Efficiency of some modified urea fertilisers for wetland rice grown on a permeable soil

Split broadcast applications of prilled urea, deep point-placed urea supergranules (USG), and broadcast sulfur-coated urea (SCU) were compared as nitrogen sources for wetland rice (Oryza sativa L.) in two field experiments on a sandy soil (Typic Ustipsamment) with a high percolation rate (approx. 110 mm/day) in the Punjab, India. The USG was consistently less effective than the split urea and averaged 1 ton ha^–1 less rice yield at the highest nitrogen rate (116 kg N ha^–1). SCU produced the highest grain yields in both experiments; it averaged 1.7 ton ha^–1 more than did the split urea at the highest N rate.The fertilisers were then compared in field microplots; percolation was permitted or prevented so that the cause of the poor performance of USG could be elucidated. USG gave higher grain yield and N uptake in microplots that were not leached than in those that were leached. In leached microplots, the grain yields were higher from prilled urea than from USG treatments provided the placement pattern of the USG matched that of the field plots. Yields were not higher from treatments in which the USG were more closely spaced. In microplots in which leaching was prevented, the broadcast prilled urea was less effective than the deep-placed USG, which gave yields approximately 60% greater than those from split urea and the same as those from SCU. Broadcast prilled urea in undrained microplots caused high levels of ammonium (40 ppm) to develop in the floodwater where high pH (8.9) and high alkalinity (4.9 meq l^–1) may have led to extensive ammonia volatilisation. The use of USG and SCU in undrained microplots reduced floodwater ammonium levels to less than 3 ppm.Urea and ammonium leaching losses measured in fallow soil columns in the laboratory were much greater from USG than from prilled urea. Leaching losses from SCU were negligible. The data suggest that SCU is the preferred N source for rice soils having a high percolation rate and that USG is a poor alternative to split applications of prilled urea.     

Effect of urea management on yield and N use efficiency of lowland rice on an acid sulfate soil

Field experiments were conducted during 1988–1989 at two adjacent sites on an acid sulfate soil (Sulfic Tropaquept) in Thailand to determine the influence of urea fertilization practices on lowland rice yield and N use efficiency. Almost all the unhydrolyzed urea completely disappeared from the floodwater within 8 to 10 d following urea application. A maximum partial pressure of ammonia (pNH₃) value of 0.14 Pa and an elevation in floodwater pH to about 7.5 following urea application suggest that appreciable loss of NH₃ could occur from this soil if wind speeds were favorable. Grain yields and N uptake were significantly increased with applied N over the control and affected by urea fertilization practices (4.7–5.7 Mg ha^–1 in dry season and 3.0–4.1 Mg ha^–1 in wet season). In terms of both grain yield and N uptake, incorporation treatments of urea as well as urea broadcasting onto drained soil followed by flooding 2 d later were more effective than the treatments in which the same fertilizer was broadcast directly into the floodwater either shortly or 10 d after transplanting (DT). The¹⁵N balance studies conducted in the wet season showed that N losses could be reduced to 31% of applied N by broadcasting of urea onto drained soil and flooding 2 d later compared with 52% loss by broadcasting of urea into floodwater at 10 DT. Gaseous N loss via NH₃ volatilization was probably responsible for the poor efficiency of broadcast urea in this study.     

Incorporation of urea for transplanted rice as affected by floodwater and growing season

