BULLETIN 553 OCTOBER 1983 Fall - Spring Nitrogen Combinations for Wheat Grain Production 40 ALABAMA AGRICULTURAL EXPERIMENT STATION GALE A BUCHANAN. DIRECTOR AUBURN UNIVERSITY AUBURN UNIVERSITY. ALABAMA in cooperation with the UNIVERSITY of GEORGIA COLLEGE of AGRICULTURE EXPERIMENT STATIONS ATHENS, GEORGIA E BROADUS BROWNE, DIRECTOR CONTENTS Page INTRODUCTION .......................................... 3 MATERIALS AND METHODS .................................. 4 6 6 9 11 14 15 17 RESULTS AND DISCUSSION ..... ........................... Fall-applied N and Yields ............................. Influence of Preceding Crop on Wheat and Fertilizer N Requirements ........................ Spring N, Total N, and Optimum N Combinations ...... Soil and Plant N vs. Nitrogen Deficiencies ............... Rainfall vs. Yield ................................. SUMMARY AND CONCLUSIONS ............................... APPENDIX .............................................. 19 FIRST PRINTING 5M, OCTOBER 1983 Information contained herein is available to all persons without regard to race, color, sex, or national origin. Fall- Spring Nitrogen Combinations for Wheat Grain Production J. T. TOUCHTON 1 and W. L. HARGROVE 2 INTRODUCTION NITROGEN FERTILIZER efficiency is defined as the percentage of applied N used by the current crop. For many nonleguminous crops, including wheat, N efficiency will generally average between 40 and 60 percent. The relatively low N fertilizer efficiency is often due to N losses through various mechanisms, such as leaching and denitrification, and unavailability through immobilization. Leaching and denitrification losses are directly related to excessive rain and wet soils. Unavailability due to immobilization results from low N-containing crop residues, such as grain sorghum and corn stalks. Frequent rains and continuously wet soils, which are common during the winter months in the Southeast, create conditions favorable for poor N efficiency. To help avoid poor efficiency of N applied to wheat, split N applications are commonly recommended. Generally, one-fourth to one-third of the N fertilizer is applied at planting (20 to 30 pounds per acre N), and the remainder (60 to 80 pounds per acre) is applied in late winter or early spring just prior to the period when wheat begins to take up large quantities of N. The second application of N probably should correspond with early jointing, which generally occurs between early February and early March in Alabama and Georgia. The quantity of available N (ammonium and nitrate) in the 'Associate Professor, Department of Agronomy and Soils, Alabama Agricultural Experiment Station, Auburn University, Alabama. 2Assistant Professor, Department of Agronomy, Georgia Station, Experiment, Georgia. 4 ALABAMA AGRICULTURAL EXPERIMENT STATION soil, and the quantity of unavailable N (primarily organic N) that will be released to the available form during the growing season depends on the previous summer crop. If the previous summer crop is a legume, such as soybeans, relatively large quantities of high N residues may be left in the field after harvest. High N residues degrade fairly rapidly and will often supply some N during late fall or early winter. If the previous summer crop is a high N efficient nonleguminous plant, such as grain sorghum, soil N levels at sorghum harvest will probably be low. In addition, the sorghum residues left in the field are generally low in N (less than 1 percent N), and can immobolize soil N. Theoretically, 3,000 to 6,000 pounds of sorghum residue will immobilize 30 to 60 pounds of plant available N. In the Southeast, wheat harvested for grain is commonly grown in double-cropping systems. Most of the wheat is double cropped with soybeans, but considerable acreage is double cropped with nonleguminous crops, such as grain sorghum, cotton, and corn. Since the fertilizer N requirements for wheat following legumes may be less than the requirement for wheat following nonlegumes, N fertilizer recommendations for wheat probably should be related to the previous crop. The primary objective of this study was to determine if N fertilizer recommendations for wheat should depend on the previous summer crop. Other objectives were to determine optimum fall-spring N combinations, and to determine if soil or tissue N levels could be used as an indication of