb N which is typi- cal for ammonium sulfate and calcium nitrate (Fig- ure 1 1c). The value of the extra wheat grain yield A m @ $010,“, N . ‘ . _ . _ . . . ‘N. 00d‘: gm i ‘ ' ' ' ' ' eg: Urea *1” Inlacn §1s-: - - - - - - ~ - - - - = ~ - - - ' - ~ - - - - - - -*‘"""°“- - '5 a o I I I I I I Y I WI’ WI 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 Wlleat Gnil Value (Slba) A - — . @ solsntn _ - - ‘N’ Rvitfl-AEE- ‘lfd-I‘ £20: ' ' ' ' “ QQZAIIIOIIiII Nitrate "120llnlacn g f (344m) +aubu¢m J 515-- gw? - - - - - - - - ~ - - - - - - ~ ~ o! I '+ e . ' 5 "" ‘ '1‘ - -+. . 5 -~;g.--__ - - - - - — - - - - - - - - - - ~ - - ~+-. . a '1 * _ * - * é J _-;II--;Ir-_*_*_*_ 0 q I I I W I I I I I fi 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 Wheat Grain Value (Slim) '" @ $03055 N 3 gm? ' ' ' ' ' ' ' ' QQSAIIQIiII Sulfate ' ' ' .41-flicker»; ' g I (21-0-0) 315i - - - — -- a 7 - c 310:‘ ' ' ' ‘ ' ° ' ‘ ' ‘l I + . .+ . ‘ i 3 _* +..+..+'°NI- 5 - - — - 75* 1_' * ' ' - ' ' ° ' ‘ ' ' ‘ ' ' ' ' ' ' ‘ ‘ ' ' ‘ ‘ i’ 0 I l f I I I I 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 Wheat Grain Value (S/bu) Figure 11. Winter wheat grain yield increases required to re- cover the expense of nitrogen (N) fertilizer applications (in- cludes $0.15lbu grain hauling). obtained due to fertilization determines the prof- itability of fertilizer applications. This profitabil- ity is sensitive to the market value of wheat grain, and affected by the cost of the fertilizer material and application rate. When winter wheat grain is priced at $2.00/bushel, grain yield increases due to the application of 34-0-0 ($0.25/lb N) of 6 and 12 bushel/acre are required to recover expenses at fertilizer rates of 40 and 80 lb N/acre, respectively (Figure 1 1b). However, when the market price for wheat grain hits $3.50/bushel, lower grain yield increases due to 34-0-0 application of 3 and 6 bushel/acre are required to recover fertilizer ex- penses at rates of 40 and 80 lb N/acre, respectively. Thus, 75% higher grain prices reduced the wheat grain yield increase due to fertilization required to recover expenses to 50 percent compared to the lower price of $2.00/bu. Additionally, the wheat grain yield increase required to recover fertilizer expense for 80 and 120 lb N/acre as 34-0-0 at $0.25/lb N at a grain market price of $3.00/bushel is 7 and 11 bushel/acre, respectively (Figure 1 1b). Conse- quently, a 33% lower N application rate reduced the wheat grain yield increase required to recover ex- penses by 36 percent. Furthermore, the wheat grain yield increase required to recover fertilizer expense for 80 lb N/acre as urea at $0.20/lb N (Figure 11a) and ammonium nitrate at $0.25/lb N (Figure 1 1b) at a grain market price of $3.00/bushel is 6 and 7 bushel/acre, respectively. Therefore, a 20% lower fer- tilizer nutrient cost reduced the wheat grain yield increase required to recover expenses by 14 per- cent. Generally, at reasonable rates of application and grain incomes, grain yield increases due to fertilization ranging from 3 to 8 bushels per acre were required to recover fertilizer expenses. Fertilizer Materials for Winter Wheat Production The nutrient contents of commonly used dry and fluid fertilizer materials are listed in Tables 4 and 5, respectively. Fertilizers that are injected into soil or banded with the seed at planting, or foliar applied to foliage, should have a low salt index. This index is used to compare solubilities of chemical compounds. Most nitrogen (N) and potassium (K) compounds are very soluble with a high salt index (range from 50 to 120) , while phosphorus (P) compounds have a low salt index (range from 10 to 30). When applied too close to the seed or on foliage, the fertilizer ma- terials with a high salt index will burn the leaves and can kill young emerging wheat plants. Ammo- nium phosphates and P fertilizers have a relatively low salt index, so they are best suited to band or injection fertilizer applications for winter wheat at planting. Nevertheless, band and injection fertilizer applications of these materials should seldom ex- ceed 100 lb/acre, and should notbe in direct contact with the seed. Anhydrous ammonia, N H2 (82-0-0), is the N fer- tilizer formulation used most, with over 90% of all N fertilizers consisting of or derived from ammo- Table 4. Composition and nutrient content of dry (granular) fertilizer materials commonly used in the Texas Blackland Prairie. Dry Analysis (%) lbs/ton Material N P205 K20 S Zn N P205 S Zn Urea 46 0 0 0 0 920 0 0 0 Ammonium nitrate 34 0 0 0 0 670 0 0 0 Ammonium sulfate 21 0 0 24 0 420 0 480 0 Diammonium phosphate 18 46 0 2 0 360 920 40 0 Calcium nitrate 16 0 0 0 0 320 0 0 0 Ammonium phosphate sulfate 16 20 0 12 0 320 400 240 0 Potassium nitrate 13 0 44 0 0 260 0 0 0 Monoammonium phosphate 11 48 0 0 0 220 960 0 0 Triple superphosphate 0 46 0 2 0 0 920 220 0 Ordinary superphosphate 0 20 0 12 0 0 400 30 0 Potassium chloride 0 0 60 0 0 0 0 0 0 Zinc chloride 0 0 0 0 48 0 0 0 960 Zinc sulphate 0 0 0 18 36 0 0 360 720 Zinc nitrate 9 0 0 0 22 240 0 0 440 Table 5. Composition and nutrient content of fluid fertilizer materials commonly used in the Texas Blackland Prairie. Fluid Analysis % IQQQI Material N P205 K20 S N P205 S Density Anhydrous ammonia 82 0 0 0 4.21 0 0 5.13 Urea ammonium nitrate (UAN 32) 32 0 0 0 3.54 0 0 11.06 Aqua ammonia 20 0 0 0 1.52 0 0 7.60 Urea 15 0 0 0 1.50 0 0 10.00 Ammonium nitrate 20 0 0 0 2.10 0 0 10.50 Ammonium polysulfide 20 0 0 _40 1.94 0 3.88 9.70 Ammonium polyphosphate 10 34 0 0 1.10 3.74 0 11.00 nia compounds. Ammonia is a colorless gas that can be hazardous since it is normally stored at high pressure in tanks. It can cause severe irrita- tion of the eyes, nose, throat and lungs, and even suffocation, and can burn skin on contact. Be- cause anhydrous ammonia can be dangerous to handle, water solutions of ammonia and other soluble N formulations are more widely used. Anhydrous ammonia dissolved in water forms aqua ammonia (20-0-0). Aqua ammonia is a low- pressure liquid that is safer to handle, but has a lower N analysis than ammonia. Ammonia con- taining fertilizers must be injected below the soil surface in order to minimize losses of N via am- monia volatilization (Table 6). Aqua ammonia also needs to be injected below the soil surface, only not as deeply as anhydrous ammonia. Further- more, improper application of ammonia when in- jector shanks leave open trenches that expose fer- tilizer to the atmosphere can cause significant losses on N via ammonia volatilization. Blackland Prairie soils that are high in montmorillonitic clays swell when wet and shrink when dry often resulting in soil cracking. Volatilization losses of ammonia from N fertilization can occur when sub- sequent soil cracks develop above the anhydrous ammonia band injected below the soil surface. Urea, CO(NH2)2 (46-0-0), is a widely used N fer- tilizer that is also used as a protein source in live- stock feeds. Urea contains the highest N percent- age of commonly used solid fertilizers. Thus, it is cheaper per pound of N than other solid N fertil- izer materials. It is very soluble (6.5 lb material/ gallon), less leachable through the soil profile than nitrate (N O3‘) fertilizers, but is more leachable than ammonium (NH 4*) N fertilizers (‘Table 6). Biuret, a manufacturing contaminant of urea fertilizer, is one hazard of urea fertilization because biuret is toxic to sensitive plants in concentrations of more than 1 percent. Urea is a popular material for fluid N fertilizer solutions since it is high in N and quite soluble in water. However, due to the threat of biuret contamination, urea of high pu- rity, and low biuret (LB urea) content must be used in foliar N solutions containing urea. When urea fertilizer (especially urea solutions) is topdressed to residue or alkaline soils, it should be incorpo- rated into the soil immediately to avoid large N losses via volatilization. Research by the Texas Agricultural Experiment Station at El Paso has shown that the