Incorporation of urea into puddled rice soils is known to reduce ammoniacal-N buildup in floodwater and the subsequent loss of N as ammonia. Little is known, however, about seasonal and temperature effects on the effectiveness of basal urea incorporation in puddled soils. A field experiment was conducted in northern Vietnam on an Aquic Ustifluvent in the spring season (February to June) and summer season (July to November) to determine the effect of the presence of floodwater and method of fertilizer incorporation on floodwater ammoniacal-N, floodwater urea-N, andpNH₃ following urea application. During the 4 d following basal urea application, floodwater temperature at 1400 h was 7 to 15°C higher in summer (July) than that in spring (February), and floodwater pH at 1400 h was 0.5 to 1.0 higher in summer than that in spring. ThepNH₃ was much higher in summer than that in spring, suggesting a high potential for ammonia volatilization in summer. The movement of transplanters through the field did not reducepNH₃, irrespective of floodwater depth (0 or 5 cm) and season. Harrowing and subsequent transplanter movement partially reducedpNH₃ in the summer;pNH₃ reduction, however, was greater when floodwater depth was 0 rather than 5 cm during harrowing and transplanting. This partial reduction ofpNH₃ in summer did not result in a corresponding increase in rice yield, presumably because N losses were only slightly reduced and because yield was constrained by additional factors, such as the adverse climate. In spring, the removal of floodwater before urea application and incorporation increased grain yield by 0.2 Mg ha^–1, even thoughpNH₃ was consistently low and was not reduced by urea incorporation. This result suggests that water management and tillage during basal urea application may influence rice growth and yield in ways other than reduced N loss.     

Effect of green manure on the yield,N uptake and floodwater properties of a flooded rice,wheat rotation receiving15N urea on a highly permeable soil

Field studies were conducted for two years on a rapidly percolating loamy sand (Typic Ustochrept) to evaluate the effect of green manure (GM) on the yield,¹⁵N recovery from urea applied to flooded rice, the potential for ammonia loss and uptake of residual fertilizer N by succeeding crops. The GM crop ofSesbania aculeata was grownin situ and incorporated one day before transplanting rice. Urea was broadcast in 0.05 m deep floodwater, and incorporated with a harrow. Green manure significantly increased the yield and N uptake by rice and substituted for a minimum of 60 kg fertilizer N ha^–1. The recovery of fertilizer N as indicated by¹⁵N recovery was higher in the GM + urea treatments. The grain yield and N uptake by succeeding wheat in the rotation was slightly higher with GM. The recovery of residual fertilizer N as indicated by the¹⁵N recovery in the second, third and fourth crops of wheat, rice and wheat was only 3, 1 and 1 per cent of the urea fertilizer applied to the preceding rice crop. Floodwater chemistry parameters showed that the combined use of the GM and 40 kg N ha^–1 as urea applied at transplanting resulted in a comparatively higher potential for NH₃ loss immediately after fertilizer application. The actual ammonia loss as suggested by the¹⁵N recoveries in the rice crop, however, did not appear to be appreciably larger in the GM treatment. It appeared the ammonia loss was restricted by low ammoniacal-N concentration maintained in the floodwater after 2 to 3 days of fertilizer application.     

Nitrogen-15 and sulfur-35 balances for fertilizers applied to transplanted rainfed lowland rice

A field experiment was conducted on a poorly-drained Aeric Paleaquult in northeastern Thailand to determine the effect of N and S fertilizers on yield of rainfed lowland rice (Oryza sativa L.) and to determine the fate of applied¹⁵N- and³⁵S-labeled fertilizers. Rice yield and N uptake increased with applied N but not with applied S in either sulfate or elemental S (ES) form. Rice yield was statistically greater for deep placement of urea as urea supergranules (USG) than for all other N fertilizer treatments that included prilled urea (PU), urea amended with a urease inhibitor (phenyl phosphorodiamidate), and ammonium phosphate sulfate (16% N, 8.6% P).The applied¹⁵N-labeled urea (37 kg N ha^–1) not recovered in the soil/plant system at crop maturity was 85% for basal incorporation, 53% for broadcast at 12 days after transplanting (DT), 27% for broadcast at 5–7 days before panicle initiation (DBPI), and 49% for broadcast at panicle initiation (PI). The basal incorporated S (30 kg ha^–1) not recovered in the soil/plant system at crop maturity was 37% for sulfate applied as single superphosphate (SSP) and 34% for ES applied as granulated triple superphosphate fortified with S (S/GTSP). Some basal incorporated¹⁵N and³⁵S and some broadcast¹⁵N at PI was lost by runoff. Heavy rainfall at 3–4 days after basal N incorporation and at 1 day after PI resulted in water flow from rice fields at higher elevation and total inundation of the 0.15-m-high¹⁵N and³⁵S microplot borders. Unrecovered¹⁵N was only 14% for 75 kg urea-N ha^–1 deep placed as USG at transplanting. This low N loss from USG indicated that leaching was not a major N loss mechanism and that deep placement was relatively effective in preventing runoff loss.In order to assess the susceptibility of fertilizer-S to runoff loss, a subsequent field experiment was conducted to monitor³⁵S activity in floodwater for 42 days after basal incorporation of SSP and S/GTSP. Maximum³⁵S recoveries in the floodwater were 19% for SSP after 7 days and 7% for S/GTSP after 1 day. Recovery of³⁵S in floodwater after 14 days was 12% for SSP and 3% for S/GTSP.This research suggests that on poorly drained soils with a low sorption capacity, a sizeable fraction of the fertilizer S and N remains in the floodwater following application. Runoff could then be an important mechanism of nutrient loss in areas with high probability for inundation following intense rainfall.     