fertilizer N requirements. MATERIALS AND METHODS These studies were conducted for 4 years (1977-81) at the University of Georgia's Southwest Georgia Branch Experiment Station (Plains) and Northwest Georgia Branch Experiment Station (Calhoun), and for 2 years (1980-82) at Auburn University's Sand Mountain Substation (Crossville), Prattville Experiment Field (Prattville), and Wiregrass Substation (Headland). Classification of soils at each location is listed in table 1. Except for N, which was a treatment variable, fertilizer and lime applications were based on soil test recommendations. The first wheat crop followed soybeans at Calhoun and grain sorghum at Plains. In subsequent years, wheat followed soybeans and grain sorghum at each location. Sorghum and soybean areas were adjacent and were alternated annually. Each year, grain FALL-SPRING NITROGEN FOR WHEAT TABLE 5 EACH LOCATION 1. CLASSIFICATION OF SOILS AT Location Series Surface Sandy clay loam Silt loam Fine sandy loam Sandy clay loam Subgroup Rhodic Paleudults Fraqiaquic Paleudults Typic Hapudults Rhodic Paleudults Plains .............. . Greenville Calhoun ............ Cedarbluff Crossville ........... Hartsells Prattville ........... Lucedale Headland .... ...... Dothan Fine sandy loam Plinthic Paleudults sorghum was sidedressed with 80 pounds per acre N (prilled ammonium nitrate) 2 to 4 weeks after emergence. Nitrogen was not applied to the soybeans. After soybean and sorghum harvest in late October or November, the soil was tilled and wheat was drilled (6 2/-inch row spacings) at a seeding rate of 90 pounds per acre. Variety and planting dates are listed in table 2. Fertilizer N treatments consisted of fall-spring combinations. Fall N rate were 0, 10, 20, 30, and 40 pounds per acre in Georgia, and 0, 12, 24, and 36 pounds per acre in Alabama. Spring N rates were 0, 20, 40, 60, and 80 pounds per acre in Georgia, and 0, 24, 48, and 72 pounds per acre in Alabama. Prilled ammonium nitrate was the N source. Fall N was applied at planting or shortly after planting, and spring N was applied between mid-February and mid-March, table 2. Fall and spring N treatments were arranged in a complete factorial within a randomized complete block design replicated four times. Data collected included soil and tissue N levels (Kjeldahl TABLE 2. WHEAT VARIETY, PLANTING DATE, AND SPRING N APPLICATION DATE FOR EACH YEAR AND LOCATION Year 1978.......... Location Calhoun Variety Ga. 1123 Planting 11-01-77 Spring N 03-16-78 Plains 1979 ... .. .Calhoun Plains Calhoun Plains Calhoun Plains Crossville Prattville Headland ,. Crossville Prattville Headland Oasis Oasis Oasis Oasis Omega 78 McNair 1813 McNair 1813 Coker 747 McNair 1813 Coker 762 Coker 762 Coker 747 Coker 762 11-28-77 11-21-78 11-17-78 11-15-79 11-29-79 11-12-80 12-04-80 10-23-80 10-24-80 11-27-80 11-09-81 11-02-81 11-05-81 03-06-78 03-14-79 02-14-79 02-25-80 02-05-80 03-03-81 02-26-81 03-02-81 03-03-81 02-24-81 03-04-82 02-20-82 02-24-82 1980 .......... , 1981 . ....... 1982 ....... 6 ALABAMA AGRICULTURAL EXPERIMENT STATION procedure) just prior to spring N applications and grain yields at maturity. Wheat was harvested with a combine and grain yields were adjusted to a test weight of 60 pounds per bushel at 13 percent grain moisture. Statistical analyses used to separate treatment means included analysis of variance, appendix table, least significant differences at the 10 percent level of probability, and multiple regression. RESULTS AND DISCUSSION Optimum N rate and combination varied among location, previous crops, and years. As a result of variability associated with yields even within previous crops, locations, and years, it is difficult to select an optimum N rate or combination. In this report, suboptimum N rates and combinations were determined at the 10 percent level of probability. Any yield value significantly less than the absolute highest yield is considered to be suboptimum. Yield values not significantly less than the absolute highest are considered to be equal to the highest yield. In the remainder of this report, fall and spring N treatments will be indicated by giving pounds per acre fall N-pounds per acre spring N. For example, a 10-40 N treatment represents 10 pounds per acre fall N and 40 pounds per acre spring N. Fall-applied N and Yields Although fall-applied N appeared to be critical