addition of 6 to 12% calcium (Ca) as calcium chloride (CaClz) to urea fertilizer solu- tions markedly reduces volatilization losses of N, and increased plant uptake of foliar applied urea (Fenn, 1986). Ammonium nitrate, NH 4NO3 (34-0-0), is an ex- tensively used, and relatively inexpensive solid N fertilizer that is readily soluble in water (about 37 lb material/gallon). Half of the N is in the ammo- nium form and the other half is in the nitrate form. Following application, the nitrate-N portion is readily available for plant uptake and is mobile, readily moving into the soil, while the ammonium- N form is not mobile and is converted to nitrate-N via nitrification prior to plant uptake. Ammonium is less subject to N losses via denitrification and leaching compared to the nitrate-N form (‘Table 6). Ammonium nitrate is classified as a hazardous material since it is a strong oxidizing agent which is explosive. The presence of carbon (C) or petro- leum products, high temperature, and pressure exceeding 500 psi can cause ammonium nitrate detonation. Ammonium nitrate is also very hygro- scopic in that it readily absorbs water, thus when it is exposed to moisture, ammonium nitrate be- comes a solid mass of granules that is difficult if not impossible to apply broadcast. For these rea- sons, ammonium nitrate should not be stored for periods of time exceeding four to six months. Urea ammonium nitrate, UAN 32 or Solution 32 (32-0-0), is a fluid N solution containing 16% urea and 16% ammonium nitrate. Both urea and ammo- nium nitrate are common components of fluid N fertilizer solutions due to their high solubilities in water. These solutions can be sprayed onto the soil surface or injected into the soil at or near plant- ing, or sprayed directly onto the foliage of estab- lished stands of wheat. We have applied up to 80 lb N/acre (22.6 gallons/acre) of UAN 32 to wheat plant foliage in February resulting in temporary chlorosis or yellowing of leaf tissue. However, this leaf burn was not noticeable one to two weeks af- Table 6. Common nitrogen (N) fertilizer formulations used in the Blackland Prairie and their relative potentials for N losses to the environment. Relative Potential for Losses Via Nitrogen Fertilizer Nitrogen Form(s) Leaching Volatilization Denitrification Ammonia NHS Low‘ High Low Urea NH, Medium High Low Ammonium Nitrate NH 4,N03 High Low High Urea Ammonium Nitrate NH2,NH4,NO3 Medium Low Medium Ammonium Sulfate NH 4 Low High Low Calcium Nitrate N03 High Low High Ammonium Phosphate NH 4 Low Medium Low ‘After conversion to nitrate (N03), leaching potential is high for all fertilizer sources. 15 ter application. Thus damage was only cosmetic since subsequent grain yield was unaffected. Care should be exercised that these solutions are not exposed to temperatures below 32 degrees F which can cause salting-out. Salting-out occurs at below freezing temperatures when crystals be- gin to form in the N solution due to the decrease in solubility of the dissolved N components with declining temperature. Furthermore, nitrogen solutions are very corrosive, rapidly destroying copper, brass, zinc, galvanized steel, and concrete materials, and are moderately corrosive to carbon steel and cast iron. Therefore, N solutions should be stored in stainless steel, aluminum alloy, or polyethylene plastic tanks, and should not remain stored in sprayer tanks and booms for extended lengths of time. Also, stainless steel nozzles are suggested for applications of fluid N solutions. Nitrogen fertilizer sources including urea (46- 0-0), ammonium nitrate (34-0-0) and urea ammo- nium nitrate (UAN 32, 32-0-0) have been examined for winter wheat and oat production at Dallas and Prosper . In 1994, one soft red winter wheat vari- ety (Coker 9803), one hard red winter wheat vari- ety (‘TAM 300), and one winter oat variety (H-833) were subjected to 80 lb N/acre broadcast applica- tions of urea, ammonium nitrate, or UAN 32 at planting or at jointing in mid February (Knowles and Hipp, unpublished data). The application of 8O lb N/acre increased grain yields of Coker 9803, TAM 300, and H-833 oat variet- ies by 15.1, 7.2, and 10.0 bu/acre, respectively, com- pared to unfertilized plots (Figure 12a). Grain yield of the H-833 oat was approximately 7 bu/acre higher than Coker 9803 which had grain yield approxi- mately 1 1 bu/acre higher than TAM 300. Broadcast N fertilizer applications at planting (10 November) and jointing (15 February) were equally as effective to correct N deficiencies of the small grain crops examined in this study. Different broadcast appli- cations of 80 lb N/acre as either 34-0-0, 46-0-0, or 32- 0-0 fertilizer formulations were equally as effective to obtain the highest grain yields in this study (57.6, 46.3, and 67.4 bu/acre for Coker 9803, TAM 300, and H-833 oat, respectively). Furthermore, N fertilizer application of 80 lb N/acre increased grain protein concentration of TAM 300 and H-833 oat by 2.0 and 1.7 percent, respectively, compared to unfertilized plots (Figure 12b). Grain protein concentration of Coker 9803 was unaffected by fertilization, and was approximately 1 percent less than TAM 300 and H- 833 oat. The three N fertilizer formulations were equally as effective for producing the highest grain protein levels observed in this study (14.2, 15.7, and 16.0% for Coker 9803, TAM 300, and H-833, respectively).These results are consistent with other field experiments conducted at Dallas that showed 34-0-0, 46-0-0, and 32-0-0 were equally effective N fer- tilizer sources for correcting N deficiencies of small grain crops when broadcast at an adequate rate sometime between planting and the jointing stage of growth. 16 1 Irmau QCOIQIQQ” Ell-m: on] 80 lb Nllcre Applied ae- - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~ - - - .. 1 _=_ 2a,,- _ 521a,...»- '- g d W :3: f; 2s - ~ a '3 - EI. s“ - .5 5,,_ ._‘ T /(::: T T) None ammo: (3444) Urea (4444) UAN a: (3244) Nitrogen (N) Fertilizer Formulation ' ITAM see Ucohrsoes Bil-as: on] 80 lb N/acre Applied 16- ' b‘ ZEE e1, a _ g I b a ‘i B - <5 . ‘- None 1111414030444) Urea (i444) UAN s: (1244) Nitrogen (N) Fertilizer Formulation Figure 12. Effect of urea, ammonium nitrate, and UAN 32 nitro- gen (N) fertilizer formulations on grain yield and protein con- centration of TAM 300 hard red winter wheat, Coker 9803 soft red winter wheat, and H-833 winter oat at Prosper. Ammonium sulfate, (N H 4) 2S0 4 (21-0-0), and cal- cium nitrate, Ca(NO3)2 (16-0-0), are two less com- monly used solid N fertilizer materials due to their relatively low N content which makes them exp en- sive N fertilizer sources on a per pound of N ba- sis. Ammonium sulfate, however, is a good sulfur (S) source for S deficient soils since this material contains 24% sulfur. Ammonium sulfate applica- tions can promote soil acidification lowering pH of non-calcareous soils, while calcium nitrate ap- plications impart an alkaline residue to the soil that can slightly raise soil pH. Calcium nitrate is very soluble in water, and ammonium sulfate is somewhat souluble in water. However their low N content and high salt indexes make them poor components of fluid N solutions. Common ammonium phosphate fertilizer sources include granular (dry) monoammonium phosphate, MAP (1 1-48-0), diammonium phos- phate, DAP (18-46-0), and fluid ammonium polyphosphate, APP (10-34-0). High application rates (in excess of 100 lb material/acre) of DAP (18-46-0) in direct seed contact can burn germi- nating wheat plants resulting in death of the young plant and thin wheat plant stands. Both DAP and MAP are somewhat soluble in water, however DAP has a higher solubility (4 lb material/gallon) than MAP (3 lb/gallon). The MAP and DAP formulations are commonly used in the manufacture of other dry bulk blends and fluid fertilizer solutions. Re- search in Oklahoma on calcareous clay soils has shown that banded MAP can provide more phos- phorus to wheat plants than DAP banded at an identical application rate due to the