Use of large granular urea (LGU) to improve efficiency of broadcast urea in wetland rice cultivation

Laboratory and greenhouse experiments were conducted to determine whether the efficiency of broadcast urea in wetland rice cultivation can be improved by using large granules which penetrate the puddled soil. In laboratory experiments the penetration increased with increasing granule size. Penetration was improved by having only a waterfilm on the soil and by the granules entering the soil with speed.In pot experiments with rice, N concentrations in the floodwater were lower with large granular urea (LGU, 6 to 8 mm diameter) dropped from a height of 2 m or shot with force into the puddled soil than with either prilled urea (PU) or LGU placed on top of the soil (+0cm). N concentrations in the floodwater were reduced even further by placement of LGU at 1 and 4 cm depths (–1 and –4cm, respectively). At all rates of N, the N uptake by grain plus straw increased with decreasing N concentrations in the floodwater. The apparent recovery of N in grain plus straw increased in an experiment on sandy soil from 61 to 85% in the order PU +0cm, LGU +0cm, LGU dropped, LGU –1cm, LGU shot and LGU –4cm. In an experiment on clay soil apparent recovery increased from 47 to 90% in the order PU +0cm, LGU +0cm, LGU dropped, LGU –0cm, LGU shot, LGU –1cm and LGU –4cm. LGU placed at –1 and –4cm resulted in significantly greater N uptake by grain plus straw than the other treatments.The experiments showed that the efficiency of broadcast urea is improved by using large urea granules, at least when conditions are favourable for penetration into the puddled soil.     

Effect of crop residue management on nitrogen dynamics and balance in a lowland rice cropping system

Two field experiments were conducted in a rice–fallow–rice cropping sequence during consecutive dry and wet seasons of 1997 on a Fluvic Tropaquept to determine the fate and efficiency of broadcast urea in combination with three residue management practices (no residue, burned residue and untreated rice crop residue). Ammonia volatilization losses from urea (70 kg N ha^–1) broadcast into floodwater shortly after transplanting for 11 d were 7, 12 and 8% of the applied N from no residue, burned residue and residue treated plots, respectively. During that time, the cumulative percent of N₂ + N₂O emission due to urea addition corresponded to 10, 4.3 and nil, respectively. The ¹⁵N balance study showed that at maturity of the dry season crop, fertilizer N recovery by the grain was low, only 9 to 11% of the N applied. Fifty to 53% of the applied ¹⁵N remained in the soil after rice harvest, mainly in the upper 0–5 cm layer. The unaccounted for ¹⁵N ranged from 27 to 33% of the applied N and was unaffected by residue treatments. Only 4 to 5% of the initial ¹⁵N-labeled urea applied to the dry season rice crop was taken up by the succeeding rice crop, to which no additional N fertilizer was applied. Grain yield and N uptake were significantly increased (P=0.05) by N application in the dry season, but not significantly affected by residue treatments in either season.     