for optimum yield in some experiments, tables 3-5, the data indicate that fall N cannot be substituted for spring N. Only in 1 of the 24 experiments did fall N without spring N result in optimum yields. The one exception occurred at Headland in 1981 when soybeans were the previous crop, table 5. In three of the experiments where low rates of spring N without fall N resulted in optimum yields (Calhoun in 1980 and 1981, table 4; and Plains in 1979, table 3), equal or higher rates of fall N without spring N did not result in optimum yields. Each of these three situations occurred when soybeans were the previous crop. Fall-applied N was required for optimum yield in 8 of the 24 experiments. Requirements for fall N occurred three times when wheat followed soybeans (Plains in 1980, table 3; and Crossville and Prattville in 1981, table 5), and five times when wheat followed grain sorghum (Plains in 1979 and 1980, table 3; Calhoun in 1981, table 4; and Crossville and Headland in 1981, table 5). FALL-SPRING NITROGEN FOR WHEAT TABLE 3. WHEAT GRAIN YIELDS AT PLAINS, GEORGIA, AS AFFECTED.BY PRECEDING CROPS AND APPLIED NITROGEN 7 Spring N, lb./ acre 0 Bu. 1978 0 ....... 20 4026 60.... Yield/acre, by previous crop and fall N, lb/acre Soybeans Grain sorghum 10 Bu. 20 Bu. 30 Bu. 40 Bu. 0 Bu. 18 25 -31-3231 41 44 40 49=58_ 45 2 5f 54 =-49-5 -1 25 34 34 41 32 37 42 36 41 43 4 14 21 34 40 22 27 32 38 45 27 42 52 49 7 14 30 3 4 22 29 37 44 53 27 54 5 10 Bu. 22 28 29 20 Bu. 23 27 27 0 30 Bu. 22 28 32 30 440 Bu. 27 29 80 ..... 1979 28 31 0 ....... _ 20 ....... 4 40 ....... 60........50-__-48 804 1980 25 0 ....... 29 28 20........29 40 40 ........ 35 60........ 44 80 ........ 43 1981 50 0 ....... 45 57 20........ 52 40........55 60 a1 m M 12 28 39 5 5 24 12 29 28 37=39 5 43 31 _31 36 45 151 35 36 44 40 47 42 42 _5 48 55 56 55 56 0 6 58 55_52 34 52 53 453- ........- Values located within the shaded area are not statistically lower (P < the absolute highhest yield. 0.10) than In each of these eight experiments, the lowest rate of fall-applied N (10 to 12 pounds per acre) was sufficient for optimum yield. In four of the eight experiments, however, the lowest rate of fallapplied N did not result in the least amount of total N (fall ± spring) needed for optimum yield. These responses occurred when sorghum was the preceding crop at Plains in 1979, table 3, and Crossville in 1981, table 4, and when soybeans preceded wheat at Crossville and Prattville in 1981, table 4. The 10-80 combination (90 pounds per acre total N) resulted in 48 bushels of wheat per acre and the 20-60 combination (80 pounds per acre total N) resulted in 45 bushels at Plains. Similar responses were obtained with the other three experiments. In addition to the eight experiments where fall N was required for optimum yields, there were seven experiments where tlhe use of fall N reduced the total N fertilizer requirements for optimum 8 ALABAMA AGRICULTURAL EXPERIMENT STATION yield. This response occurred with both previous crops at Plains in 1981, table 3, and Calhoun in 1980, table 4; with soybeans at Prattville in 1982, table 5; and with sorghum at Prattville in 1981 and at Headland in 1982. At Plains, the 0-80 combination (80 pounds per acre total N) resulted in 58 bushels per acre wheat yields while the 40-20 combination (60 pounds N) resulted in 63 bushels when wheat followed sorghum. The same relationships were found with the other six experiments, which suggests that the use of fall N may reduce total fertilizer N requirements even though optimum yields may be obtained without any fall-applied nitrogen. In summarizing wheat yield responses to fall N, there was a beneficial effect of fall N in 15 of the 24 experiments. In 8 of these 15 experiments, fall N was required for optimum yield regardless of spring N rate; three responses occurred when soybeans were the previous crop and five occurred when sorghum was the previous crop. In the other seven experiments, fall N was not required for optimum yield, but applying fall N allowed significant reductions in the amount of spring N needed, thus TABLE 4. WHEAT GRAIN YIELDS AT CALHOUN, GEORGIA, AS AFFECTED BY PRECEDING CROPS AND APPLIED NITROGEN Spring N, lb./acre 0 Bu. Yield/acre, by previous crop and fall N, lb./acre Soybeans Grain sorghum 10 Bu. 20 Bu. 30 Bu. 40 Bu. 0 Bu. 10 Bu. 20 Bu. 30 Bu. 40 Bu. 1978 0 ........ 