Nitrification and nitrate movement from ammonium sulphate and urea in sandy soils as affected by their previous applications

Two successive applications of urea and ammonium sulphate (AS) at varying intervals were given in two soils, one of which was salt affected. The nitrification and nitrate leaching after both the applications of fertilizers was studied. The nitrification of first application of AS was faster than urea on both soils. However, the nitrification rate of both fertilizers was slow in salt effected soil. The same trend of results was observed with second application of fertilizers. However, the nitrification of second application given within 6 weeks of the first application proceeded at a much faster rate than that of the first application. The amount of NO ₃ ^- that moved down with periodic water application was related with nitrification rate and the amount of fertilizer nitrified at the time of water application.     

Transformation of calcium cyanamide and its inhibitory effect on urea nitrification in some tropical soils

Transformation of calcium cyanamide and its inhibitory effect on urea nitrification was studied in some tropical soils. Three soils, from Onne, Mokwa and Samaru, representing different agro-climatological zones of Nigeria were incubated with calcium cyanamide, urea or a mixture of both for eight weeks at 30 °C and at field capacity moisture content. The recovery of inorganic N (NH ₄ ⁺ plus NO ₂ ^- plus NO ₃ ^- )from calcium cyanamide varied from 64% to 87% in different soils. Most of the inorganic N accumulated was in NH ₄ ⁺ form. Nitrification of the accumulated NH ₄ ⁺ in all the soils was slow.Urea (75 mg N kg^–1 soil) was completely nitrified within a week in the Samaru and Mokwa soils whereas in the Onne soil the rate of nitrification was slow. Addition of CaCN₂ at the rate of 10 mg N kg^–1 soil generally delayed ammonification of urea and nitrification was severely inhibited in all the soils. Addition of CaCN₂ at the rate of 20 mg N kg^–1 soil further reduced the ammonification of urea and completely inhibited the nitrification. High recovery of inorganic N from calcium cyanamide and its effective reduction of nitrification of urea make it suitable source of N for plants in the tropics, provided it is managed to avoid phyto-toxicity.     

Effect of the nitrification inhibitors 1-amidino-2-thiourea and dicyandiamide in combination with urea and ammonium sulfate

The efficiency of the nitrification inhibitors dicyandiamide (DCD) and 1-amidino-2-thiourea (or guanylthiourea = GTU) in reducing losses from N fertilizers was investigated in two greenhouse experiments where leaching of nitrate-N was induced by percolation at 3 and 5 weeks after fertilization.At an application rate of 10% by weight of fertilizer-N (e.g. 10 kg GTU/ha), GTU in combination with ammonium sulfate (AS) had effects similar to those of DCD (e.g. 15 kg DCD/ha) with regard to nitrate leaching, plant yields and nitrogen uptake. However, in combination with urea (U), GTU was more effective than DCD when applied at the same ratio except with a humic sandy clay soil (pH 7.3, 4.4% organic C), where GTU did not perform as effectively. Nitrate leaching was reduced by as much as 50% using U/GTU instead of U/DCD, and plant yield increased by 30%.At temperatures between 17 and 25°C, the combination U/GTU could protect a high percentage of the nitrogen from being nitrified and leached over a 3 to 5 weeks period. The superiority of GTU over DCD was demonstrated especially in the treatments with 5 weeks of preincubation, despite the considerably lower application rate.     

Transformations and losses of fertilizer nitrogen on no-till and conventional till soils