Incorporation of urea in puddled rice soils as affected by tillage implements

Thorough incorporation of urea into the soil is known to reduce the high N losses associated with the traditional practice of broadcasting urea onto puddled lowland rice fields. Few studies have, however, examined the effectiveness of farm-level implements for incorporating broadcast urea during final land preparation in small rice fields. A field experiment was conducted at two sites in the Philippines to compare the effectiveness of several commonly available and experimental tillage implements for basal incorporation of urea. The relative effectiveness of N incorporation and relative susceptibility of N to ammonia volatilization losses were assessed from floodwater (urea + ammoniacal)-N and partial pressure of ammonia (pNH₃) following urea application to puddled, unplanted soil.Conventional water buffalo- and single axle tractor-drawn comb harrows were equally, but only partially, effective in reducing floodwater (urea + ammoniacal)-N andpNH₃ by 42 to 56% of the values for broadcast prilled urea (PU) without incorporation. Removal of the comb harrow from the single axle tractor did not reduce the effectiveness of PU incorporation, indicating that the cagewheel rather than the comb harrow was largely responsible for fertilizer incorporation. An experimental conical puddler was slightly more effective than the conventional comb harrow. The movement of transplanters through the field did not effectively incorporate PU. A power weeder, frequently used by researchers to incorporate fertilizer in small experimental plots, was less effective than traditional comb harrows, reducing floodwater (urea + ammoniacal)-N by only 35%. No tillage implement for incorporating urea reducedpNH₃ as effectively as did the liquid urea band injector.Results suggest that sizeable losses of fertilizer N still occur following incorporation of PU with tillage implements commonly used by small rice farmers. Considerable scope remains for the improvement of fertilizer incorporation in puddled rice soils.     

Fate of nitrogen fertilizer applied to lowland rice on a Sulfic Tropaquept

A field experiment was conducted on an acid sulfate soil in Thailand to determine the effect of N fertilization practices on the fate of fertilizer-N and yield of lowland rice (Oryza sativa L.). A delayed broadcast application of ammonium phosphate sulfate (16-20-0) or urea was compared with basal incorporation of urea, deep placement of urea as urea supergranules (USG), and amendment of urea with a urease inhibitor. Deep placement of urea as USG significantly reduced floodwater urea- and ammoniacal-N concentrations following N application but did not reduce N loss, as determined from an¹⁵N balance, in this experiment where runoff loss was prevented. The urease inhibitor, phenyl phosphorodiamidate (PPD), had little effect on floodwater urea- and ammoniacal-N, and it did not reduce N loss. The floodwater pH never exceeded 4.5 in the 7 days following the first N applications, and application of 16-20-0 reduced floodwater pH by 0.1 to 0.3 units below the no-N control. The low floodwater pH indicated that ammonia volatilization was unimportant for all the N fertilization practices. Floodwater ammoniacal-N concentrations following application of urea or 16-20-0 were greater on this Sulfic Tropaquept than on an Andaqueptic Haplaquoll with near neutral pH and alkaline floodwater. The prolonged, high floodwater N concentrations on this Sulfic Tropaquept suggested that runoff loss of applied N might be a potentially serious problem when heavy rainfall or poor water control follow N fertilization. The unaccounted-for¹⁵N in the¹⁵N balances, which presumably represented gaseous N losses, ranged from 20 to 26% of the applied N and was unaffected by urea fertilization practice. Grain yield and N uptake were significantly increased with applied N, but grain yield was not significantly affected by urea fertilization practice. Yield was significantly lower (P = 0.05) for 16-20-0 than for urea; however, this difference in yield might be due to later application of P and hence delayed availability of P in the 16-20-0 treatment.     

