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Arrowleaf clover (Trifolium vesiculosumSavi ‘Yuchi’) is a winter annual legume that can be used to extend the grazing of warm‐season perennial grasses in forage production systems. Stand establishment from overseeding often fails because of unfavorable climatic conditions and nutrient deficiencies in soils. The objectives of this research were to determine the effects of P and K fertilization on yield and nutrient uptake of arrowleaf clover and accumulation of soil P and K. Arrowleaf clover was seeded in field plots in the fall of 1978. Fertilizer treatments consisted of an incomplete factorial arrangement of multiple rates of P, K, and P plus K. Rates of P and K included a control that did not receive fertilizer; 20, 40, and 80 kg P ha−1; 37, 74, and 148 kg K ha−1; and P‐K combinations of 20:37, 40:74, and 80:148 kg ha−1. The soil type was a Taloka silt loam (Mollic Albaqualfs). Yield and N, P, and K uptake in forage were determined in four consecutive years starting in 1979. Soil test indices were measured prior to seeding in 1977, in the fall of 1978, and after four annual fertilizations in 1982. Regression equations with linear, quadratic, and interaction terms were calculated to describe dependent variable response surfaces for each year and the average over years. Phosphorus fertilization increased yield each year. Yield was not increased with K fertilization alone, but there was a positive P × K interaction in the 1982 season indicating soil K levels were decreasing. Predicted maximum yields were 6561, 6490, 6516, and 6860 kg ha−1due to applications of 50.8, 52.4, 54.0 and 57.2 kg P ha−1, respectively, at fixed rates of 0, 37, 74, and 148 kg K ha−1. Phosphorus increased N, P, and K uptake and when applied in combinations with K, some interactions occurred. The soil test index for P increased slowly (0.5 kg ha−1yr−1) with P rates of 20 kg ha−1annually; however, a rapid accumulation of soil P occurred with four annual applications of 80 kg P ha−1(16.5 kg ha−1yr−1). Approximately 107, 16, 6, and 3 annual applications of 20, 40, 60, and 80 kg P ha−1, respectively, would be required to increase soil test values for P to a sufficient level. Soil test values for K were increased from 133 to 308 kg ha−1with four annual applications of 148 kg K ha−1. However, after four annual applications of 80 kg P ha−1in combination with 148 kg K ha−1, the soil test value for K was only increased to 192 kg ha−1due to the marked response from P fertilization and subsequent increase of K removal in forage.

Incubation studies were conducted to determine the effect of temperature, moisture content, organic C, soil depth, and time on transformations of added 15NO3‐N to a Sonoran Desert soil in open and closed systems. In open system studies, increasing temperature from 20° to 37°C, adding organic C, and increasing moisture content from field capacity to saturation increased denitrification losses 16, 22, and 2%, respectively. There were no apparent differences in immobilization of 15NO3‐N due to temperature, time, moisture content, or soil depth without organic C amendments. However, immobilization of 15NO3‐N soils with organic C amendments was increased markedly with increased temperature and time, but initial moisture content had little effect. At C/N ratios of 150:1 only traces of 15N appeared in NH4‐N, however, at C/N ratios of 15:1 to 45:1 ammonification and immobilization occurred simultaneously. Closed system investigations with organic C and 15NO3‐N amendments showed 79% loss in 5 days with the remaining 21% accumulating as organic‐15N. Denitrification losses in open systems with soil moisture contents representative of saturated conditions were 70% of the loss observed under closed systems at the same moisture content.

The effects of 0.0, 0.01, 0.1, and 1.0Mconcentrations of sodium, copper and calcium chloride salts added to a Pima clay loam with and without 15NH4Cl on mineralization of soil nitrogen, nitrification, and immobilization were measured over a 49‐day period. Dilute concentrations of salt, 15NH4Cl, and dilute salts plus 15NH4Cl stimulated mineralization of soil N. The “priming effect” was shown to be real and not just a simple exchange. Nitrification of native ammonium nitrogen and 15N‐labeled ammonium decreased with increasing concentrations of salt. High concentrations of copper and calcium chloride salts inhibited nitrification of 15N‐labeled ammonium more than sodium salts. Immobilization of 15NH4+‐N was decreased significantly by high concentrations of salt. Gaseous loss processes were decreased with increased salts.