A field study was conducted in 1982 to measure the effect of no-till (NT) and conventional till (CT) systems on N transformation after surface and subsurface applications of N fertilizers. Urea, urea-ammonium nitrate (UAN) solution, (NH₄)₂SO₄ (AS), and CA(NO₃)₂ were applied to NT and CT plots (5.95 m²) at a rate of 448 kg N ha^–1. A comparison of fertilizer N recovered in soils receiving incorporated or surface applied N was used to estimate NH₃ volatilization while denitrification was estimated from fertilizer N recovered in the presence and absence of nitrapyrin with incorporated N. Immobilization was assessed in microplots (0.37 m²) after surface application of (¹⁵NH₄)₂SO₄ to NT and CT systems at a rate of 220 kg N ha^–1.The results indicate little difference between NT and CT systems on urea hydrolysis rates and immobilization of surface applied fertilizer N. Approximately 50% and 10% of the surface applied N was recovered in the inorganic and organic fractions, respectively, on both tillage systems. The N not recovered was likely lost from plot areas through soil runoff. Incorporation of UAN, urea and AS resulted in 20 to 40% greater inorganic N recovery than from surface application. Nitrification rates were greater under the NT than the CT system. The similarities in concentration in the various N pools observed between the two tillage systems may be partially due to the short length of time that NT was imposed in this field study (<1 year) since other researchers using established tillage systems (>5 y) indicate that NT tends to promote decreased efficiency of fertilizer N.     

Effects of temperature on nutrient release from slow-release fertilizers

We studied the effect of temperature on the release of N, P, and K from slow-release fertilizers (SRF). The study was conducted in micro-lysimeters filled with moist peat medium. Increasing the temperature from 4 to 12°C slightly increased N release from three different slow-release N (SRN) carriers with different particle sizes and coating thicknesses. At 21°C the rate of release was significantly different than the other two temperatures. Urea formaldehyde (UF), sulphur coated urea (SCU) and coated calcium nitrate (CCN), incubated in sphagnum moss peat, released between 3 and 20% of the applied N in six weeks. For eight synthetic and organic NPK carriers, the release pattern was similar to UF and SCU. However, the leaching losses of N from the NPK fertilizers were up to twenty times more than for the SRN products. Except for Osmocote^® and Duna, which released 30–40% of the applied N as mineral-N within six weeks, all other slow-release and slowly mineralized NPK carriers acted like readily water-soluble compound NPK. Temperature did not affect the nutrient release from NPK fertilizers.     

Nitrogen transformations near urea in soil: Effects of nitrification inhibition,nitrifier activity and liming

A laboratory incubation experiment was conducted to gain a better understanding of N transformations which occur near large urea granules in soil and the effects of dicyandiamide (DCD), nitrifier activity and liming. Soil cores containing a layer of urea were used to provide a one-dimensional approach and to facilitate sampling. A uniform layer of 2 g urea or urea + DCD was placed in the centre of a 20 cm-long soil core within PVC tubing. DCD was mixed with urea powder at 50 mg kg^–1 urea and enrichment of soil with nitrifiers was accomplished by preincubating Conestogo silt loam with 50 mg NH ₄ ⁺ -N kg^–1 soil. Brookston clay (pH 5.7) was limited with CaCO₃ to increase the pH to 7.3. The cores were incubated at 15°C and, after periods of 10, 20, 35 and 45 days, were separated into 1-cm sections. The distribution of N species was similar on each side of the urea layer at each sampling. The pH and NH ₄ ⁺ (NH₃) concentration were very high near the urea layer but decreased sharply with distance from it. DCD did not influence urea hydrolysis significantly. Liming of Brookston clay increased urea hydrolysis. The rate of urea hydrolysis was greater in Conestogo silt loam than limed Brookston clay. Nitrite accumulate was relatively small with all the treatments and occurred near the urea layer (0–4 cm) where pH and NH ₄ ⁺ (NH₃) concentration were high. The nitrification occurred in the zone where NH ₄ ⁺ (NH₃) concentration was below 1000µgN g^–1 and soil pH was below 8.0 and 8.7 in Brookston and Conestogo soils, respectively. DCD reduced the nitrifier activity (NA) in soil thereby markedly inhibiting nitrification of NH ₄ ⁺ . Nitrification was increased significantly with liming of the Brookston soil or nitrifier enrichment of the Conestogo soil. There was a significant increase in NA during the nitrification of urea-N. The (NO ₂ ^– + NO ₃ ^– )-N concentration peaks coincided with the NA peaks in the soil cores.A practical implication of this work is that large urea granules will not necessarily result in NO ₂ ^– phytotoxicity when applied near plants. A placement depth of about 5 cm below the soil surface may preclude NH₃ loss from large urea granules. DCD is a potential nitrification inhibitor for use with large urea granules or small urea granules placed in nests.     