Time and mode of nitrogen fertilizer application to tropical wetland rice

In experiments with transplanted rice (Oryza sativa L.) at the International Rice Research Institute, Philippines, two methods of split application of urea and ammonium sulfate were compared with deep, point placement (10 cm) of urea supergranules and broadcast application of a slow-release fertilizer sulfur-coated urea (SCU). Comparisons were made in the wet and dry seasons and were based on rice yield and N uptake. Urea- and ammonium-N concentrations and pH of the floodwater were measured to aid interpretation of the results.Split applications of urea were generally less efficient than ammonium sulfate. The split in which the initial fertilizer dose was broadcast and incorporated into the soil before transplanting was more effective than the split in which the fertilizer was broadcast directly into the floodwater 21 days after transplanting. Both split applications were inferior to the urea supergranules and SCU, in terms of both yield and N uptake efficiency; average apparent N recoveries ranged from 30% for the delayed split urea to 80% for the urea supergranule.Broadcast applications of urea and ammonium sulfate produced high floodwater concentrations of urea- and ammonium-N, which fell to zero within 4–5 days. Floodwater pH was as high as 9.3 and fluctuated diurnally due to heavy algal growth. Ammonia volatilization and algal immobilization of N in the floodwater were probably responsible for the poor efficiency of the split applications; the supergranules and SCU on the other hand produced low floodwater N concentrations and were efficiently used by the rice crop.     

Determination of dinitrogen emission and retention in floodwater and porewater of a lowland rice field fertilized with15N-urea

This paper reports a study on the distribution of dinitrogen between the atmosphere, floodwater and porewater of the soil in a flooded rice field after addition of¹⁵N-labelled urea into the floodwater.Microplots (0.086 m²) were established in a rice field near Griffith, N.S.W., and labelled urea (80 kg N ha^–1 containing 79.25 atoms %¹⁵N) was added to the floodwater when the rice was at the panicle initiation stage. Emission of nitrous oxide and dinitrogen was measured directly during the day and overnight, using a cover collection method and gas chromatographic and mass spectrometric analytical methods. Ammonia volatilization was calculated with a bulk aerodynamic method from measurements of wind speed and floodwater pH, temperature and ammoniacal nitrogen concentration. Seven days after urea application the¹⁵N₂ content of the floodwater and soil porewater was determined and total fertilizer nitrogen loss was calculated from an isotopic balance.Throughout the experimental period gas fluxes were low; nitrous oxide, ammonia and dinitrogen flux densities were less than 5, 170 and 720 g N ha^–1 d^–1, respectively. The greatest dinitrogen flux density was observed two days after urea addition and this declined to ~ 100 g ha^–1 d^–1 after seven days.The data indicate that, of the urea nitrogen added, 0.02% was lost to the atmosphere as nitrous oxide, 0.9% was lost by ammonia volatilization, and 3.6% was lost as dinitrogen gas during the 7 days of measurement. At the end of this period 0.028% and 0.002% of the added nitrogen was retained as dinitrogen gas in the floodwater and soil porewater respectively. Recovery of the¹⁵N applied as nitrogen gases, plant uptake, and soil and floodwater constituents totaled about 94% of the nitrogen added.     

Increasing productivity and protein content using early-maturing rices and efficient nitrogen management

Germplasm with shorter duration than that of the currently grown varieties is being generated to maximize productivity of irrigated rice. However, short-duration varieties often produce yields lower than the medium- and long-duration varieties. Experiments were conducted during the 1980–82 dry and wet seasons to increase productivity through the use of very early-maturing rices and the improved management of nitrogen (N) fertilizers.Results over three years showed that IR58 and IR9729-67-3 (growth duration 100 ± 5 days) yield as well as or higher than IR36 although earlier maturing. They generally had a higher productivity (kg ha^–1 day^–1) than IR36 (110 ± 5 days).Three years' data suggest that the improved timing of broadcast applications of urea in split doses increased grain yield comparable with the basal incorporation of slow-release sulfur-coated urea (SCU) or deep point-placement of urea supergranules (USG).Results on elite breeding lines showed that the early-maturing IR9729-67-3 produced higher protein yield ha^–1 than longer duration varieties such as IR8 and IR42 in the dry season. Furthermore, contrary to earlier results, single basal incorporation of slow-release SCU increased the protein yield of rice by 53 kg ha^–1 and deep point-placement of USG by 43 kg ha^–1 over split application of prilled urea.     