Nitrogen and P applied together in deep‐band applications have been reported to increase crop production over that obtained with conventional applications. Little information is available on the continuous use of this method of application. Therefore, the objective of this study was to compare the effects of N, P, and N plus P knifed in bands with conventional broadcast methods on grain yield, N and P uptake in grain, and concentration in wheat forage when applied several years at the same location. Winter wheat (Triticum aestivumL.) was grown at three locations from 1980 to 1983. Two N sources, anhydrous ammonia (AA) and ammonium nitrate (AN), one P source, ammonium polyphosphate (APP), and one nitrification inhibitor source, nitrapyrin (NI), were used. Rates of N, P and NI were 112, 30, and 0.56 kg ha−1, respectively. Methods of N and P fertilization included broadcast (BC) and knifed (KF) in bands 15 cm deep with 45‐cm spacings. Crops responded to N fertilization at Altus (southwest location), N plus P at Stillwater (northcentral), and P at Haskell (northeast). Effects of methods of application varied with locations and years. The first year, at Stillwater was the only occurrence of N and P knifed in that resulted in higher yield and N and P uptake in grain compared with broadcast methods. However, with continued fertilization in the same plots with subsequent years, the positive effect of fertilizer in deep placed bands diminished, and broadcast applications were more efficient. Broadcast P applications were more efficient in later years of the study at Haskell where the greatest response to P fertilization was obtained. No long‐term benefits due to placement of N and P together or in combination with NI in deep bands were observed at any of the three locations.

Phosphorus fertilization has been reported to influence micronutrient availability. The influence of applied P on extractable micronutrients depends mainly on the chemical characteristics of the soil and the P source. Soils in the western Great Plains typically have high soil pH and are saturated with Ca, which has a marked effect on micronutrient availability. Therefore, the objectives of this study were to determine the effects of different P sources and rates on (a) soil pH, (b) Bray and Kurtz no. 1 P, and (c) DTPA-extractable Fe, Mn, Zn, and Cu.

Many modifications in procedures for N determination by Kjeldahl digestion exist. However, few modifications include the quantitative recovery of NO−3–N, which is essential in 15N research. A digestion procedure is described in which reduced Fe and H2SO4pretreatment is used for the quantitative recovery of NO−3–N. The procedure utilizes an aluminum block digester with a programmable control unit. The recommended program employes a stepwise temperature ramp up to 400°C and continues digestion 1 h after clearing for a total digestion time of 7.25 h. Two other digestion programs were compared with the recommended program. One program used a temperature ramp identical to the first, but digested 2 h after clearing for a total time of 8.25 h. A third digestion program used a shortened temperature ramp, attained a final temperature of 400°C, and was completed 1 h after clearing for a total time of 5.90 h. Wheat (Triticum aestivumL.) grain samples with up to 0.5 mg additional NH+4–N and NO−3–N were used to evaluate these procedures. The recommended procedure provides complete recovery of grain N, and added N with superior levels of precision in comparison to the other digestion procedures evaluated. The digestion procedure using 5.90 h did not yield accurate analysis of grain N, indicating incomplete digestion. The 8.25‐h digestion did not result in improvement of grain N results or added N recovery.