lower dissolu- tion pH for MAP (Follett, Murphy, and Donahue, 1981). Granular ammonium phosphate sulfate is derived from a mixture of dry ammonium sulfate and dry monoammonium phosphate (MAP) and has a fertilizer analysis of 16-20-0. This material is a good source of sulfur (12% S) on S deficient soils and can be banded with a grain drill at plant- ing at rates up to 300 lb material/acre. Blackland Prairie wheat producers commonly band 100 lb 18- 46-0 with the seed using a grain drill at planting, then topdress the remainder of the required N as 34-0-0 sometime prior to February 15. Calcium phosphates, Ca(H2PO4)2, include granu- lar (dry) ordinary or normal superphosphate (0-20- 0) and triple superphosphate (0-46-0) fertilizers, are also common P fertilizer materials. Although ordi- nary superphosphate has a low concentration of P relative to other P fertilizers, it is a good source of sulfur. Ordinary superphosphate and triple super- phosphate are only sparingly soluble in water (0.15 lb material/gallon). Triple superphosphate contains only trace amounts of sulfur from contaminating sulfuric acid in phosphoric acid used to manufac- ture triple superphosphate. It is Widely used in the manufacture of other fertilizer grades in dry bulk blends or for direct application. Agronomically, there is little difference in the availability of P from either ordinary or triple superphosphate (Follet, Murphy, and Donahue, 1981). Potassium nitrate, KN O3 (13-0-44), and potassium chloride, KCl (0-0-60) are two common dry K or pot- ash fertilizer formulations. Potassium fertilizer ap- plications are rarely suggested since applications are not profitable for optimum grain yield of winter wheat grown in the Texas Blackland Prairie. Potas- sium nitrate is highly soluble in water (2 lb mate- rial/gallon) and has a relatively low salt index ('74), thus it is commonly used to manufacture other grades of foliar-applied fluid fertilizer solutions for high-value vegetable and cotton crops. Potassium chloride, or muriate of potash, is lower in cost due to its relatively high K content, and is more soluble (2.3 lb material/gallon) than other K carriers, but has a relatively high salt index (1 15). This relatively high salt index limits its usefulness in the manufac- ture of foliar applied fluid fertilizer solutions. It is an excellent source of chloride (Cl) containing about 47% Cl by weight. Recently, research by Dr. Travis Miller in Hill and Bosque Counties (Texas) has indi- cated that spring applications of chloride can in- crease winter wheat grain yields by about 5 bushel/ acre compared to unfertilized wheat by suppress- ing common fungal diseases (Taylor, 1993). Spring topdress application of potassium chloride at 4O lb Cl per acre seemed to reduce leaf rust severity by up to 75% for winter wheat varieties susceptible to 17 this fungal disease, and subsequently may have in- creased grain yield by 5 to 10 bushel/acre. Conse- quently, response to chloride is greater for leaf rust susceptible wheat varieties, and seems to be related to chloride levels in the soil. Moreover, foliar appli- cations of chloride in fluid fertilizer solutions should be avoided since chloride materials have a very high salt index, thus they can act as a desiccant drying out plant leaves possibly defoliating wheat plants un- der hot, dry conditions. Applications of zinc (Zn) fertilizer are seldom re- quired for profitable winter wheat production in the Texas Blackland Prairie. On rare occasions, Zn defi- cient soils are encountered that require preplant broadcast applications ranging from 1 to 4 lb Zn/ acre based on a preplant soil test report. Excessive P fertilization and/ or preplant available P levels in alkaline, calcareous soils can intensify soil Zn defi- ciencies. Commonly available dry inorganic zinc fer- tilizer sources include zinc chloride, ZnClz (48% Zn), zinc sulphate, ZnSO4 (36% Zn), and zinc nitrate, Zn(NO3)2 (22% Zn). These Zn fertilizers are highly soluble in water and can be either broadcast and incorporated or banded dry at or near planting. They may also be foliar applied later in the season as a fluid fertilizer solution that could be tank mixed with pesticides. Water solubility of zinc sulfate, zinc ni- trate, and zinc chloride is 7.4, 27, and 36 lb material per gallon water, respectively. Organic Zn chelates are also available that can be soil or foliar applied, however these formulations are very expensive and wheat grain yield increases due to their application are not profitable for Blackland Prairie winter wheat grain production. Winter Wheat Developmental Growth Stages Recognition and proper identification of the stage of growth of winter wheat at any point dur- ing the growing season is necessary for intelligent crop management decisions. Producers need to have a basic understanding of growth stages of winter wheat since many herbicides and fungicides are most effective when applied within a narrow range of the wheat plants development. Water avail- ability and timing of fertilizer application to coin- cide with specific growth stages can improve the efficiency and profitability of both. For instance, winter wheat nitrogen (N) uptake is most rapid from tillering through the booting developmental growth stages with 80% of the total N uptake oc- curring before grain filling. We have found that optimum window of time for N fertilization result- ing in maximum N uptake efficiency by winter wheat coincides with the developmental growth stages from tillering through jointing. Several growth stage scales including the Feekes Scale have been developed to designate specific stages of development of wheat as they progress from emergence through tillering and jointing, then to boot, heading, and grain ripening (Baur, Smika, C) Jointing growth stage. E) Pre-booting growth stage. Note 2nd joint is visible on stems. F) Booting growth stage. Note extended (curled) flag leaves (March). Inset 1a. Winter wheat developmental growth stages ranging from four to five leaf through booting. 18 G) Boot swollen. Pre-heading growth stage. H) Close-up of booting growth stage just prior to heading. lee l K) Milky ripe growth stage. L) Basal stem tissue sampled for nitrate-N analysis. Inset 1b. Winter wheat developmental growth stages ranging from booting through milky ripe, and basal stem tissue sampled at four to five leaf for nitrate-N analysis. 19 and Black, 1983). A typical progression of seasonal developmental growth stages of winter wheat grown for grain production in the Texas Blackland Prairie is illustrated in photographs located in Inset 1 of this bulletin. Winter wheat for grazing is normally planted from August through Septem- ber and the progression of early developmental growth stages depends on grazing intensity and duration. The developmental growth stages of win- ter wheat grown for grain production were based on the following events (calendar dates are ap- proximate and may vary two to four weeks either side of those given): A. Planting (October 15-November 15): Seed is sown into moist soil or prior to rainfall (Feekes Stage 0). B. Emergence (November 25): It is first possible to see emerged plants (Feekes Stage 1). C. 3-4 Leaf (December 15): Three fully-expanded leaves visible on the main stem and a fourth im- mature leaf is also developing on the main stem (Feekes Stages 2-4). 