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Many laboratory experiments have been conducted to study ammonia volatilization from N fertilizers, but few experiments have been conducted under field conditions. The objective of this study was to evaluate ammonia volatilization in the field from ammonium sulfate and ammonium nitrate applied to the surface of a calcareous soil, Houston Black Clay, that belongs to the fine, montmorillonitic, thermic family of Udic Pellusterts. Volatilization was evaluated indirectly by measuring N uptake by Coastal bermudagrass (Cynodon dactylonL.) and directly by measuring ammonia as it volatilized. In the field plot study, estimated losses ranged from 3 to 10% of the applied N for ammonium nitrate, from 36 to 45% of the applied N for pelleted ammonium sulfate, and from 25 to 55% of the applied N for liquid ammonium sulfate at rates of 140 and 280 kg N/ha. Direct measurements of ammonia losses from pelleted ammonium sulfate were 47, 43, 59, and 50% of the applied N in late summer, 1975, and 39, 30, 27, and 35% of the applied N in spring, 1976, at rates of 33, 66, 140, and 280 kg N/ ha. The lower loss of ammonia in the spring was apparently in response to lower soil temperatures at that time. Consistent diurnal fluctuations in ammonia losses occurred at both times. These fluctuations corresponded closely with daily fluctuations in the atmospheric relative humidity. These results agree with earlier laboratory studies in that ammonium sulfate loses considerably more ammonia than ammonium nitrate when applied to the surface of a calcareous clay soil.

Soils of the Wilson‐Crockett‐Burleson and the Burleson‐Heiden‐Crockett associations in the Blackland and Prairie have poor water relations and low fertility. The result is low crop production in many cases. The poor water relations are caused, in part, by a claypan that is often near the soil surface. The objective of this study was to impose various tillage and fertilizer treatments on a representative low‐producing area of these soils to determine the feasibility of renovation to improve forage production by Coastal bermudagrass (Cynodon dactylonL.). The soil at the experimental site was a Crockett sandy clay loam (Uderic Paleustalf, fine, montmorillonitic, thermic). The site was chiseled about 15 cm deep with chisels on 1 m centers the first year of the study only. Fertilizer treatments were imposed on the chiseled area and an adjacent area not chiseled. Forage yields were measured. Chiseling increased forage production throughout the study. When N application was 140 kg/ha and greater, forage production from the chiseled site was up to 3.3 times greater than from the area not chiseled. Breaking the claypan probably allowed greater infiltration of rainwater and applied fertilizer, thereby improving the environment for forage production. On an adjacent area, tilled to a depth of 1.5 m, forage production was the same on deep‐tilled and nontilled areas during the first year of the study. During the second year, forage production was increased by deep‐tillage and increased further when the deep‐tilled areas were treated with gypsum at 11 metric ton/ha.

The apparent recovery of N fertilizer by Coastal bermudagrass [Cynodon dactylon(L.)] growing on swelling clay soils has usually been quite low (approximately 40 to 50%), possibly due to biological immobilization of large quantities of applied N fertilizer. The objective of this study was to determine if the net immobilization might be reduced after several years of continued fertilization, thereby improving the recovery of N by Coastal bermudagrass. The 6‐year study was set up with variable N rates (NH4NO3was the N source), some applied only the first 3 years of the study, some applied only the last 3 years and some fertilized throughout the study. Measurements were taken of N removed in forage, soil NO3−‐N at the 504 kg N/ha rate, and soil organic N at the end of the study. Immobilization (as determined by the increase in soil organic N) accounted for over 50% of the applied N, about equal to the amount of N going into the harvested portion of the grass. Other potential losses of N such as leaching and denitrification were apparently insignificant. There was no indication of inorganic N accumulation in the later years of the study. However, some improvement in N availability was noted in the last three years of the study on those plots receiving fertilizer throughout the 6‐year study. The results from this study suggest that immobilization of fertilizer N on Coastal bermudagrass pastures would be a substantial sink for applied N fertilizer for an unknown but large number of years. Continuous generous fertilization programs would result in a gradual buildup of soil organic N with a resulting gradual improvement in N recovery by forage.