33 20 ....... .37 40 ....... 44 60 ....... .49 8 80.... ..... 1980 0 ........ 20 ....... 26 33 36 38 49 5 27 ....... " 32 42 46 5= 49 34 41 35 51 44 48 34 47 44 55 55 39 -43 24 25 31 32 ... 38 34 40 ....... ..... 60. .. A4 80 .. 1981 4= 4 42 41 46 59 55 37 A2 4 38 . 4 4 4 .~t) 34 52 54 56 62 64 41 4 39 60 61 .... 67 23 37 3 4 36 23 34 45 62 60 ---=_~-------- 30 39 45 42 31 35 58 53 35 4 4- 37 .... 43 45 r8 40 50 56 5 1 41 4 i4 47 48 61 69 1 0....... 38 20 ....... 50 6 40 ....... ... 60 ....... 80 ....... 48 57 56 7 43 49 59 58 4 71 0 7 8 Values located within the shaded area are not statistically lower (P < 0.10) than the absolute highest yield. FALL-SPRING FALL-SPRING NITROGEN FOR WHEAT 9 -NI'TROGEN OR HEAT9 TABLE 5. WHEAT GRAIN YIELDS AT CROSSVILLE, PRATIVILLE, ANT)HEADLAND, ALABAMA, AS AFFECTED BY THE PREVIOUS CROP AND APPLIED N Yield/acre, by. previous crop And fall N, lb./acre SpringcN, 0 Bu. Crossville 1981 Soybeans Grain sorghum 12 Bu. 24 Bu. 36 Bu. 0 Bu. 12 Bu. 24 Bu. 36 Bu. 0 ........ 24............ ..... 48.......... 72........... Crossville 1982 0 ............ 24 ............ 48 ........... 72........ 31 42 38 43 14 27 38 40 2 21 31 40 44 4 24 37 42 47 4 28 39 46 5 4 17 23 21 15 21 27 18 23 21 15 14 22 Prattville 1981 0 ............ 24............ 48............ 72............ Prattville 1982 35 50 66 65 18 41 62 64 70 21 43 47 58 47 75 76 26 45 48 49 54 58 67 71" 75" 39 0............ 24 48 ............ 72............ ............ 35 43 48 43 49 48 42 50 52 13 34 42 48 35 37 19 37 47 52 40 42 56 55 21 32 40 42 25 40 42 52 38 48 '64 26 41 45 48 45 54 5 Headland 1981 0..............41 24 ............. 48 .............. 72 ............. Headland 1982 0..............19 24 .............. 48.............. 72.............. 50 54 49 55 55 51 24 31 41 49 54 51 32 36 40 47 52 55 51 49 53 59 59 30 40 44 32 36 45 50 16 26 38 39 45, Values located within the shaded area are not statistically lower (P < 0.10) than the absolute highest yield. permitting the use of less total N with a fall-spring combination than with spring N alone. This response occurred three times when soybeans were the previous crop and four times when sorghum was the previous crop. Influence of Preceding Crop on Wheat and Fertilizer N Requirements At optimum N rates and combinations, generally there was not much difference between yields of wheat following soybeans and 10 10 ALABAMA ALABAMA AGRICULTURA AGRICULTURAL EXPERIMENT. STATION XEIEN TTO yields following sorghum. Even when using the absolute highest yield in each experiment to determine which of the two previous crops resulted in the highest wheat yield, the only large.difference occurred at Crossville in '1982. In this experiment, yield of wheat following soybeans was 40 percent higher than when sorghum was the previous.crop. To obtain relatively equal. wheat yields, higher N rates were generally required when sorghum was the previous crop than when wheat followed soybeans. There were only three exceptions where the optimum N rate (fall plus spring combination that resulted in the lowest N rate required for optimum yield) for wheat following sorghum was less than or equal to the optimum N rate required for wheat following soybeans. These exceptions occurred at Prattville in 1981 and at Crossville and Headland in 1982. At Crossville, the N combination resulting in the lowest rate of total N required for optimum yield was 0-48 regardless of previous crop. As previously mentioned, this was the only experiment in which sorghum resulted in lower wheat yields than soybeans. At Headland, optimum N combinations were 0-72 and TABLE 6. YIELDS OF THE PRECEDING CROPS AT EACH LOCATION AND YEAR Previous crop location and year Yield/acre Soybeans Bu. Sorghum Bu. Plains 1977 .. .. .. .. .. .. .. .. .. .. .. .. ......1978.............................. 1979 .............................. ..... .. .. . ......... 1980 ......... 1 32 41 -35 115 90 Calhoun .............. 1977........ ........ 1978 ............................. 1979.............................. 1980 61 49 -- 80 56 ............................ Crossville 1980.............................. 9 52 1981.............................. Prattville 1980................10 1981 ................. Headland 1980 .................. 1981................45 'Yield data not available. 