5-6 Leaf (January 15): Five fully-expanded leaves are visible on the main stem and the sixth imma- ture leaf is also developing on the main stem (Feekes Stages 5-6). E. Tillering (January 15): Occurs at or near the 5-6 leaf stage when leaf tillers develop above leaf sheath bases (in the axil of leaves) on the main stem (Feekes Stages 2-5). F. Jointing (February 15): The stage of first intern- ode elongation when the first swollen node is vis- ible on the main or largest stem just above the soil surface. This swollen node is visible only af- ter stripping lower leaves from the main stem, and usually occurs just before expansion of the fifth or sixth leaf on the main stem (Feekes Stages 6-9). Booting (March 15): About nine fully-expanded leaves are visible on the main stem (the lower 2 leaves may be shed at this stage). The uppermost (flag) leaf is fully extended just above its swollen leaf sheath containing the wheat head (spike, ear) in its axil (Feekes Stage 10). . Heading (April 15): The green head (inflorescence) is fully emerged from the base of the flag leaf. Awns (fine bristles growing from kernels of awned vari- eties) are fully extended about the wheat head (Feekes Stage 10.1). I. Anthesis, Flowering, and Soft Dough (May 1): The head or inflorescence is still green and anthers emerge from kernels on the central part of the head. Pollination occurs, grain filling begins, and lower leaves begin to turn yellow as wheat be- gins senescence (Feekes Stage 10.5). J. Milky Ripe (May 15): Kernels from the central part of the head are still easily deformed when pressed between the fingers and a milky liquid exudes under such pressure (Feekes Stages 11- 11.2). 2O K. Hard Dough or Ripe (June 1): Kernels on the head lose their green color. It is no longer possible to deform the kernel by pressure between the fingers, but the kernel can still be cut by pressure from the fingernail. Upon breaking the kernel open, the in- terior displays a white, floury appearance (Feekes Stages 11.3-11.4). L. Maturity or Harvest (June 15): Kernels are com- pletely ripe and the wheat straw has died (senes- cent). More precise growth scales exist, however, the preceding developmental stages of growth are ad- equate to define optimum windows for winter wheat crop management decisions in the Texas Blackland Prairie. These developmental growth stages are more reliable than calendar dates since they accurately define precise one to two week periods of plant development during the winter wheat growing season. Calendar dates approxi- mate developmental growth stages within two to four weeks at best since growth stages are highly influenced by seasonal variations in climate (weather) and timing of production practices (e.g. planting date). Timing and Methods of Fertilization for Blackland Prairie Winter Wheat Production High rates of fertilizer N at planting can cause excessive vegetative growth that can increase the susceptibility of winter wheat to early season win- terkill and fungal diseases. Under limited mois- ture, excessive fall vegetative growth of wheat can also deplete available soil moisture and cause plants to suffer from moisture stress. A sugges- tion for grain production is to, apply 15 to 30 lb N/ acre at planting, then topdress the remainder prior to jointing. For grazing and grain, an application of 40 to 50 lb N/acre at planting is suggested to obtain adequate vegetative growth for fall and win- ter grazing, depending on available soil moisture. For intensive grazing from early fall seedings, sug- gested topdressing of one half the total N required should be applied in late fall and one half applied in late winter, especially when grain harvest is not planned (Gray, Welch, and Hodges, 1976). Graz- ing should be deferred to at least one week after rainfall has incorporated topdressed N fertilizer into the soil. Phosphorus should be applied and incorporated or banded into the soil before or at seeding so it will be in the root zone of young seed- lings to increase early season growth and vigor and to help develop winter hardiness. Fertilizer-P can be broadcast on the soil surface then incorpo- rated into the soil preplant or drilled in a band within theseed row at planting. An increase in broadcast P rates by 15 to 2O lb PzOs/acre over drilled P rates determined from a soil test is sug- gested. If grazing is planned, P should be band- applied in the seed row rather than broadcasted and incorporated. Drill-applied N fertilizer that does not contain P (has a high salt index) should be separated from the seed by at least 1 to 2 inches, and should not be applied at rates greater than 15 lb N/acre for wheat or 30 lb N/acre for oats (Gray, Welch, and Hodges, 1976). Nitrogen fertilizer use efficiency by winter wheat is highest when N applications are timed to corre- spond to periods when crop use of N is high. Winter wheat N uptake is most rapid from tillering through booting growth stages with 80% of the total N accu- mulation occurring before grain filling (Doerge, Roth, and Gardner, 1991). Much of the N fertilizer applied to Blackland Prairie winter wheat is topdressed as ammonium nitrate (34-0-0) in late winter or early spring. Growers often delay N appli- cation to evaluate winter survival and potential pro- ductivity of their wheat crop before investing in N fertilizer. However, wet field conditions following prolonged rainy periods can delay N fertilizer appli- cations to winter wheat until late spring. Eight years of data from field experiments con- ducted at Dallas and Prosper were examined to determine optimum timing of N fertilizer applica- tions for efficient use of applied N by plants throughout the winter wheat growing season (Knowles et al., 1994). Ammonium nitrate (34-0-0) was topdressed at three rates (0, 40, and 80 lb N/ acre) at or near planting, and at the tillering, joint- ing, booting, and heading growth stages, plus split applications at these dates. Conventional tillage was used for seedbed preparation, and winter wheat followed wheat, oats, cotton, or fallow, de- pending on the year of the study. Optimum grain yield resulted from single N topdressings of 40 lb N/acre at jointing, 8O lb N/acre applied at plant- ing through jointingfor split applications of 40 lb N/acre at planting and at jointing (Figure 13a). Fertilizer N topdressed following booting resulted in grain yields that were 15 to 20% lower than ob- tained with N topdressed prior to booting. This was attributed to low amounts of rainfall follow- ing booting which reduced movement of N fertil- izer into the soil in time for uptake by winter wheat plant roots (Figure 2). Sufficient split N applica- tions were equally as effective as sufficient single N applications for optimum winter wheat grain yield, however split applications of N fertilizer are more expensive because of extra fertilizer appli- cator trips over the field. Nitrogen uptake by winter wheat proceeds very slowly until tillering begins, then peaks at a maxi- mum of about 1.9 lb N/acre/day during the joint- ing growth stage (Figure 13b). Adequate soil-N reserves (or N fertilizer) should be made available to wheat plants so that N deficiencies do not oc- cur during this period of peak vegetative growth and N uptake. Lower N rates were required for optimum winter wheat grain yield when fertilizer- N was topdressed during the period between tillering and jointing wheat growth stages. There- fore, the most efficient time to topdress N fertil- 21 55 I +00 rum/u. A +40 lbmN/Ae g °° ' ' " " " ' ° " ‘ " ‘ ' ' ' ' ' ' ' ' ' ' ' ' ‘ ‘ ' " ' ' ' ' ' ' "l ‘r I é =5 - 2 U - 5'- so - a .5 3 45-} fi 3 : i 4° '_ 5-6 ha! Joint Boot 35" nscts mus I mn1s_ MARI!