Few attempts have been made to directly measure ammonia losses from surface applications of urea under field conditions. The objective of this research was to measure NH3volatilized from urea in the field and to compare these losses to (i) losses measured in the laboratory, and (ii) losses estimated by crop response in the field. Direct field measurements were made using an automated apparatus designed to collect NH3lost without significantly altering environmental conditions on the treated area. Laboratory measurements were made using an apparatus similar to systems described previously by other workers. Ammonia losses were estimated from crop response in the field by measuring N uptake by Coastal bermudagrass (Cynodon dactylonL.). Losses of NH3in the field measured directly were small (0 to 9% of the applied N) compared to those obtained in the laboratory (13 to 31% of the applied N) and those estimated in the first year of the N uptake study (0 to 39% of the applied N). However, in the second year of the field plot study, N uptake by forage from several urea fertilizer treatments was greater than from Ca(NO3)2. When the results were combined for both years, N uptake by forage was similar from Ca(NO3)2and urea treatments. Considering the small ammonia losses measured directly in the field, differences in crop response between urea and Ca(NO3)2may have been due to some factor other than NH3volatilization. Results indicate that losses of N by NH3volatilization under field conditions may not be as severe as previously thought, at least under the soil and environmental conditions of this study. Results also demonstrate the importance of directly measuring NH3losses in the field.

In order to accurately evaluate the N mineralization from decomposing crop residue, residue should be uniformly labeled, i.e., it should contain a relatively constant atom % 15N in all N compounds within the plant. Our objective in this study was to estimate the uniformity of 15N labeling by comparing the 15N content of different plant parts of winter wheat (Triticum aestivumL.) and grain sorghum [Sorghum bicolor(L.) Moench]. Labeling with 15N was accomplished by applying a single, highly enriched but low rate of N to the soil early in the growth stage of the respective crops. Small enclosed microplots were used for incorporating the 15N enriched fertilizer into the initial crop and for following decomposition in subsequent years. At crop maturity, recovery of the initial K15NO3application avg. 71.9% for all soil texture‐crop combinations. Data from the 15N labeling study indicated that relatively uniform enrichment of various plant parts was obtained when K15NO3was applied to the soil at the jointing and eight‐leaf stages for wheat and sorghum, respectively. On average, atom % 15N values for all plant parts ranged from 6.962 to 8.171 and 2.801 to 3.274 for winter wheat and sorghum, respectively. These results imply that segregation of 15N‐labeled plant material into various plant parts for use in subsequent crop residue decomposition experiments is unnecessary.

The management of crop residues and their contribution to the supply of plant nutrients is intimately tied with N cycling in the soil. Any attempt to utilize crop residues to the fullest extent requires an understanding of how the system functions. The objective of this research was to determine the extent and rate of N mineralization from 15N‐labeled winter wheat (Triticum aestivumL.) and grain sorghum [Sorghum bicolor(L.) Moench] residues. In addition, the influence of soil texture and fertilizer N on net N mineralization also was evaluated. Nitrogen mineralization from decomposing crop residues was followed by means of plant and soil sampling during subsequent cropping seasons. Plant N uptake and soil inorganic N data showed that 12 to 15% of wheat residue N and 12 to 33% of sorghum residue N were mineralized after one cropping season. Net mineralization differences between crop residues may be attributed to both the C/N ratios and chemical composition of these plant materials. Soil texture and the addition of fertilizer N mainly influenced the initial rate of mineralization, particularly with the wheat residue. There was also evidence of N immobilization of the recently mineralized 15N followed by remineralization later in the growing season for wheat and sorghum residues decomposing in the medium‐textured soils. This immobilization occurred during a period of rapidly increasing soil temperatures. On avg, 79% of the mineralized wheat residue N and 82% of the mineralized sorghum residue N were utilized by wheat and sorghum plants, respectively. Based on these results, the greatest proportion of N mineralized from decomposing wheat and sorghum residues appears to coincide with plant N demand by the subsequent crop, irrespective of differences between C/N ratios of these residues or soil texture.