Fate of nitrogen (15N) from oxamide and urea applied to turf grass: A lysimeter study

Slow release N fertilizers are receiving increasing attention for use on turf grass, but their fate in the plant-soil system is still poorly understood. We aimed to quantify the uptake and recovery of N by a mixture of grasses when applied as either urea or oxamide in different diameter granules using a tracer technique (¹⁵N). The effects of the N source on soil biomass, root density and amount of readily available organic C in soil were also evaluated.In a first experiment oxamide in 4–5 mm diameter granules was compared with urea. The initial N absorption, 40 days after fertilization (d.a.f.), was higher for urea (23.5%) than for oxamide (12.1%), but after 64 days absorption efficiencies were about the same (11%) for both fertilizers. Fertilizer-derived N lost by leaching was much greater from the urea-fertilized soil (1.57 g), compared with losses from oxamide-fertilized soil (0.05 g). The total residual fertilizer N remaining in the system at the end of the experiment was 26.7% of applied urea N and 39.6% of applied oxamide N. Cumulated absorption efficiencies, calculated after dismantling the lysimeters, were 43.1% for urea and 54.8% for oxamide (roots included). A priming effect caused by a larger uptake of soil N because of the better root development was found in the oxamide-treated lysimeter. Fertilization with oxamide also caused an increase in the amount of soil microbial biomass.In a second experiment, the efficiencies and fertilizer N uptake rates from oxamide applied at two different granule sizes (1–2 mm and 5–10 mm) were evaluated. The amount of soil N taken up by the grass was linearly related to root density (r = 0.92).     

Fate of nitrogen applied in different fertilizers to the surface of a calcareous soil in Syria

¹⁵N-labelled ammonium sulphate or¹⁵N-labelled urea were each applied in solutionat a rate of 30 kg N ha^-1 to the surface of 20soil cores (52 mm internal diameter × 100 mm deep)located on a field experiment at the ICARDA station,Tel Hadya, Syria. Recovery of ¹⁵N-label in theammonium, nitrate, organic and/or urea-N pools in thesoil was measured on days 0, 1, 2, 5 and 13 afterapplication. Total recovery of ¹⁵N was initially100%, but by day 13 after application it had declinedto 51% with urea and 73% with ammonium sulphate.Ammonium nitrate labelled either as ammonium or asnitrate was also applied to the soil surface of 8other cores at the same time. ¹⁵N recovery in thefour soil N pools was measured only on day 12 afterapplication. Total recovery of ¹⁵N-label was 75%with labelled ammonium and 57% with labelled nitrate.Volatilization of ammonia from this calcareous soil(pH 8.1) is one probable mechanism of N loss fromammonium and urea fertilizers: with nitrate bothleaching beyond the base of the core (i.e. 100 mm) and denitrification were responsible for Nlosses. These large losses of N immediately afterapplication have implications for fertilizermanagement practices.     

Large granules,nests or bands: Methods of increasing efficiency of fall-applied urea for small cereal grains in North America