‘ APlIIS 0 30 60 90 120 150 Nitrogen Application Date (Daya After Planting) 2 i 1 % Seaaonal Nitrogen 3. I Uptake: 3 80 to 120 lb N/acne w: .3 a b s" - E ‘i 1i g 5-6 Leaf Joint Boot DEC 15 @ l5 FEB l5 MAR 15 APR 15 0 #1 ~ so ao 0o 12o 15o Nitrogen Application Date (Days Alter Planting) Figure 13. Effect of timing of two nitrogen (N) fertilizer rates on winter wheat grain yields measured from 1985 to 1992, and daily N uptake (flux) typical of winter wheat grown at TAES-Dallas. izer for optimum winter wheat grain yield was be- tween the tillering and jointing growth stages (January 15 through February 15) . Nitrogen ap- plied during this time period was incorporated into the soil by rainfall such that N was available to wheat plant roots in time for rapid spring veg- etative growth following jointing. Fertilizer-N ap- plied at or near planting was more subject to N losses via immobilization by decomposing crop residues, leaching, and denitrification, while N applied after booting would have the greatest po- tential for carry-over of fertilizer-N into the next cropping season which should cause environmen- tal concerns. Several factors can affect optimum timing of N fertilizer applications for winter wheat. Late spring foliar-N applications can be beneficial re- sulting in foliar N uptake by wheat plant leaves when lack of rainfall prevents movement of dry N fertilizer into the root zone of plants. Preplant urea applied to the soil surface can be less effective for optimum winter wheat grain yield due to N losses via volatilization and leaching compared to am- monium nitrate applied preplant. Urea applied preplant can also be less effective than urea am- monium nitrate, or foliar N applied in early spring because of early season N losses. Lower rates of N fertilizer are required when N is banded below the soil surface in no-till winter wheat production systems. Furthermore, split N applications could be advantageous for wheat production on sandier soils located in production regions other than the Texas Blackland where the probability of N losses by leach- ing is high, for winter wheat that is grazed, or for wheat grown in no-till previous crop residues. Interactions of Fertilizer With Disease and Insect Pests of Winter Wheat Excessive rates of N fertilizer applied at or near planting can result in lush vegetative growth that is more subject to winter freeze damage and fun- gal diseases. Powdery mildew and leaf or stem rust infection of susceptible winter wheat varieties can be magnified by excessive N fertilization (Figure 14a). However, powdery mildew infection seemed to have less of a negative effect on subsequent win- ter wheat grain yield than N deficiencies at low rates of N fertilizer (Figure 14b). Excessive veg- etative growth resulting from excessive rates of N fertilizer is more attractive to insect pests such as greenbug and armyworm. On the other hand, diseases such as take-all of wheat and insect pests such as spider mite and Russian wheat aphid ap- pear to prefer N deficient wheat plants. Chloride 10- -----V¢rl=m-~--- § ‘ ‘Collin Em: EISturrIy 0- - - - - - - - - - - - - - - . . . . . . . . . _ . - . . - . ~ - - - - - - - - - a 7O - ' ' ‘ ' ' ' ' ‘ ' ' ' ' ‘ ' ‘ ' ' ‘ ‘ ° ' ' ' ' ' ' ' ' ' ° ' ' Variety‘ ico- Icmin-IZMivEJsi-iay 1 - - - - - - - -. - - - - - - c‘ ~ ‘be ¢¢ll gbgli 35C" ' ' ' ' ' ' ' ' ' Baltic‘ j 6 be q "4"“ ::: ' 5:3 b .5 _ =30 0 32C- .“ ,::: ::: ,::: I 4O 8O 12C Nitrogen Applied (lbl-lAc.) Figure 14. Effect of nitrogen fertilizer applications on powdery mildew severity (a) and winter wheat grain yield (b) at TAES- Dallas. (Cl) fertilization as either potassium chloride or ammonium chloride topdressed at 40 lb Cl/acre in spring seemed to decrease leaf rust infection of susceptible winter wheat varieties. Interactions of fertilizer nutrients with insect and disease pests of wheat have not been studied extensively since this is a relatively new field of study with many opportunities for future research. Interactions of Fertilizer With Winter Wheat Varieties Occasionally, during years with high yield po- tential, winter wheat varieties will respond differ- ently to N fertilizer application rates. During the 1991 to 1992 growing season, four N rates (0, 40, 80, and 120 lb N/acre) were topdressed as ammo- nium nitrate (34-0-0) at planting to 5 winter wheat varieties grown at Prosper (Knowles and Hipp, unpublished data). These N rates were applied to one soft red (Pioneer 2548) and four hard red (‘TAM 201, 2180, Siouxland 89, and Collin) winter wheat varieties. Averaged across varieties, the N appli- cation rate of 80 lb N/acre resulted in maximum winter wheat forage and grain yield (Figure 15a, 15b). Averaged across N ratesLPioneer 2548 had the highest grain yield followed by TAM 201, 2180, Siouxland 89, and Collin. Siouxland 89 had the highest straw yield followed by Collin, 2180, TAM 201, and Pioneer 2548. The N by variety interac- tion was significant (P < 0.05), with maximum win- ter wheat grain yield resulting from application of 40 lb N/acre to Pioneer 2548, TAM 201, and Siouxland 89; or 80 lb N/acre applied to 2180 and Collin winter wheat. An economic analysis of the data to determine optimum grain yield showed 40 lb N/acre applied to Pioneer 2548 and 80 lb N/ acre applied to 2180 resulted in profitable grain yield increases while grain yield increases due to N fertilizer application to TAM 201, Siouxland 89, and Collin did not recover the expense of N fer- tilization (Figure 15c). Overall, Pioneer 2548 and 2180 required higher rates of N fertilizer for opti- mum grain and forage yield compared with TAM 201, Siouxland 89, and Collin. During the 1992 to 1993 growing season, three N rates (0, 40, and 80 lb N/acre) were topdressed as 34-0-0 at planting to 6 winter wheat varieties grown at Prosper (Knowles and Hipp, unpub- lished data). Phosphorus was banded with the seed at planting as triple superphosphate (0-46-0) at two rates (0 and 50 lb PzOs/acre). These fertilizer treat- ments were applied to two soft red (Pioneer 2548 and Coker 9543) and four hard red (TAM 201, 2180, TAM 300, and Siouxland 89) winter wheat variet- ies. Averaged across varieties, fertilizer applica- tions of 80 lb N/acre plus 50 lb PzOs/acre resulted in maximum winter wheat grain yield (Figure 4a, 4b). Grain yields were not significantly different between the six varieties, and the fertilizer by va- riety ‘ interaction was not significant. Economic analysis of grain yield showed fertilizer applica- m, _. I NoNApplietLEw ltLN/ac. Elan u» um. Shaun» rum-WI Wheat Grain Yield (bu/acre) l . 7 2100 Collin TAM 201 Slouxland-OO Pleneer-2548 Variety ,_ _ , . . , . _ , _ _ _ _ , _ _l!~,~_'! +P.=='.'~_<'-9§"-'=r f"??- § E 5 i b 3 5 u: E TAII 201 2180 Slouxland-BI Pioneer-HAS Collin Variety 180 160i l. N0 Nlhpplled Q00 lb N/ac. lblNl-ac. 832010 Nlac. _ -140; ~ H T‘°°'_ 1;; ' c g 80 — .1; - 2 - I 8°“ 15;:- 40-4 3.: ' =0 - :33: - a ~- \ o A Slouxland-OI Pioneer-2548 Collin Variety Tl“ 201 21 8O Figure 15. Effect of nitrogen (N) fertilizer applications on winter wheat yield in a wheat-wheat rotation at Prosper. Income was $3.50/bu hard and $3.00Ibu soft red wheats; expenses were $0.25llb N and production costs were $117/acre. tions of 80 lb N/acre resulted in profitable grain yield increases for all six varieties (Figure 4c). However, the higher grain yields obtained with phosphorus fertilizer applications were not prof- itable since yield increases did not recover the ex- pense of P fertilization. Although rainfall was not limiting and leaf rust was minimal in 1992-93, winter wheat grain yield was lower compared with 1991-92. In 1991-92, win- ter wheat followed winter wheat with a prior one- year fallow period. This fallow period resulted in higher residual preplant soil-N for winter wheat due to higher mineralization rates of organic soil- N into more available inorganic soil-N (nitrate-N). In 1992-93, winter wheat followed cotton after a 23 one month fallow period prior to planting, thus re- sidual soil-N was very low. The response to applied N fertilizer was greater due to the short period of time between cotton harvest/plow-down and win- ter wheat planting. Therefore, N fertilizer rates re- quired for optimum winter wheat grain yield were highly dependent upon previous crop and the length of the fallow period prior to planting wheat. During the 1993 to 1994 growing season, three P rates (0, 40, and 80 lb P2O5/acre) were banded with the seed at planting as triple superphosphate (0-46-0) to 4 winter wheat varieties grown at Pros- per (Knowles and Hipp, unpublished data). These P rates were applied to two soft red (Coker 9803 and AgriPro Mallard) and two hard red (TAM 300 and 2163) winter wheat varieties. Nitrogen was applied to every plot at 80 lb N/acre as 34-0-0 topdressed at planting. Band application of 40 and 80 lb PzOs/acre at planting did not increase grain yield of Coker 9803 and Mallard soft red winter wheat , nor did it increase