34 58 16 .38 85 10 25 15 FALL-SPRING NITROGEN FOR WHEAT 11 12-48 when soybeans and sorghum, respectively, were the preceding crop; and at Prattville the optimum combinations were 24-48 and 12-24 for soybeans and sorghum, respectively. At both Prattville and Headland the preceding sorghum crop was a failure, table 6, and N applied to the sorghum (80 pounds per acre N) was evidently utilized to some effect by the subsequent wheat crop. In addition, the preceding soybean crops at Prattville (10 bushels per acre) and at Headland (15 bushels per acre) were poor, and N produced by these crops may have been low. The only other low previous crop yield occurred at Crossville in 1980. This low soybean yield (9 bushels per acre) did not result in higher N fertilizer needs when wheat followed soybeans as it did at Headland and Prattville when the preceding crop yields were low. The only locations in which more than 10 to 12 pounds per acre of fall N were needed to obtain the lowest total N for optimum yields of wheat following soybeans occurred when the preceding soybean yields were less than 15 bushels per acre. Spring N, Total N, and Optimum N Combinations There was a positive relationship between wheat yields and spring N rates. As previously discussed, the amount of spring N required for optimum yields was dependent on fall N and the previous crop. Spring N requirements also varied among years and locations. In table 7, the fall-spring combination resulting in the lowest total N rate required for optimum yield is listed along with the fall-spring combination which resulted in the absolute highest yield. Since the two yields are not significantly (P < 0.10) different, the treatment combination for each experiment that resulted in the lowest total fertilizer N requirement should be the optimum N combination for a specific location and year. Neither the optimum N combination nor the total N rate required for optimum yield was related entirely to wheat yield levels. For example, for wheat following soybeans at Plains in 1981, the optimum combination was 10-40 (50 pounds per acre N), and in 1980 the optimum combination was 10-60 (70 pounds per acre N). The 50-pound-per-acre N rate in 1981 resulted in 63-bushel-per-acre wheat yields and the 70-pound rate in 1980 resulted in only 47 bushels. In other words, the amount of fertilizer N needed is not necessarily established by yield potentials. Since N fertilizer requirements cannot be identified prior to harvesting wheat, and since optimum N combinations vary from 12 TABLE ALABAMA AGRICULTURAL EXPERIMENT STATION 7. FALL-SPRING COMBINATION RESULTING IN THE LOWEST TOTAL N RATE REQUIRED FOR OPTIMUM WHEAT YIELD, AND THE FALL-SPRING COMBINATION WHICH RESULTED IN THE ABSOLUTE HIGHEST YIELD FOR EACH YEAR AND LOCATION Year and previous crop Plains 1978 Sorghum ........ 1979 Soybeans ......... Sorghum ........ 1980 Soybeans ........ Sorghum ........ 1981 Soybeans ...... Sorghum ........ Calhoun 1978 Soybeans ........ 1980 Soybeans ........ Sorghum ........ 1981 Soybeans ........ Sorghum ........ Crossville 1981 Soybeans ........ Sorghum ......... 1982 Combi1 nation Fall Lb. Applied N/acre Spring Lb. 60 80 20 60 60 80 60 80 80 80 40 80 20 80 80 60 20 40 40 60 40 80 80 80 24 48 48 72 48 48 48 72 48 72 24 48 48 72 72 72 Total Lb. 60 120 20 80 80 100 70 110 90 90 50 80 60 120 80 100 30 50 50 90 40 80 90 110 48 72 72 108 48 84 48 84 72 96 36 72 60 108 72 84 Yield/ acre Bu. 31 34 47 54 45 52 47 52 53 53 63 65 64 68 58 63 40 45 39 45 66 71 64 71 45 51 42 53 30 38 23 27 75 76 72 79 47 50 48 52 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 2 1 2 1 2 1 2 1 2 2 0 30 0 20 20 20 10 30 10 10 10 0 40 40 0 40 10 10 10 30 0 0 10 30 24 24 24 36 0 36 0 12 24 24 12 24 12 36 0 12 Soybeans ........ .1 Sorghum ......... Prattville 1981 Soybeans ........ Sorghum ......... 1982 Soybeans ......... Sorghum ......... .1 continued FALL-SPRING NITROGEN FOR WHEAT 13 TABLE 7. N (CONTINUED) FALL-SPRING COMBINATION RESULTING IN THE LOWEST TOTAL RATE REQUIRED FOR OPTIMUM WHEAT YIELD, AND THE FALL-SPRING COMBINATION WHICH RESULTED IN THE ABSOLUTE HIGHEST YIELD FOR EACH YEAR AND LOCATION Year and previous crop Combination1 Fall Applied N/acre Spring Total Yield/ acre Lb. Headland 1981 Soybeans ........ 1 0 Lb. 24 Lb. 24 Bu. 50 Sorghum ........ 1982 Soybeans ........ Sorghum ........ 