In North America where the climate is cool enough only one crop is grown yearly, N fertilizers are sometimes applied in the previous fall rather than in the spring for fall- or spring-sown cereal grains. However, in areas where snow accumulates in winter, fall application of N fertilizers is generally inferior to spring application. Substantial nitrification takes place in winter and subsequent N loss occurs primarily in early spring by denitrification after the snow melt. Immobilization of N is also greater with fall- than spring-applied N fertilizers. Nitrogen is more efficiently retained in the soil as NH₄ and thus more effectively used by plants if formation of nitrite (NO₂) and NO₃ is reduced or prevented by inhibiting nitrification. The nitrification is reduced when urea is placed in bands, because of high pH, ammonia concentration and osmotic pressure in the soil. The rate of nitrification is further reduced when urea is placed in widely-spaced nests (a number of urea prills placed together at a point below the soil surface) or as large urea granules (LUG) by reducing contact between the nitrifying bacteria and the NH₄ released upon urea hydrolysis. A further reduction in nitrification from LUG can be obtained by addition of chemical nitrification inhibitors (such as dicyandiamide (DCD)) to LUG. The concentration of a chemical inhibitor required to suppress nitrification decreases with increasing granule size. The small soil-fertilizer interaction zone with placement of urea in nests or as LUG also reduces immobilization of fertilizer N, especially in soils amended with crop residues. The efficiency of fall-applied N is improved greatly by placing urea in nests or as LUG for small cereal grains. Yields of spring-sown barley from nests of urea or LUG applied in the fall are close to those obtained with spring-applied urea prills incorporated into the soil. Delaying urea application until close to freeze-up is also improved the efficiency of fall-applied N. This increased effectiveness of urea nests or LUG is due to slower nitrification, lower N loss over the winter by denitrification, and reduced immobilization of applied N. Fall application of LUG containing low rates of DCD slows nitrification, reduces over-winter N loss, and causes further improvement in yield and N uptake of winter wheat compared to urea as LUG alone in experiments in Ontario; in other experiments in Alberta there is no yield advantage from using a nitrification inhibitor with LUG for barley. Placement of LUG or nests of urea in soil is an agronomically sound practice for reducing N losses. This practice can eliminate or reduce the amount of nitrification inhibitor necessary to improve the efficiency of fall-applied urea where losses of mineral N are a problem. The optimum size of urea nest or LUG, and optimum combination of LUG and an efficient nitrification inhibitor need to be determined for different crops under different agroclimatic conditions. The soil (texture, CEC, N status), plant (winter or spring crop, crop geometry, crop growth duration and cultivar) and climatic (temperature, amount and distribution of precipitation) factors should be taken into account during field evaluation of LUG. There is a need to conduct region-specific basic research to understand mechanisms and magnitudes of N transformations and N losses in a given ecosystem. Prediction of nitrification from LUG or urea nests in various environments is needed. In nitrification inhibition studies with LUG and chemical nitrification inhibitors, measurements of nitrifier activity will be useful. Finally, there is need for development of applicators for mechanical placement of LUG or urea prills in widely-spaced nests in soil.     

Nitrogen losses from labelled ammonium sulphate and Urea applied to a flooded rice soil

A field study was carried out to estimate volatilization and denitrification losses of¹⁵N applied as urea of ammonium sulphate to a wet land rice soil. Nitrapyrin (a nitrification inhibitor) was also applied to some treatments along with the two N sources.The N level in floodwater increased rapidly, soon after applying fertilizer N, but decreased to lower values within a few days. At 1 week after applying urea and ammonium sulphate, N losses were 37.6% and 60.6% respectively. The corresponding figures after 4 weeks were 55.7% and 61.9% while with nitrapyrin added the corresponding values were 37.2% and 57.2% after 1 week and 52.7 and 65.0% after 4 weeks respectively indicating that losses due to dentrification are negligible.     

Efficiency of fall-applied urea for barley: Influence of date of application

Fall application of N fertilizers is often inferior to spring application for increasing yields of spring-sown cereal grains. The objective of this study was to determine the influence of date of application on efficiency of fall-applied N. Fall application dates were related to recovery of fall-applied N as mineral N in soil in spring, and related to yield and N uptake for spring-sown barley. Urea at a rate of 50 or 56 kg N ha^–1 was incorporated into the soil to a depth of 10 cm. There were 2 or 3 application dates in the fall and one in the spring at sowing. Linear regression indicated recovery of fall-applied N as soil mineral N in spring increased from 30% with urea added on 19 September to 79% with addition on 6 November, but the predictability was low (r = 0.54^**). Increase in grain yield, expressed as relative efficiency of fall- versus spring-applied N, was only 23% on 19 September but rose to 76% by 6 November (r = 0.68^**). Results were similar for N uptake in grain. Other approaches to predicting the relative efficiency of fall- versus spring-applied N for yield increase were based on fall soil temperature at 5 cm depth, instead of fall calendar date. Soil temperature on the day of N application gave inferior correlation (r = –0.55^**), but the use of number of days from application to first day of 0°C soil temperature gave a fairly close correlation (r = –0.77^**). Soil degree-days accumulated from application to first day of 0°C soil temperature gave a similarly close correlation (r = –0.78^**). In all, the efficiency of fall-applied urea was markedly increased by delaying the application into the late fall; and calendar date, number of days or soil degree-days from application to soil freezing all predicted the efficiency fairly well.(Contribution No. 599)     