A field study was conducted for 8 yr on an initially low soil test P, Woodson silt loam soil (fine, montmorillonitic, thermic Abruptic Argiaquolls) in east‐central Kansas to determine the effectiveness of ortho‐and polyphosphate fertilizers applied annually at 25 kg ha−1, every 2 yr at 50 kg ha−1, and every 4 yr at 100 kg ha−1for grain sorghum (Sorghum bicolorL.) production. The first year, yield was higher with the larger, less frequent P applications because of the low P status of the soil. Second‐and third‐year yields were not statistically different. The fourth year, when the time span was longest since the every 4‐yr P application had been applied, yield was lower with the every 4‐yr P application compared to annual P. The every 2‐yr P treatment produced intermediate results. Total grain production for the first 4 yr did not differ significantly with the P application frequency treatments. During the second 4 yr, when soil P levels were higher, the P application frequency treatments produced no significant differences in yearly or total yield. The ortho‐and poly‐phosphate P sources, in general, produced similar response applied annually, or at infrequent intervals. These data suggest that there are Argiaquolls in east‐central Kansas on which infrequent P fertilizer applications can perform similarly to annual P applications.

Nitrification inhibitors can slow the rate of nitrification in soils and reduce nitrogen losses under some soil‐climatic conditions. Carbon disulfide (CS2) and compounds which produce CS2have slowed nitrification in laboratory studies, but these compounds have not been tested extensively in the field. Therefore, field studies were conducted on irrigated corn (Zea maysL.) in 1978 and 1979 to evaluate inhibition of nitrification by CS2at 5.6, 11 and 22 kg ha−1, sodium trithiocarbonate (STC) at 11 and 22 kg ha−1, ammonium dithiocarbamate (ADTC) at 11 and 22 kg ha−1, and potassium ethyl xanthate (KEtX) at 5.6 and 11 kg ha−1as compared to nitrapyrin (2‐chloro‐6‐(trichloromethyl)‐pyridine) at 5.6 kg ha−1. Anhydrous ammonia was knifed 15 to 20 cm deep on 60 cm spacings at rates of 84, 168, and 252 kg N ha−1. All materials were evaluated on a Eudora fine sandy loam (Fluventic Hapludoll, coarse, silty mesic) in 1979 and CS2and STC were evaluated on the same soil in 1978 and on two additional soils in 1979: a Pratt loamy fine sand (Psammentic Haplustalf, sandy, mixed, mesic) and a Crete silty loam (Pachic Argiustoll, fine, montmorillonitic, mesic). The performance of STC and CS2in 1978 and 1979 indicated that these materials were not effective in inhibiting nitrification under the conditions that existed at the locations where they were applied. ADTC and KEtX were not effective inhibitors of nitrification, although they were only tested at one location in 1979. Nitrapyrin was effective in inhibiting nitrification in all studies. However, it resulted in an increase in corn yield only once—the fall N application on the Pratt loamy fine sand. The lack of yield response was attributed to the absence of conditions conducive for losses of nitrate nitrogen.

Limestone suspensions provide a means of uniformly applying finely divided liming materials to raise soil pH. Water has usually been the suspending medium, but N fertilizer solutions may be used. The objective of this research was to measure ammonia (NH3) losses that might occur while a lime and urea‐ammonium nitrate (UAN) suspension is being prepared and potential losses after the suspension is applied to soil. Ammonia volatilization was measured in the laboratory from lime‐UAN suspensions formulated with lime sources containing various amounts of CaO. Suspension pH's, rates of loss, and total NH3losses were directly proportional to the CaO content of the lime material and ranged up to approximately 90% of the ammoniacal fraction of the UAN after 4 hours. Losses were not influenced greatly by the initial pH of the UAN materials used in the study. Ammonia volatilization from the surface of two acid soils was also measured after lime‐UAN suspensions were applied at a rate of 840 kg/ha effective calcium carbonate [CaCO3; (ECC)] and 112 kg N/ha. Lime applied with the UAN increased NH3volatilization, but less on Grundy sicl (Aquic Argiudoli) than on Parsons sil (Mollic Albaqualf) with a lower cation exchange capacity (CEC). Up to 17% of total N was lost with the lime‐UAN treatments on Parsons soil with a CEC of 10.2. Results indicate substantial losses of NH3may occur while lime‐UAN suspensions are being prepared only if the lime contains CaO. After soil application, NH3losses can occur in the absence of CaO if the lime‐UAN is allowed to remain on the soil surface.