2 1 2 1 2 1 2 12 12 24 0 36 12 36 24 48 48 72 72 48 72 36 60 72 72 108 60 108 55 56 64 44 50 40 45 'Combination 1 is the fall-spring combination that resulted in the lowest N rate required for optimum wheat yields. Combination 2 is the fall-spring combination that resulted in highest absolute wheat yield. year to year and location to location within years, attempts were made to identify N combinations that would have resulted in optimum yields regardless of year or location. The fall-spring combinations resulting in the highest total N rates used would have resulted in maximum yield regardless of year or location, but these combinations would have resulted in excessive use of N fertilizer in each experiment. Several procedures including multiple regression were used to help identify optimum fall-spring N combinations. The most logical combinations were obtained from averaging the data in table 7 across locations and years for each fall-spring combination and each previous crop. The values were rounded to the nearest 5-pound level and listed in table 8. When the average fall-spring N combinations, table 8, are compared back to the various combinations that resulted in optimum yields, tables 3-5, the following observations can be made: 1. When wheat followed soybeans: The low N rate combination (10-45) resulted in suboptimum wheat yields in 5 of the 12 experiments (Plains in 1980, Calhoun in 1978, Crossville in 1981, Prattville in 1981, and Headland in 1982). The high N rate combination (20-60) did not result in suboptimum wheat yields in any of the 12 experiments. The average of the two combinations (15-53) resulted in suboptimum wheat yields in 3 of the 12 experiments (Calhoun in 14 ALABAMA AGRICULTURAL EXPERIMENT STATION TABLE 8. FALL-SPRING N COMBINATION (AVERAGED ACROSS YEARS AND LOCATIONS) RESULTING IN THE LOWEsT TOTAL N REQUIRED FOR OFTIMUM YIELDS AND ABSOLUTE HIGHEST YIELDS Previous crop Combination Fall Lb. Nitrogen/acre Spring Lb. 45 60 53 50 70 61 Total Lb. 55 80 68 60 95 80 Soybeans .............. 1 2 Mean 1 2 Mean 10 20 15 10 25 19 Sorghum .............. 1Combination 1 is the fall-spring combination that resulted in the lowest total N required for optimum yields. Combination 2 is the fall-spring combination that resulted in absolute highest yields. 1978, Prattville in 1981, and Headland in 1982). In two of these three experiments, however, the preceding soybean yields were less than 15 bushels per acre. 2. When wheat followed grain sorghum: The low N rate combination (15-20) resulted in suboptimum wheat yields in 6 of the 12 experiments (Plains in 1979, 1980, and 1981; Calhoun in 1981; Crossville in 1981; and Prattville in 1982). The high N rate combination (25-70) did not result in suboptimum wheat yields in any of the 12 experiments. The average of the two combinations (20-60) resulted in suboptimum wheat yields in 3 of the 12 experiments (Plains in 1980, Calhoun in 1981, and Prattville in 1982). Soil and Plant N vs. Nitrogen Deficiencies Just prior to spring N applications, soil (0 to 6-inch depth) and whole plant samples were collected for N analysis. The inorganic soil N (ammonium, nitrate, and nitrite) levels ranged from 6 to 20 pounds per acre. Differences found, however, were among locations and years and not between previous crops or among rates of fall N. No relationship was found between soil N levels and the amount of fertilizer needed by the wheat. Nitrogen in the plant tissue samples, tables 9 and 10, generally increased with increasing rates of fall N. Nitrogen levels in the tissue were usually slightly higher when soybeans were the previous crop. At Prattville in 1981, however, the N levels were much FALL-SPRING FIALLSPRING TABLE NITROGEN NITROGEN FOR FO;R WIIAT1 _WHEAT 15 9. NITROGEN IN WHEAT TISSUE JUST PRIOR TO SPRING N APPLICATIONS AT PLAINS AND CALHOUN, GEORGIA, AS AFFECTED BY PREVIOUS CROP AND FALL-APPLIED N Plant N, by previous lb./I crop and year Fall N, Fl.!acreSoybeans 1978 Pct. acre 1979 Pct. 1.90 2.67 3.09 3.48 3.23 2.92 3.00 3.27 3.38 1980 Pct. 2.47 2.62 2.37 2.99 2.80 2.67 2.66 2.66 2.90 1981 Pct. 4.44 4.48 4.63 4.74 4.68 4.80 4.81 5.06 4.97 1978 Pct. 1.91 2.17 Sorghum 1980 1979 Pct. Pct. 1.72 1.83 1981 Pct. 4.59 4.78 Plains 0...........10 ..........20...........30..........40..........-Calhoun 2.91 0.......... 10.......... 3.88 3.96 20.......... 30.......... TABLE 10. 