Urea briquette applicator for transplanted rice

A plunger-type, completely hand-operated applicator prototype, made of polyvinyl chloride (PVC), for deep placement of urea briquettes (UB), i.e., pillow-shaped urea supergranules with edges, in line transplanted rice has been developed for use by small-scale rice farmers. The field evaluation of the applicator was conducted in the Philippines during the 1989 dry season. The applicator consistently placed UB at proper depth (7 to 8 cm), which resulted in low concentrations of urea N (<7 ppm) in about 4 cm of floodwater 1 day after placement. These findings indicated that the prototype worked properly. Average work output of the applicator was 0.20 ha workday^–1 and may increase with practice. The yields of irrigated transplanted rice in the field trials show that agronomic efficiencies of hand-placed UB and applicator-placed UB were equal and were superior to those of split-applied prilled urea.     

Efficiency of modified urea fertilizers for tropical irrigated rice

An objective of the International Network on Soil Fertility and Fertilizer Evaluation for Rice (INSFFER) network is to field evaluate deep-point placement (urea supergranules) and slow-release (sulfur coated urea) N fertilizers in irrigated rice. These N sources were compared for performance with split application of prilled urea at 19 sites in Asia in wet season 1981.SCU or USG differed significantly in response curves from prilled urea at 12 of the 17 sites where N response was observed. Over these 17 sites, 22–25% less N as SCU or 29–31% less N as USG provided the same yield increment as the comparatively higher level of N as prilled urea.High profit N rates were derived for 5 sites. The optimal N levels for SCU or USG were less than for prilled urea. However, in one case for both test materials prilled urea was more profitable than SCU or USG. The marginal rates of return of using SCU or USG as opposed to OPU were calculated for the 11 sites where the response functions of the test materials differed significantly from prilled urea. In other than 2 sites for SCU the MRR exceeded 2.0 for 29 and 58 kg N ha^–1, indicating the general profitability of these materials when compared to prilled urea.     

Effect of rates and sources of nitrogen application on yield and nutrient uptake of Citronella Java (Cymbopogon winterianus Jowitt)

Two field experiments were conducted for two crop cycles each of two years (1985–87 and 1986–88) on an entisols to study the effect of rate and sources of N application on yield and nutrient uptake of Citronella Java (Cymbopogon winterianus Jowitt). Fresh herbage and essential oil yields were significantly influenced by application of N up to 200 kg ha^–1 yr^–1, while tissue N concentration and N uptake increased only to 150 kg N ha^–1. The oil yields with Neem cake coated urea (urea granules coated with Neem cake) and urea super granules were 22 and 9% higher over that with prilled urea and urea supergranules were significantly increased up to 200 kg N ha^–1 while with Neem cake coated urea, response was observed only to 150 kg N ha^–1! Estimated recovery of N during two years from Neem cake coated urea, urea supergranules and prilled urea were 38, 31 and 21%, respectively.     

Slow-release urea fertilizers — effect on floodwater chemistry,ammonia volatilization and rice growth in an alkali soil

In experiments with transplanted rice (Oryza sativa L.) at the Central Soil Salinity Research Institute, Karnal, India, two methods of application of granular urea, wholly as basal dose U(W) or in splits U(S) were compared with deep, point placement (8 cm) of urea supergranules and broadcast application of two slow-release sources, sulphur-coated urea (SCU) and lac-coated urea (LCU). Comparisons were made in wet season 1984 and 1985 on the basis of ammoniacal N concentration and pH of floodwater, ammonia volatilization, rice yield and N uptake.In 1984 the highest peak concentrations of ammoniacal N (AN) in the floodwater, > 12g m^–3, and ammonia volatilization losses 54% of applied N were produced in U(W). Application of N in splits U(S) reduced peak AN levels 5g m^–3 and losses to 45.1%. LCU was ineffective in reducing peak AN levels ( 7.5g m^–3) or losses (43.6%). However SCU and USG were effective in reducing peak AN levels to < 2g m^–3 and N losses to 16.9 and 3.4% respectively. Total ammonia volatilization losses as well as the initial rate of loss correlated very well with the peak levels (second day) of AN, NH₃ (aq.) as well as equilibrium vapour pressure of NH₃. Floodwater pH was between 9.5 and 10.0.Split application of granular urea was generally more efficient in terms of yield and N recovery (41.4%, average of two years) as compared to whole application (29.5%). LCU was ineffective in improving grain yields or N recovery (30.9%). SCU was ineffective in improving grain yields but improved N recovery to 57.9%., USG increased grain yields only in first year by 19% over U(S) and improved N uptake to 60.5%. A negative linear relationship was established between N uptake by rice at harvest and AN levels in floodwater two days after fertilization which can be used as an index to evaluate fertilizers.     