Initial and residual effects of nitrogen fertilizers on grain yield of a maize/bean intercrop grown on a Humic Nitosol and the fate and efficiency of the applied nitrogen

Initial and residual effects of nitrogen (N) fertilizers on grain yield of a maize/bean intercrop grown on a deep, well-drained Humic Nitosol (66% clay, 3% organic carbon) were evaluated. Enriched (¹⁵N) N fertilizer was used to study the fate of applied N in two seasons: using urea (banded) at 50 kg N ha^–1 in one season, and¹⁵N-enriched urea (banded), calcium ammonium nitrate (CAN, banded), and urea supergranules (USG, point placement) were applied in the other season (different field) at 100 kg N ha^–1. Nitrogen fertilizer significantly (P = 0.05) increased equivalent maize grain yield in each season of application with no significant differences between N sources, i.e., urea, CAN, and USG. Profitmaximizing rates ranged from 75 to 97 kg N ha^–1 and value: cost ratios ranged from 3.0 to 4.8. Urea gave the highest value: cost ratio in each season. Most (lowest measurement 81%) of the applied N was accounted for by analyzing the soil (to 150 cm depth) and plant material. Measurements for urea, CAN, and USG were not significantly different. The high N measurements suggest low losses of applied N fertilizer under the conditions of the study. Maize plant recovery ranged from 35 to 55%; most of this N (51–65%) was in the grain. Bean plant recovery ranged from 8 to 20%. About 34–43% of the applied N fertilizer remained in the soil, and most of it (about 70%) was within the top soil layer (0–30 cm). However, there were no significant equivalent maize grain increases in seasons following N application indicating no beneficial residual effect of the applied fertilizers.     

Improvement of the N fertilizer efficiency with dicyandiamide (dcd) in citrus trees

Nitrification inhibitors such a dicyandiamide (DCD) help to reduce leaching losses by retaining applied N in the ammoniacal form. Research objectives were to evaluate dicyandiamide added to ammonium sulphate-nitrate (ASN) as a nitrification inhibitor in cultivated soils (Xeropsamments) and its effect on N uptake by citrus (Citrus sinensis (L.) Osbeck). Under field conditions, fertilization of adult trees with ASN (600 g N tree^–1) either with or without DCD (2% DCD-N) was compared (ASN+DCD and ASN, respectively). The NH ₄ ⁺ -N concentrations in plots fertilized with ASN+DCD were significantly higher than ASN plot in the 0-15 cm layer during 5–105 day period. Nitrification started immediately after N application in both treatments (ASN and ASN+DCD). In all three soil layers analyzed, NO^– ₃-N concentrations were higher in the ASN plots than in the ASN+DCD during the first 20 days. This indicates that nitrification of NH⁺ ₄ from ASN was more rapid in the absence of DCD. On the other hand, fertilization with ASN+DCD kept higher levels of NO^– ₃-N in soils than ASN during the rest of experience period (40–160 days). Addition of DCD to ASN showed a higher N concentration in the spring-flush leaves with respect to the trees fertilized with ASN, during the growth cycle. These results suggest that the use of a nitrification inhibitor permitted a more efficient utilization of fertilizer N by citrus trees. The plants treated with DCD added to ASN showed a higher yield in number of units per tree and a better fruit colour index than those treated with ASN alone.     