grain yield of TAM 300 and 2163 hard red winter wheat, compared to unfertilized plots at this location (Figure 16). Preplant soil analysis of available P by the TAEX Soil and Plant Tissue Testing Laboratory indi- cated that a response to P fertilization was un- likely (TAEX-P was 160 ppm). Grain yield of TAM 300 (39.9 bu/acre) was lower than grain yields of 2163 (53.9 bu/acre), Mallard (52.1 bu/acre), and Coker 9803 (54.0 bu/acre). Band application of granular P fertilizer at a rate of 4O lb PzOs/acre at planting has increased win- ter wheat grain yields by 5 to 10 bu/acre compared to unfertilized wheat plots at Prosper (Figure 4b). Fertilizer P applications have been most impor- tant for obtaining maximum forage yield of small grain pasture. Grain yield response to P fertiliza- tion is highest when cold soil conditions caused by inclement weather limit the availability and wheat plant uptake of native soil-P. When P fertil- izer is broadcast applied, normally an additional 10 to 20 lb PzOs/acre is required compared to banded P applications. This broadcast P also needs l-No P Applied C140 u» P205lac. E100 n» P205/ac. I 8o__ . . . . . . . . . . . . . . - - - - - - - - - - - - - - - - - ~ '5 C) é 560- » - ~ - - - - - - - - - - - - - - ~ ~ - - - ~ ~ "a ab‘ ~- 1, ‘u’ ‘be .999. “c abe bc '5 .. . ; d . . . . . s-"l- d d 13:11 g 11.1: "jj: t! - III: II... 32o- IIIII IIIiI s . . . . . . . . . . Q _ _ __ 3 iiii: 1:11: o_ I/Iilii I iiii; ZI-I-I-i- TAM 300 2163 Mallard Coker 980 Wlnter Wheat Variety Figure 16. Effect of phosphorus (P) fertilizer applications on winter wheat yield in a wheat-wheat rotation at Prosper. to be incorporated into the surface 4 to 6 inches of soil so that immobile P fertilizer is positionally avail- able to winter wheat plant roots early in the season. Generally, in years of average to above average rainfall, the soft red winter wheats that have been bred for superior disease resistance (e.g.: leaf rust) had higher yields and net income than did hard red winter wheat varieties grown in North Texas. However, in a dry year, at equivalent N and P fer- tilizer rates, hard red winter wheats will normally outyield soft red winter wheats.Variety by N and/ or P fertilizer interactions are not normally seen in Texas Blackland Prairie wheat production; how- ever, this interaction has been observed when sea- sonal rainfall is abnormally high and winter wheat has an unusually high grain yield potential. Most years, when winter wheat N fertilizer requirements are determined based on preplant soil nitrate and actual grain yield potential, all wheat varieties can be managed the same with 40 to 80 lb N/acre ap- plied at or near planting (1 18 to 235 lb 34-0-0/acre, or 100 lb 18-46-0 at planting plus 100 to 217 lb 34- 0-0 topdressed between the tillering and jointing wheat growth stages) resulting in optimum win- ter wheat grain yield. No-till grain production, short crop rotations, and/or early season grazing (with livestock removed prior to jointing for grain harvest) can increase this N requirement by an additional 30 to 40 lb N/acre. Nitrogen fertilizer should be banded below no-till decomposing crop residues for optimum efficiency. Other research has shown that topdress N fertilizer applications delayed as late as the jointing growth stage (around Feb. 15) and can still result in maximum grain yield of winter wheat grown with conven- tional tillage practices. Split N fertilizer applica- tions have not been superior to sufficient single N fertilizer topdress applications when the single ap- plication occurs sometime between planting and jointing for optimum grain production. The actual quantity of N and P fertilizer applied should be determined from a preplant soil test to avoid costly excessive fertilizer application rates. Soil and Plant Tissue Testing for Blackland Prairie Winter Wheat Production Soil testing is an important tool for predicting nutrient requirements of winter wheat at planting. Early soil sampling prior to planting winter wheat will allow for analysis time (one to two weeks by mail for TAEX-College Station) and for subsequent purchase and application of recommended fertil- izers. When plant tissue analysis is employed in conjunction with soil testing, the response of wheat to applied fertilizer can be monitored dur- ing the growing season. Often, plant tissue analy- sis can identify a nutrient deficiency during the growing season. This deficiency may then be cor- rected with a fertilizer application to eliminate further yield reductions. 24 Proper collection of soil samples is extremely important. The accuracy of the laboratory analy- sis is limited by the degree to which the soil sample is representative of the field being sampled. If ar- eas in the field are visibly different or have been managed differently, they should each be sampled as a separate unit. These uniform sampling units will probably vary in extent from about 5 to 20 acres each. A soil survey map provided by the USDA Natural Resource Conservation Service (former Soil Conservation Service) can be used to separate a field into uniform areas by soil type. If these maps are unavailable, careful observations should be made relating to differences in slope, erosion, crop growth, soil texture, soil color, and, if known, cropping and chemical (fertilizer, pesti- cide, manure, fertilizer, etc.) history. Sampling eroded knolls and low spots, small gullies, exposed subsoil, manure spots, chemical spills, field en- trances and roads, fence rows, and terrace water- ways should be avoided. When fertilized fields are sampled avoid coring directly into the fertilized band. As a general rule, any area that is extremely different and is large enough to be managed sepa- rately should be sampled separately. For soil samples, a shovel, spade, soil auger, or soil sampling tube should be employed to a depth of 6 inches. Surface litter (e.g.: previous crop resi- dues) should be removed and sampling to the same depth each time is important. Soil samples from a 2 to 6 inch depth should be collected to avoid er- rors in phosphorus laboratory analysis due to stratification of fertilizer-P that can be present in abnormally high concentrations within the surface 2 inches of soil. The soil sample from each uni- form area should be composed of 10 to 15 subsamples taken at random from different places within the defined unit. Soil subsamples should be broken-up if cloddy and placed in a clean plas- tic container (galvanized buckets may contaminate the soil sample with zinc), mixed thoroughly, and approximately 1 pint taken out for the composite soil sample representative of the field or sampling unit. Soil samples should not be dried with heat. Soil samples can be mailed to the testing labo- ratory in soil sample bags provided by the labora- tory or in sealable “zip-lock” plastic sandwich bags carefully packed in a cardboard box. Five differ- ent soil test options are currently available from the Texas A&M University soil testing laboratory. These options include a $10 routine analysis (pH, NOa-N, P, K, Ca, Mg, Na, S, and total salts), a $15 routine plus micronutrients (Zn, Fe, Cu, Mn) analy- sis, a $25 routine plus micronutrients plus boron, aluminum, and lime requirement analysis, a $30 Routine plus micronutrients plus boron, alumi- num, and lime requirement, plus organic matter analysis, and a $25 routine plus detailed salinity analysis. The detailed salinity analysis is prima- rily used for irrigated crop production on saline- sodic west and south Texas soils, and is normally not necessary for wheat production in the Texas Blackland Prairie. The boron, aluminum, and lime requirement soil test is used on acidic soils pre- dominantly in east Texas. Alkaline, calcareous Blackland soils are already high in lime, thus a soil test for additional lime requirement is not nec- essary. Organic matter concentration in soil can be important for determining application rates of certain herbicides. This soil test can be run alone or