2.69 2.24 2.44 2.50 - 2.20 3.15 2.91 2.74 2.63 2.78 2.79 2.63 4.81 4.85 4.97 4.40 4.71 4.71 2.74 2.85 3.10 3.38 2.37 2.77 4.46 2.63 4.41 NITROGEN IN WHEAT TISSUE AT THE ALABAMA LOCATIONS IN 1981 As AFFECTED BY THE PREVIOUS CROP AND FALL-APPLIED N Fall N, lb./ acre Crossville Soybeans Pct. Plant N, by location and previous crop Prattville Headland __________ _________ Sorghum Pct. 3.96 Soybeans Pct. 2.80 Sorghum Pct. 3.50 Soybeans Pct. 3.39 Sorghum Pct. 2.95 0 .......... 36...........4.96 4.52 12...........4.85 24.......... 4.66 4.20 3.97 4.24 2.78 2.82 3.53 3.34 3.50 3.32 3.62 3.78 3,04 3.11 3.07 2.91 higher when sorghum was the previous crop. The greatest differences between tissue N levels occurred among years and locations. Although there was a positive relationship between tissue N levels and yields, tissue N levels appear unsuited as an indicator for spring N requirements, primarily because of the variations that exist from- year to, year and location to location. For example, Calhoun, the 1980 data suggest when wheat followed soybeans that wheat grain yields would not respond to spring applied N if tissue N level exceeded 2.66 percent, but in 1978 and 1981, critical N levels were 4.53 and 5.06 percent, respectively. at Rainfall vs. Yield There is no doubt that rainfall and other climatic conditions are the primary factors that determine yield levels. With winter crops such as wheat, excessive and not deficient rain is, probably TABLE 11. RAINFALL DURING THE WHEAT GROWING SEASON AT EACH LOCATION Rainfall/month, by location and growing season Month 1978 In. November........... 3.0 December........... 1.8 January............. 7.3 February............ .7 March............. 4.8 r- Calhoun 1979 In. 5.5 1.5 6.5 5.2 7.9 Plains 1981 In. 2.4' 6.9 .8 6.2 4.5 Crossville 1981 In. 1.4 1.9 1.4 8.9 5.1 2.1 2.7 1981 In. 2.9 .9 1.2 7.4 5.5 4.4 1.9 1982 In. 2.8 6.6 10.1 6.9 2.9 9.3 .2.1 Prattville 1981 In. 2.2 1.8 1.5 7.7 5.2 5.9 2.7. 1982 In. 0.4 7.1 3.4 9.61 2.9 8.9 1.4 Headland 1981 In. 1.9 1.2 1.5 9.6 6.2 1982 In. 1.3 8.2 5.1 9.4 3.3 5.0 -3.0 0 cI 1980 In. 3.1 1978 In. 2.6 2.0 9.7 2.6 4.8 1979 In. 4.6 2.6 2.3 2.8 2.1 1980 In. 1.4 .7 3.3 2.0 9.8 4.3 3.5 .7 April............... May ....... 2.9 . 11.2 3.7 14.8 5.8 7.4 5.2 1.9 F 3.5 3.0 3.6 6.2 1.9 1.2 .9 4.8~ 3'.7 m m m x z -I z FALL-SPRING NITROGEN FOR WHEAT 17 responsible for low wheat yields in most years. In the years that these experiments were conducted, it appears that 25 to 30 inches of rain during the wheat growing season (November-May) was the desirable quantity, table 11. The highest wheat yields at each location were obtained in 1981 when total rain during the growing season was 28, 24, 24, 28, and 25 inches for Calhoun, Plains, Crossville, Prattville, and Headland, respectively. Poorest yields were recorded in years with the most rain, at Calhoun in 1980, Plains in 1978, and all Alabama locations in 1982. In these years, total rain ranged from 31 to 41 inches. The exceptionally dry year at Plains in 1979 (18 inches of rain) may have restricted yield, but yields in 1979 averaged approximately 20 bushels per acre higher than in 1978 when rainfall exceeded 31 inches. It is difficult to single out a period in which excessive rain will cause the greatest yield reduction, but it appears that high rainfall in January, especially when coupled with high rainfall in December, may be the most devastating. Since N efficiency is poorer in wet years than dry years, it would appear that fertilizer N requirements would be less in dry than wet years. Data from these experiments do not indicate that higher N rates will necessarily be required in the wetter years. At Plains, the wettest year was 1978 (32 inches), but N fertilizer requirement when sorghum was the previous crop was as high or higher in the other 3 years which were much drier (18 to 25 inches of rain). At Calhoun, the wettest year was 1980 (39 inches), and regardless of the previous crop, less N fertilizer was required in 1980 than in 1978 (25 inches) or 1981 (27 inches). Although disease data were not recorded in these experiments, it is well known that disease problems can be more severe in wet thah dry years. Plant disease may have limited yields in some of the wet years, thus decreasing the N fertilizer requirement in situations where N efficiency should theoretically be poor and fertilizer needs should theoretic(lly be high. In other words, it appears that N fertilizer