Current changes in residue management systems for winter wheat (Triticum aestivumL.) include anhydrous NH3injection combined with herbicide application, planting, and other field operations. The feasibility of NH3application during planting is dependent upon potential NH3toxicity to germinating seeds. A laboratory experiment was conducted to study anhydrous NH3movement in soils by injecting 124 and 180 mg N as NH3into a Hollister clay (fine, mixed, thermic Pachic Paleustolls), Grant silt loam (fine‐silty, mixed, thermic Udic Argiustolls), and Norge clay loam (fine‐silty, mixed, thermic Udic Paleustolls) at 0.124 and 0.181 water (kg kg−1) contents. Soil samples were taken 1, 7, and 14 d after N application. Soil NH+4‐N and calculated NH3(aq) concentrations following NH3injection generated a normal distribution curve with N concentrations highest near the injection point and decreasing with increasing distance. Calculated NH+4diffusion coefficients decreased with time and water content and were soil dependent. Field studies were conducted in 1983 to determine the potential germination hazard from NH3applied at 0, 56, 112, 168, 224, and 280 kg N ha−1while planting, and in 1984 to winter wheat planted 0, 1, 3, 5, 6, and 7 d after applications of 0, 100, and 200 kg N ha−1. Stand counts, forage NO‐3‐N concentration, and yield were not influenced by NH3applications during planting in 1983, but grain N concentration increased linearly up to 24.1 mg g−1with 280 kg N ha−1. In the 1984 experiment, increasing NH3rates did not decrease seedling stand, even though analysis of soil samples reflected increased soil pH and soil NH+4‐N concentration. Anhydrous NH3placement at a sufficient distance from newly seeded rows will ensure the success of systems which include NH3injection in conjunction with other field operations in residue management systems.

Efficient N applications are necessary to maximize bermudagrass (Cynodon dactylonL.) dry matter yield and N uptake in forage. Significant losses of NH3from surface applications of urea and NH4‐N fertilizers by volatilization have been reported. Field experiments were conducted over a three‐year period under a wide range of climatic and soil conditions in Oklahoma to determine the efficiency of N applications for dry matter production and N uptake in bermudagrass forage. Four locations were selected in 1978 and one each in 1979 and 1980, representing four typical soil types for bermudagrass production with a pH range of 4.7 to 6.9. Anhydrous ammonia (AA) and ammonia passed through a Cold‐Flo adapter (ACF) were injected into bermudagrass sod and urea‐ammonium‐nitrate (UAN), urea, and (NH4)2SO4(AS) were broadcast. Rates of N were 0, 112, 224, and 448 kg·ha−1initially as well as 224, 336, and 448 kg·ha−1in split applications. An additional experiment was established in 1980 to evaluate the effect of S on yield and N uptake. Rates of S were 0 and 64 kg · ha−1and sources were elemental S, CaSO4· 2H20, MgSO4, and AS. There were two harvests at each location in 1978, three in 1979, and two in 1980. Yield and N uptake from AA and ACF applications were not significantly different, but generally were lower than with broadcast UAN, urea, and AS. Yield and N uptake generally were lower from urea than UAN and AS. Relative efficiency of N sources obtained from averaging across N response experiments were in the order of AS > UAN > urea > AA > ACF. Split application of N in 1978 and 1980 did not increase yield, but in 1979 an increase in yield was obtained with more even rainfall distribution during the growing season. Sulfur fertilization decreased N:S ratios in forage, but did not increase yield or N uptake or improve N efficiency. Potential losses of NH3by volatilization from broadcast urea, UAN, and AS in these experiments were considered minimal.

Influences of ammonium, nitrate, organic N sources and organic C amendments on microbial respiration, N transformations and gaseous losses were measured under natural environmental conditions in two field experiments. Microbial respiration was increased by addition of wheat straw and barley grain. Addition of N as (15NH4)2SO4and K15NO3alone did not increase CO2evolution significantly. Significant immobilization of added inorganic‐15N occurred with few exceptions in all treatments except nitrate alone. Organic‐15N in soils amended with wheat straw increased with increasing N rates with both inorganic N sources. Inorganic‐15N was practically void in both soils at the end of 12 months. Mineralization of 15N‐labeled barley grain occurred but at a very low rate and no significant accumulations of inorganic‐15N were observed. Gaseous losses of N at the end of 12 months varied from 73 to 94% of the inorganic‐N added alone, with the greatest loss occurring from K15NO3. The incorporation of wheat straw stimulated immobilization of added N and reduced gaseous losses to 30 to 86% of the total N applied and the smallest loss occurred with (15NH4)2SO4. The average loss of mineralized organic‐15N derived from barley grain for the two soils was 19% of the total N added. Most of the gaseous losses occurred within the first 3 months, the period of the measurable rainfall. Analyses of subdivided soil sections (0–5 and 5–10 cm) showed that 95 to 100% of 15N remaining in the soil was in the upper 5‐cm layer indicating that leaching was not appreciable in any treatment.