in addition to routine soil tests for an additional $5 to $10, rather than the $30 complete soil test. Ordinarily, the routine $10 soil test will detect nu- trient deficiencies that should be corrected by fer- tilizer application prior to planting winter wheat in the Texas Blackland Prairie. However, the rou- tine plus micronutrient soil test should be re- quested for soils that have not been tested previ- ously, or at four to five year intervals for soils that are tested annually. This $10 to $15 annual soil testing expense can pay for itself in reduced fer- tilization expenses by averting fertilizer nutrient applications that are not required for maximum yield and avoiding excessive fertilizer application rates that can lower crop yield and have potential to pollute the environment. However, an accurate grain yield prediction on the soil test form is es- sential since wheat producers who overestimate their yield potential will receive guidelines that overestimate their true fertilizer requirements. The preplant soil test level below which P fertili- zation is suggested for the TAEX soil extractant is 26 parts per million (ppm) extractable P for winter wheat grown in the Texas Blackland Prairie. If the Olsen-P soil extractant is used for laboratory soil analysis the soil test level below which P fertiliza- tion is suggested is 20 ppm extractable P. When extractable soil-P is deficient in Blackland Prairie soil, P fertilizer application of 40 lb PzOs/acre banded with a grain drill at planting is suggested. If avail- able soil-P is deficient and P fertilizer is broadcast applied and incorporated into the surface 4 to 6 inches of soil prior to planting, 60 lb PzOs/acre is suggested for winter wheat grain production. After the available N level in soil is obtained from the laboratory, Table 6 can be used to determine the base requirement for N fertilizer. Microbial decom- position of previous crop residues such as wheat, corn, cotton, or sorghum can immobilize enough soil- and fertilizer-N to require an additional 20 to 40 lb N/acre. Generally, this additional N require- ment will be reflected in lower preplant soil nitrate- N (NOa-N) levels (see Table 2). Winter wheat used for grazing will have a 20 to 40 lb N/acre higher N requirement than wheat for grain production only. Since soil test values are determined from labora- tory analysis of the N03 ion, soil-N levels from re- cent anhydrous ammonia, urea, or other ammonium (N H Q fertilizer applications may not be reflected in the soil test report. If these ammonium-based fertil- izer materials have not had time to undergo nitrifi- cation, soil test N levels may be erroneously low. Additionally, previous legume residues (eg: clover) that have not had time to undergo mineralization 25 and make soil-N available as NOa-N may result in erroneously low preplant soil-N levels. Soil-N defi- ciencies of winter wheat after legumes are often short-lived in spring, when these residues mineral- ize available N for uptake by winter wheat later in the growing season. Furthermore, soil nitrate-N is mobile and moves readily with soil moisture within the soil profile. Therefore, soil nitrate-N is often con- centrated in the surface 6 inches of soil during peri- ods of drought or may be leached below the surface 6 inches of soil during wet, rainy periods. This mo- bility of the NOgion can also create confusion in in- terpretation of adequate preplant soil-N levels and misleading N fertilizer recommendations. Figure 17 shows two typical soil test reports from the Texas A&M University Soil Testing Labo- ratory for winter wheat grown in the north Texas Blackland. The top report is for both grazing and grain production and the bottom report is for grain production only. Preplant soil P, K, Ca, magnesium (Mg), zinc (Zn), iron (Fe), manganese (Mn), cop- per (Cu), and sulphur (S) levels ranged from me- dium to very high. For this typical Blackland Prai- rie soil sample, only preplant nitrate-N (N Os-N) was deficient. Therefore, two split applications to total 100 pounds N/acre are suggested for winter wheat that is grazed, or two split applications to total 70 pounds N/acre for wheat produced only for grain. Our experience has shown that split N applications are unnecessary for grain production only, consequently one application of 70 pounds N/acre sometime between preplant and jointing would probably be adequate for optimum grain yield in the Texas Blackland Prairie. Additionally, since soil P concentrations are very high, applica- tion of 2 lb Zn/acre is suggested at planting. How- ever, since Zn was not actually deficient in this soil test report (0.27 ppm, medium), and previous research at this site indicated no response to Zn, winter wheat probably would not benefit from the application of Zn. The cost of Zn application most likely would not be recovered from a possible in- significant grain yield increase obtained with Zn fertilization, thus we would not suggest the expen- diture. Tissue testing is the second major tool available to monitor the in-season N status of a winter wheat crop. Analysis of plant tissue collected early in the growing season can indicate if N deficiencies could be expected at a later time. If an N deficiency is detected early in the season, prior to the joint- ing growth stage, application of N fertilizer will alleviate further winter wheat grain yield reduc- tions. Nitrate-N content of this lower portion of the stem of small grains has been found to be indica- tive of the N nutritional status of wheat plants (Knowles, Doerge, and Ottman, 1991). The portion of stem below the soil level and above the seed, or the lower two inches of wheat stem is used for ni- trate-N analysis (see photo L in Inset 1b). Thirty to forty representative stems are randomly sampled to provide sufficient plant material for Both Grazing and Grain Production SOIL TEST REPORT pkg; 1 TEXAS AGRICULTURAL sxmnusxou SERVICE -- was TEXAS A a M UNIVERSITY srsrsa soIL TESTING LABORATORY, COLLEGE STATION rx. 71643 LARRY onaua luv‘ 001365 sxrznsrou soIL CHEMIST FOR: TIM KNOWLES _ I». 2212222222.; 1:41:42: °*LL*5' Tx~ count! = COLLIN 75252 couuIY|- oas ISAHPLE ID‘ 3-10-90 SOIL ANALYSIS ISOIL TEST RATINGS — PPM ELEMENT (AVAILABLE FORM)| I Pa I IPaosPao-| I I I I I I I I l I AcIoIIY INITROGENI nus IPOTASSIUHICALCIUM IuAsuasIonIsALIuIIYI zzuc I IRON IMANGANESEI COPPER I soorun IsuLPnoa 0.3 I 10. I 95. I 365- I 8830 I 256. I 390. I o.27I 2o.aaI 9.05 I 0,41| 33, I 5, IuoozaArzLY| v:aY I van! I vzar I vzar I axon I none I MEDIUM] nxcu I urea I nxen I vzaY I uIcu I ALKALINE I Low I axon I axon | axon I I I I I I I Low I (PPM x 2 - LBS/ACRE 6 INCHES DEEP) caor AND YI:Ln RANGE: nnmAI (60-79 BU/A) ennzxnc a GRAIN Yooa YIzLo GOAL: so BU/A - susessro FsaIILIzsa RATE LBS/A: 100 - 0 - 0 N P205 K20 APPLY ALL or was Anov: sueezsrzo FzaIILIzma PRBPLANT AND INCORPORATE INTO soIL ouaruc FINAL ssnoaso PazPAaAmIon. (on sAuoY soILs, APPLY 1/2 oF ABOVE sucessrso NITRO- esu PREPLANT AND was REMAINING 1/2 or was nzraocsu IN LATE FALL). TOPDRESS nIIn AN ADDITIONAL 50 TO 10 LBS-N/A AFTER LIVESTOCK REMOVAL AND PRIOR T0 JOINTING. (ass was aIcasa RATE FoLLonIue BEAVIER GRAZING PRESSURES AND soon MOISTURE) NOTE: IF SULFUR Is sueszswso, APPLY ALL or IT wIIa raxs PRE-JOINT TOPDRESS uxraoeau. BROADCAST 2 LBS/ACRE ZINC . FURTHER INFORMATION AND ASSISTANCE CAN BE OBTAINED FROM YOUR COUNTY EXTENSION AGENT : L. xznuzra WHITE 210 so. no DONALD, cooaraoosz MCKINNEY TX. 75959 Grain Production Only SOIL TEST REPORT PAGE 1 TEXAS AGRICULTURAL EXTENSION SERVICE —— THE TEXAS A 8 M UNIVERSITY SYSTEM SOIL TESTING LABORATORY, COLLEGE STATION TX. 77843 LARRY UNRUH EXTENSION SOIL CBEMIST INV§ 001365 FQR; TIM KNQ"LEs DATE RECEIVED 2 11/12/90 17360 COIT RD» DATE PROCESSED: 11/19/90 DALLA$' gx_ . COUNTY : COLLIN 75252 COUNTY}: 085 LAB T 3 OQZOZ FEE : $14.00 |sAuPLs Iol 3-10-90 SOIL ANALYSIS ISOIL TEST RATINGS - PPM ELEMENT (AVAILABLE FORH)| I I I I I I I zxuc I IRON IHANGANESEI COPPER I sooxuu ISULPHUR I pg I |PaosPno-I I I I I ACIDITY INITROGENI ans IPOTASSIUHICALCIUH IHAGNBSIUMISALINITY I a.s I 10. I 95. I ass. I aaso I zsa. I 390. I o.21I 2o.aaI a.os I o.41I 33. I 59 IHODERATELYI VERY I vnax I vsax I vnnx I nIca I noun I MEDIUMI area I axon I axon I vmax I axsa I ALKALINE I LOW I axon I nIea I HIGH I I I I I I I LOW I (PPM X 2 I LBS/ACRE 6 INCHES DEEP) CROP AND YIELD RANGE: WHEAT (60-79 B0/A) GRAIN ouLY Youn YIzLn GOAL: so B0/A socczsrsn FERTILIZER RATE LBS/A: 70 - o - 0 N P205 K20 APPLY 1/3 0F was ABOVE soeessrnn nrwnocau PREPLANT (ALONG wIIa ALL oF THE PHOSPHORUS AND POTASSIUM, IF soeczsrsn) AND INCORPORATE DURING FINAL szzoazo PREPARATION. IoPnnsss was REMAINING nxmaoczu (PL0s ALL s0LFon, IF sueesswsn) PRIOR TO JOINTING. BROADCAST 2 LBS/ACRE zxnc . FURTHER IuFonnAIIou AND ASSISTANCE CAN an OBTAINED FROM Youn COUNTY axwzusxon AGENT . xzunatn WHITE 210 so. ac DONALD, cooaraoosz MCKINNEY TX- 75069 Figure 17. Typical soil test report from the Texas A&M Soil Testing Laboratory with suggested fertilization of winter wheat grown for either grazing and grain or grain only at Prosper. 