requirements are higher in dry than wet years, which indicates that climatic effects on yield potentials are more important than climatic effects on N efficiency. SUMMARY AND CONCLUSIONS Wheat following sorghum will yield as high as wheat following soybeans, but N fertilizer requirements (both fall and spring) 18 ALABAMA AGRICULTURAL EXPERIMENT STATION will be higher (15 to 30 pounds per acre) when wheat follows sorghum. Soil N levels cannot be used to determine the amount of fertilizer N needed, regardless of the previous crop. Tissue analysis may identify a N deficiency, but N levels in the plant tissue cannot be used to predict N fertilizer requirements. High rainfall years do not increase N fertilizer requirements. Yield potentials will, however, be low in wet years, especially when excessive rain occurs in January and February. The adverse effect of wet weather on yield potential exceeds and masks the adverse effects of wet weather on N efficiency. The data actually indicate that N fertilizers rates can be reduced in wet years. Fall-applied N is critical when wheat follows sorghum. Unless soybean yields are low (less than 20 bushels per acre) optimum wheat yields can be obtained without any fall N. However, even in years when fall N is not required for optimum wheat yield, applying fall N will reduce spring N requirements, thus permitting the use of less total N with a fall-spring combination than with spring N alone. Although fall-applied N may reduce the amount of spring N needed, it cannot replace spring N. Many conclusions about optimum N combinations can be drawn from the data presented in this report, including the following guide for safe N application for wheat: 1. Wheat following soybeans-15 pounds per acre at planting followed by 50 pounds just prior to jointing, which normally occurs in February or early March, unless soybean yields were low (less than 20 bushels per acre), in which case N rates should be increased to 20 pounds at planting and 60 pounds prior to jointing. 2. Wheat following sorghum-25 pounds per acre in the fall followed by 70 pounds just prior to jointing. FALL-SPRING NITROGEN FOR WHEAT APPENDIX GRAIN YIELD STATISTICAL PROBABILITY LEVELS FOR FALL N, SPRING N, AND FALL-SPRING INTERACTIONS Year and Soybeans Grain sorghum 19 location Fall N Spring N Int. Fall N Spring N Int. 1978 Calhoun........0001 Plains ..........1979 Plains..........08 1980 Calhoun....... .20 Plains ......... .0001 1981 Calhoun....... .38 Plains ........... 02 Crossville....03 Prattville....0001 Headland....16 . .0001 .05 - -- .008 .0001 .18. .68 .20 .03 .20 .09 .93 .0001 .36 .08 .12 .84 .0001 .006 .0001 .24 .006 .50 .29 .0001 .001 .0001 .001 .00(1 .0001 .0001 .0001 .001 .0001 .0001 .0001 .0001 .0001 .0001 .0001 .0001 .0001 .0001 .01 .003 .0001 .0001 .26 .77 .02 .18 .73 .05 .53 .0001 .0001 .004 .0008 .19 ,.005 .002 1982 Crossville....66 Prattville.....0001 Headland .... uu-rrv~rr 0008 r-rv AUtl$UIN NIV(S11'i 'Vith an agric1tural research unit in every mc najor soil area. Auiburn Iiiersit% II scr \es the needs of field Cro( livest)ck, p. forestry and hor- ticultural producers ill each region in Allhama. Every citizen of the State has a strake in this researCh prograln, since any a~dvantageL~c from neW~ and more()r eca~nnomici N a\s t prod)Cucing a;nd halndling farm proClucts di reCtly I1Cenelts the consuming pulic. 0 OO L Rt-eafl to9' ® Main Agricultural Experiment Station, Auburn. 3 E. V. Smith Research Center, Shorter. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. Tennessee Valley Substation, Belle Mina. Sand Mountain Substation, Crossville. North Alabama Horticulture Substation, Cullman. Upper Coastal Plain Substation, Winfield. Forestry Unit, Fayette County. Chilton Area Horticulture Substation, Clanton. Forestry Unit, Coosa County. Piedmont Substation, Camp Hill. Plant Breeding Unit, Tallassee. Forestry Unit, Autauga County. Prattville Experiment Field, Prattville. Black Belt Substation, Marion Junction. The Turnipseed-Ikenberry Place. Union Springs. Lower Coastal Plain Substation, Camden. Forestry Unit, Barbour County. Monroeville Experiment Field, Monroeville. Wiregrass Substation, Headland. Brewton Experiment Field, Brewton. Solon Dixon Forestry Education Center, Covington and Escambia counties. 20. Ornamental Horticulture Field Station, Spring Hill. 21. Gulf Coast Substation, Fairhope.