Effects of type and amount of applied nitrogen fertilizer on nitrous oxide fluxes from intensively managed grassland

Five field experiments and one greenhouse experiment were carried out to assess the effects of nitrogen (N) fertilizer type and the amount of applied N fertilizer on nitrous oxide (N₂O) emission from grassland. During cold and dry conditions in early spring, emission of N₂O from both ammonium (NH ₄ ⁺ ) and nitrate (NO ₃ ^– ) containing fertilizers applied to a clay soil were relatively small, i.e. less than 0.1% of the N applied. Emission of N₂O and total denitrification losses from NO ₃ ^– containing fertilizers were large after application to a poorly drained sand soil during a wet spring. A total of 5–12% and 8–14% of the applied N was lost as N₂O and via denitrification, respectively. Emissions of N₂O and total denitrification losses from NH ₄ ⁺ fertilizers and cattle slurry were less than 2% of the N applied. Addition of the nitrification inhibitor dicyandiamide (DCD) reduced N₂O fluxes from ammonium sulphate (AS). However, the effect of DCD to reduce total N₂O emission from AS was much smaller than the effect of using NH ₄ ⁺ fertilizer instead of NO ₃ ^– fertilizer, during wet conditions. The greenhouse study showed that a high groundwater level favors production of N₂O from NO ₃ ^– fertilizers but not from NH ₄ ⁺ fertilizers. Inereasing calcium ammonium nitrate (CAN) application increased the emitted N₂O on grassland from 0.6% of the fertilizer application rate for a dressing of 50 kg N ha^–1 to 3.1% for a dressing of 300 kg N ha^–1. In another experiment, N₂O emission increased proportionally with increasing N rate. The results indicate that there is scope for reducing N₂O emission from grasslands by choosing the N fertilizer type depending on the soil moisture status. Avoiding excessive N application rates may also minimize N₂O emission from intensively managed grasslands.     

Ammonium transformation in paddy soils affected by the presence of nitrate

Coupled nitrification and denitrification is considered as one of the main pathways of nitrogen losses in paddy soils. The effect of NO₃ ^– on NH₄ ⁺ transformation was investigated by using the ¹⁵N technique. The paddy soils were collected from Wuxi (soil pH 5.84) and Yingtan (soil pH 5.02), China. The soils were added with either urea (18.57 mol urea-N enriched with 60 atom% ¹⁵N excess) plus 2.14 mol KNO₃-N (natural abundance) per gram soil (U+NO₃) or urea alone (U). The KNO₃ was added 6 days after urea addition. The incubation was carried out under flooded condition in either air or N₂ gas headspace at 25°C. The results showed that in air headspace, ¹⁵NH₄ ⁺ oxidization was so fast that about 10% and 8% of added ¹⁵N in the treatment U could be oxidized during the incubation period of 73 hours after KNO₃ addition in Wuxi and Yingtan soil, respectively. The addition of KNO₃ significantly inhibited ¹⁵NH₄ ⁺ oxidation (p<0.01) in air headspace, while it stimulated ¹⁵NH₄ ⁺ oxidation in N₂ gas headspace, although the oxidation was depressed by the N₂ gas headspace itself. Therefore, the accumulation of NO₃ ^– would inhibit further nitrification of NH₄ ⁺ at micro-aerobic sites in paddy soils, especially in paddy soils with a low denitrification rate. On the other hand, NO₃ ^– would lead to oxidation of NH₄ ⁺in anaerobic bulk soils.     