236 analysis. Sample uniform areas of the overall field unit(s). Stem tissue samples taken at the five to six leaf or tillering wheat growth stages can be tested in time to make necessary N fertilizer ap- plications efficiently. If a N deficiency is detected after jointing, N fertilizer applications prior to booting will help avoid further winter wheat grain yield reductions. After the wheat stem tissue nitrate-N level is ob- tained from the laboratory, Table 7 can be used to determine the base requirement of N fertilizer. When basal stem tissue nitrate-N concentrations fall be- low these levels, N fertilizer applications are re- quired to avoid any further grain yield losses. Ex- cessive stem nitrate-N levels can be an indication of excessive vegetative growth at the expense of grain yield that can result in an increased potential for moisture stress and fungal diseases. Nitrogen fer- tilizer application to N deficient plants will increase grain yield compared to unfertilized wheat, however, some yield loss will occur compared to wheat that is not allowed to become N deficient. Stem nitrate-N levels will normally be very high at tillering, then decline rapidly following the joint- ing growth stage when nitrate-N reserves are con- verted and translocated from stem tissue to the developing wheat grain. Extended periods of cloudy weather and low solar radiation can result in a premature decline in basal stem tissue nitrate- N levels which may be mistaken for an N defi- ciency. Stem samples from wheat growing in wa- ter-logged soils can also be low indicating a N de- ficiency due to low concentrations of oxygen in water-logged soil that inhibits plant uptake of available soil-N by wheat roots. Furthermore, one to two weeks (or longer for ammonium-based N fertilizers that must undergo nitrification prior to Table 7. Predicting nitrogen (N) requirements of winter wheat using preplant soil and early season basal stem nitrate-N (N03- N) analyses. Sampling Test Suggested Date Value Fertilizer ppm NOS-N lbs N/acre lbs N/acre Preplant Soil: 0-6 in depth > 30 None 30 - 20 0 - 40 2O - 10 4O - 8O < 10 80 - 120 Basal Stem: @ 4-6 Leaf > 3500 None 3500 - 2000 0 - 40 < 2000 40 - 80 @ Jointing < 500 0 - 40 Multiply ppm NOa-N by 4.4 to convert to parts per million nitrate (NO ). Multiply ppm NOS-N by 2.0 to convert to lbs. NOa-N/acre to a 6 inch soil depth. plant uptake) following fertilizer application and incorporation into the soil by rainfall or tillage is necessary before fertilizer-N shows up as nitrate- N in wheat stem tissue. References Baur, A., D. Smika, and A. Black. 1983. Correlation of five wheat growth stage scales used in the Great Plains. USDA ARS AAT-NC-7. Northern Great Plains Research Lab., Mandan, ND. 17 pp. Box, John. 1973. Crop residue to improve Texas soils. TX Agric. Exp. Sta. MP-807. Texas A&M University, College Station. 8 pp. Doerge, T.A., R.L. Roth, and B.R. Gardner. 1991. Nitro- gen fertilizer management in Arizona. Univ. of Ari- zona Special Project No. 89-EWQI-1-9102. Univ. of AZ, Tucson, AZ. 87 pp. Donahue, R.L., R.W. Miller, and J .C. Shickluna. 1983. Soil fertility and plant nutrition. p. 208-239. In: Soils. An introduction to soils and plant growth. 5th Ed. Prentice-Hall, Inc., Englewood Cliffs, NJ. Fenn, Lloyd B. 1986. Increased agricultural benefits through cost effect utilization of urea fertilizer. TX Agric. Exp. Sta. MP-1 608. Texas A&M University, Col- lege Station. 9 pp. Follett, R.H., L.S. Murphy, R.L. Donahue. 1981. Fertiliz- ers and Soil Amendments. Prentice-Hall, Inc., Englewood Cliffs, NJ. 557 pp. Foth, H.D., L.V. Withee, H.S. Jacobs, and S.J. Thien. 1982. Microbial decomposition of organic residue in soils. p. 1 15-126. In: Laboratory manual for introduc- tory soil science. 6th Ed. Wm. C. Brown Company Publishers. Dubuque, IW. Gray, C., C.D. Welch, and R.J. Hodges. 1976. Time and methods of applying fertilizer for small grains. TX Agric. Ext. Service AGR 11-1, AGR 10. Texas A&M University, College Station. 1 pp. Hipp, Billy W. 1987a. Influence of planting date on re- sponse of winter wheat to phosphorus. TX Agri. Exp. Sta. B-1564. Texas A&M University, College Station. 4 pp. Hipp, Billy W. 1987b. Residual effects of clover on ni- trogen nutrition of crops grown on Blackland soils. J. of Plant Nutrition, 10:1811-1817. Hipp, Billy W., and Dwain Hooks. 1978. Influence of planting date and phosphorus application method on wheat response to phosphorus. TX Agri. Exp. Sta. PR-3526. Texas A&M University, College Station. 6 PP- Hipp, Billy W., Tim C. Knowles, and Benny J . Simpson. 1992. Descriptions and uses of soils of the Texas Ag- ricultural Experiment Stations at Dallas and Pros- per. TX Agri. Exp. Sta. B-1705. Texas A&M Univer- sity, College Station. 21 pp. Hipp, B.W., and B.J. Simpson. 1988. Thirty-five years of farming systems research in the Texas Blackland. TX Agric. Exp. Sta. B-1604. Texas A&M University, College Station. 24 pp. John Deere. 1991. John Deere tillage tool residue man- agement guide. Datalizer Slide Charts, Inc., Addison, IL. Knowles, T.C., T.A. Doerge, and M.J. Ottman. 1991. Im- proved nitrogen management in irrigated durum wheat using stem nitrate analysis. I. Nitrate uptake dynamics. II. Interpretation of nitrate-nitrogen con- centrations. Agron J. 83:346-356. Knowles, T.C., B.W. Hipp, P.S. Graff, and D.S. Marshall. 1993. Nitrogen nutrition of winter wheat in tilled and no-till sorghum and wheat residues. Agron. J. 85:886- 893. Knowles, T.C., B.W. Hipp, P.S. Graff, and D.S. Marshall. 1994. Timing and rate of top-dress nitrogen for rainfed winter wheat. J. Production Agriculture. 7:216-220. Matocha, J. 1995. Study looks at value of legume crops. p. 6-7. In: Southwestern Farm Press. Volume 22, No. 2. (Jan. 19, 1995). Dallas. Pigg, Calvin. 1994. No-till system slashes power needs. p. 1, 14-15. In: Southwest Farm Press. Volume 21, No. 3. (February 3, 1994). Dallas. 28 Soil Conservation Service. 1992. Picture your residue. USDA Soil Conservation Service Leaflet SCS- CRM-02. Spence, C.O., and C.D. Welch. 1977. Phosphorus fertili- zation for wheat production on Blackland and Grand Prairie Soils. TX Agri. Exp. Sta. L-1530. Texas A&M University, College Station. 6 pp. Taylor, Charles. 1993. Can chloride enhance wheat yield‘? Texas Farmer-Stockrnan. Dallas. June 1993. p. 6-7, 18. Welch, C.D., C. Gray, J .E. Cole, G.D. Alston, and B.W. Hipp. 1977. Field crop fertilization on Texas Black- land and Grand Prairie soils. TX Agric. Exp. Sta. L- 743. Texas A&M University, College Station. 4 pp. Williams, R.L. 1992. Economic factors of small grain production. p. 167. In G. Rutz (ed.). Delta Agricul- tural Digest-1992. Farm Press Publications. Clarksdale, MS. 224 pp. Texas Agricultural Experiment Station The Texas A&M University System Mention of a trademark or a proprietary product does not constitute a guarantee 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. Copies printed: 1,200