Nitrification inhibitors offer the potential to improve efficiency of N applications when climatic and soil conditions are favorable for leaching and/or denitrification. Grain sorghum [Sorghum bicolor(L.) Moench] was grown in five field experiments at three locations during 1976, 1977, 1978, and 1979, with two nitrification inhibitors and three N sources. Four of these five experiments received supplemental irrigation water. Soil textures at experimental locations were sandy, silt, and clay loams and pH's ranged from 5.2 to 7.8. Nitrification inhibitors evaluated were nitrapyrin and etradiazole. Sources of N were anhydrous ammonia and urea in 1976, urea and urea‐ammonium‐nitrate (UAN) solution in 1977, and UAN hi 1978 and 1979. Treatments consisted of 0.56 kg/ ha of nitrapyrin and multiple rates of etradiazole applied with varying rates of N. Soil samples were taken from selected experiments at varying time intervals and analyzed for urea‐, NH4+−, NO3−, and/or NO3−‐N depending on the year and experiment. Field plots were harvested with a conventional combine and yields were analyzed statistically to assess beneficial effects of treatments. Soil N forms were altered by nitrification inhibitors at various tune intervals during the growing seasons. However, the changes observed did not lead to increased efficiency of applied N or to an increase in yield of grain sorghum. It was concluded that little if any benefit can be derived from nitrification inhibitors in grain sorghum production in Oklahoma.

Gaseous losses of N from soils are mainly from denitrification and ammonia volatilization processes. Minute quantities of N gases may be lost via chemical reactions, but the amount is so small that these processes are not covered here. Although denitrification and ammonia volatilization processes have been known for many years, the magnitude of losses has been highly underestimated until recently. Denitrification and ammonia volatilization processes have been studied primarily under controlled laboratory conditions until the past few years. In soils, the presence of organisms such as Alcaligenes, Bacillus, Micrococcus, Pseudomonas, and Thiobacillus; anoxie microsites due to excessive moisture; organic C for energy; elevated temperatures; and high soil pH all have interacting affects on the magnitude of loss by denitrification. Ammonia volatilization losses are influenced mainly by soil pH, ammonia concentration, pH buffering, cation exchange capacity, moisture content, and temperature. Management practices to control denitrification and ammonia volatilization losses are discussed. Although much progress has been made, more research is required to fully understand the interactions of all the factors that govern gaseous N losses under field conditions.

Iron chlorosis in soybeans, Glycine max (L.) Merr., is a major problem in many soils in the western Great Plains. Tolerance to chlorosis differs significantly within soybean cultivars. Therefore, the objectives of this study were (a) to determine the tolerance of soybean cultivars to chlorosis; and (b) to determine the relationship of nutrient concentrations in leaves and the degree of chlorosis. A randomized complete block design consisting of four replications with a factorial arrangement of treatments (2x5x2) was used in a greenhouse experiment to study the objectives. The main effects were soil types (Quinlan cl, pH 8.1 and McLain si cl, pH 7.3), cultivars (Essex, Crawford, York, Gail, and Forrest), and fertilizer rates (0 and 0.2 kg Fe ha-1). Chlorosis was more severe in all cultivars grown in Quinlan soil than in McLain soil. York and Forrest exhibited the most severe chlorosis in Quinlan soil while York was the most chlorotic in McLain soil. Essex exhibited the least chlorotic symptoms of the cultivars tested in both soils. Chlorophyll concentration in leaves was higher in Essex than in the other cultivars. There were no significant differences in Fe concentration of leaves among cultivars grown in both soils. Phosphorus in leaves of Forrest was significantly higher than in the other cultivars when grown in McLain soil. High Ca and Mn concentrations in leaves were usually associated with the most chlorotic cultivars. Other nutrient concentrations and micronutrient ratios varied among chlorotic and nonchlorotic cultivars.