Fate of urea nitrogen applied to a banana crop in the wet tropics of Queensland

This paper reports a study in the wet tropics of Queensland on the fate of urea applied to a dry or wet soil surface under banana plants. The transformations of urea were followed in cylindrical microplots (10.3 cm diameter × 23 cm long), a nitrogen (N) balance was conducted in macroplots (3.85 m × 2.0 m) with ¹⁵N labelled urea, and ammonia volatilization was determined with a mass balance micrometeorological method. Most of the urea was hydrolysed within 4 days irrespective of whether the urea was applied onto dry or wet soil. The nitrification rate was slow at the beginning when the soil was dry, but increased greatly after small amounts of rain; in the 9 days after rain 20% of the N applied was converted to nitrate. In the 40 days between urea application and harvesting, the macroplots the banana plants absorbed only 15% of the applied N; at harvest the largest amounts were found in the leaves (3.4%), pseudostem (3.3%) and fruit (2.8%). Only 1% of the applied N was present in the roots. Sixty percent of the applied N was recovered in the soil and 25% was lost from the plant-soil system by either ammonia volatilization, leaching or denitrification. Direct measurements of ammonia volatilization showed that when urea was applied to dry soil, and only small amounts of rain were received, little ammonia was lost (3.2% of applied N). In contrast, when urea was applied onto wet soil, urea hydrolysis occurred immediately, ammonia was volatilized on day zero, and 17.2% of the applied N was lost by the ninth day after that application. In the latter study, although rain fell every day, the extensive canopy of banana plants reduced the rainfall reaching the fertilized area under the bananas to less than half. Thus even though 90 mm of rain fell during the volatilization study, the fertilized area did not receive sufficient water to wash the urea into the soil and prevent ammonia loss. Losses by leaching and denitrification combined amounted to 5% of the applied N.     

Studies on the effectiveness of various sulfur fertilizers under controlled conditions

Three factorial experiments with four replications were conducted in a greenhouse to examine the effectiveness of gypsum, elemental sulfur (ES powder) and three S containing N fertilizers, viz., ammonium sulfate (AS), urea + ES, and Ureas (20% AS and 80% urea). All experiments were conducted twice in different years.In the first experiment with uncropped soil, the effects of soil type, leaching rate (2.3 and 6.9 mm water per day) and urea addition on sulfate leaching losses were studied. Leaching losses decreased in the order Ureas > ammonium sulfate (AS) > gypsum urea + ES. Increasing the leaching rate greatly increased sulfate losses from both soils. Losses were greater in the sandy Typic Hapludoll than in the clayey Oxic Paleustalf. Sulfate adsorption was found to decrease strongly with rising the pH in both soils. Hydrolysis of urea temporarily raised the pH of the soil, thereby increasing the sulfate leaching losses.In the second experiment the effects of S rate (0–65 mg per kg soil), split application and leaching rate (0 and 2.3 mm per day) on sulfate leaching losses and apparent S recovery (ASR) by three successive cuts of ryegrass (Lolium perenne L.) were studied. Herbage yield more than doubled when S was applied. The effectiveness of the sulfate fertilizers was greater when S was split-applied than given all at once. With split applications the ASR decreased in the order: Ureas > AS > gypsum > urea + ES > ES powder. ES fertilizers were least effective, because the oxidation rate of ES to sulfate was clearly too slow.In the third experiment the effects of S rate (0–40 mg per kg soil) and split application on sulfate leaching losses and ASR in the grain of wheat (Triticum aestivum L.) were studied under leaching conditions (2.3 mm per day). Grain yield increased strongly due to S application. Split application greatly increased the effectiveness of the sulfate fertilizers and appeared to be an effective tool in satisfying the S need of the crop under leaching conditions. Again, ES fertilizers were least effective, because the oxidation rate of ES was too slow to meet the S demand of the crop.In all experiments leaching losses of sulfate from the ES fertilizers were smaller than from the sulfate fertilizers.     

Efficiency of urea and ammonium sulfate for wetland rice grown on an acid sulfate soil as affected by rate and time of application

A pot experiment was conducted in a greenhouse to assess the effect of rate and time of N application on yield and N uptake of wetland rice grown on a Rangsit acid sulfate soil (Sulfic Tropaquepts). Response of rice at N rates of 800, 1600 and 2400 mg N/pot (5 kg of soil) was compared between urea and ammonium sulfate when applied at two times: (i) full-rate basal at transplanting and (ii) one half at transplanting and one half at the PI stage. In addition, labelled¹⁵N sources were applied either at transplanting or at the PI stage to determine the nitrogen balance sheet in the soil/plant system.No significant difference in grain and straw yields between urea and ammonium sulfate at low rate was observed. At the higher N rates, urea produced higher yields than did ammonium sulfate regardless of timing. The highest yields were obtained when urea at the high N rate was applied either in a single dose or a split dose while lowest yields were observed particularly when ammonium sulfate at the same rate was applied. Split application of N fertilizer was shown to be no better than a single basal application. The occurrence of nutritional disorder, a symptom likely reflected by high concentration of Fe (II) in combination with soluble Al, was induced with high rate of ammonium sulfate.In terms of fertilizer N recovery by using¹⁵N-labelling, ammonium sulfate was more efficient than urea when both were applied at transplanting. In contrast, application at the PI stage resulted in higher utilization of urea than of ammonium sulfate. The recovery of labelled N in the soil was higher with urea than with ammonium sulfate when the two sources were applied at transplanting, while the opposite result was obtained when the same fertilizers were applied at the PI stage. The losses from urea and ammonium sulfate were not different when these fertilizers were applied at transplanting but loss from urea was higher than that from ammonium sulfate when both were applied at the PI stage.     

Plant response to the management of fluid and solid N fertilizers applied to furrow-irrigated corn

The use of fluid fertilizers has increased in recent years. Plant response to field management practices of fluid and solid N fertilizers in furrow-irrigated field studies has not been well-documented. This research studied the response of corn (Zea mays L.) to several field management practices of fluid and solid N fertilizers applied at several rates. Corn grown with sidedressed applications of the fluid fertilizers, urea ammonium nitrate (UAN) and 18-0-0+7Ca, generally had higher grain yields, higher yield efficiencies, higher ear populations, larger seed size, more kernels per ear, and a higher ear leaf N concentration than corn grown with preplant broadcast treatments of urea, ammonium nitrate (AN), and UAN. In 1988, corn grown with 280 kg N ha^–1 of AN applied preplant broadcast had a lower grain yield, yield efficiency, kernels per ear, and ear leaf N concentration, while ear population and kernel size were unchanged, in comparison to split applications of UAN at 224 kg N ha^–1. In 1989, corn grown with three split applications of UAN at 280 kg N ha^–1 had a higher grain yield and produced more kernels per ear without affecting yield efficiency, ear population, kernel size, or ear leaf N concentration compared with treatments at the 224 kg N ha^–1 rate. Use of split, side-dressed N management practices in furrow-irrigated corn should eliminate the need to use excessive N rates while maintaining grain yields and other plant responses, resulting in more efficient N use than traditionally achieved.     

Degradation of the nitrification inhibitor 1-amidino-2-thiourea in soils,and its action in Nitrosomonas pure culture and soil incubation experiments

The degradation of guanylthiourea (GTU) via 3,5-diamino-1,2,4-thiadiazole (TDZ) to dicyandiamide (DCD) was studied in selected soils. All three compounds could be determined by HPLC. GTU decomposed rapidly (within hours-days), the reaction from TDZ to DCD continued more slowly (within days-weeks). Soil type and temperature had an essential effect on the rate of degradation; conspicuous was a more rapid breakdown of GTU in presence of ammonium sulfate (AS) than in combination with urea.Each compound is a nitrification inhibitor; inNitrosomonas cell suspensions, 0.5 ppm GTU and 10 ppm TDZ achieved an effect comparable to 200 ppm DCD.The combination of these two effects—degradation in soil and inhibition of nitrification—were studied in soil incubation experiments. The three substances had inhibitory effects also in soil, however at significantly different application rates (20 ppm GTU or TDZ and 30 ppm DCD). Using these concentrations, AS/DCD and urea/GTU showed similar effects.Urea/GTU retarded nitrification by the factor 1.7 as compared to urea/DCD. AS/GTU had no advantage over AS/DCD which can be explained by the more rapid degradation of GTU in presence of AS.Urea/GTU apparently presents a promising possibility to utilize N-fertilizers more efficiently.