Residual effects of two 15N‐labeled N fertilizers, urea and oxamide, were compared during the second cropping season after they had been applied at rates of 0, 56, 112, and 168 kg N/ha. ‘Sudax SX11’ sorghum‐sudan grass hybrid (Sorghum sudanense) was grown and harvested three times during this second cropping season while the residual effects were being measured. Fertilizer N removed in plant tops during this second year of cropping contained 13–18% of the residual fertilizer N in the soil at the end of the first season and was equal to 4–6% of that applied originally in the fertilizers. At the end of the second cropping season, 22 and 26% of the initial applications of N in urea and oxamide, respectively, remained in the soil. The effect of carriers on these amounts of residual fertilizer N was not significant. The removals in crops during the second season plus those amounts remaining in the soil at the end of the second season were essentially equal to the residual fertilizer N which had been in the soil at the end of the first season. Thus, no evidence was found for loss of residual N by leaching, denitrification, or other processes during the second cropping season. Since removals of N in crops were considerably greater than the additions in fertilizers, it is hypothesized that nearly all inorganic fertilizer N was taken out of the soil and most of the residual fertilizer N was in relatively stable organic forms. Conditions for these experiments had been selected to conserve N, and as a consequence the deficits of fertilizer N reported here may be expected to be smaller than might occur under other practices.

Urea and oxamide, each labeled with 15N, were compared as fertilizers in two field experiments in adjacent locations in successive years with ‘Sudax SX11’ Sorghum‐sudan hybrid (Sorghum sudanense) as the test crop. Four cuttings were harvested during the first experiment and three during the second. Patterns of uptake of fertilizer N were in accord with the characteristics of the two carriers. In the first harvests in both experiments the amounts of N taken up from urea were markedly greater than from oxamide; but by the third harvests, yield responses and N uptake from oxamide were greater than from urea. Total recoveries of fertilizer N from the two carriers during the entire growing season were similar as greater recoveries of urea‐N in the early summer were compensated for by greater recoveries of oxamide‐N in the late summer. Of the N added in urea in the first experiment, 51% was recovered in the crops and 28% was still in the soil (0–25cm) at the end of the growing season. Corresponding figures for oxamide were 52% in the crops and 31% in the soil. In the second experiment, when fertilizer applications and planting operations were delayed until more favorable growing weather, 93% and 99% of the urea‐ and oxamide‐N, respectively, were estimated as recovered in the crops, and measurements of the amounts of fertilizer remaining in the soil were not attempted.

Evidence of the “priming effect” on the uptake of soil N by additions of rather conservative amounts of fertilizer N was examined in data from two recently reported field experiments. In these experiments, urea and oxamide each labeled with 15N were compared on adjacent locations in successive years with ‘Sudax SX11’ Sorghum‐sudan hybrid (Sorghum sudanense) as the test crop. The priming effect in the first experiment was calculated from data from four cuttings during the first year and from a residual cutting during the second year. For the second experiment, data were used from the three cuttings harvested during the year the fertilizers were applied. Additions of N fertilizer increased the uptake of soil N by 17 to 45% in the first experiment in 1966 and by 8 to 27% in the second experiment in 1967. In the residual cutting of the first experiment, increases in uptake of soil N ranged from less than 0 to 29%. The increase in uptake of soil N by the crops was speculated to be due to stimulation of microbial activity by N fertilizers which increased mineralization of soil N, thus making more soil N available for use by plants.