Relative susceptibility of conventional and experimental nitrogen sources to ammonia loss from flooded rice fields

Urea, the most common N source in Asia, is prone to high loss as ammonia when applied to tropical flooded rice (Oryza sativa L.). Chemical or physical modifications of urea could offer potential in reducing ammonia loss. Two field studies were conducted to identify conventional and experimental N-containing sources loss prone to ammonia less than prilled urea. Relative susceptibility to ammonia loss was inferred from equilibrium ammonia vapor pressure, pNH₃. For the sources studied, ammonia formation and presumably loss were least for guanylurea sulfate (GUS) and sulfur-coated urea (SCU). The slow mineralization and acidifying effect of GUS resulted in negligible ammonia concentration in the floodwater. Amendment of urea with either 5 or 10% paraformaldehyde (ureaform) reduced pNH₃, but never by more than 55%. Coating urea with phosphate rock tended to be less effective than amendment with paraformaldehyde in reducing pNH₃. There was no significant difference in the pNH₃, and presumably ammonia loss, for urea phosphate (17-44-0), urea-urea phosphate (34-17-0), and urea. About 3 days after fertilization, the floodwater pH tended to become higher with NP sources than with urea. This elevation in pH was apparently due to the stimulation of algal photosynthetic activity by added P, and it may explain the failure of a phosphoric acid amendment to urea (urea phosphate) in reducing pNH₃. Ammonia disappearance from broadcast diammonium phosphate (DAP) and ammonium phosphate sulfate (16-20-0) was complete within 3 days after N application, whereas ammonia remained in floodwater for up to 7 days following broadcast application of urea and ammonium sulfate.     

Increasing urea-N efficiency for transplanted lowland rice by pneumatic injection: Yield and economics at the farm level

To increase the fertilizer-N efficiency in lowland rice (Oryza sativa L.) cultivation, new management practices are needed. Main cause of the present low efficiency is the low N recovery by plants, as a considerable part of the N applied is lost; deep placement techniques improve the recovery. A pneumatic injector, with which urea prills can be point-placed at a depth of 5–10 cm in paddy soils, was tested in 38 on-farm trials in 1989/90, mostly during the wet season. The experiments, located in Africa and Asia, focussed on differences in grain yield between conventional methods of broadcasting urea and injection by the pneumatic injector, at recommended N-rates. The study shows that the pneumatic injector is effective as a tool to improve the N fertilizer efficiency. The average yield increases per region, resulting from the use of the injector, ranged from about 250 to 1300 kg grain ha^–1. The value of the yield increase would allow most farmers to recover the costs of the injector within one season, even if labour was hired to carry out the injections. The average labour requirement of the injector was 40 hours ha^–1. In Indonesia, injection of prilled urea gave yields similar to those obtained with urea briquettes.     

Effect of encapsulated calcium carbide and urea application methods on denitrification and N loss from flooded rice

Poor N fertilizer use efficiency by flooded rice is caused by gaseous losses of N. Improved fertilizer management and use of nitrification inhibitors may reduce N losses. A microplot study using¹⁵N-labelled urea was conducted to investigate the effects of fertilizer application method (urea broadcast, incorporated, deep-placed) and nitrification inhibitor [encapsulated calcium carbide (ECC)] treatments on emission of N₂+N₂0 and total loss of applied N on a grey clay near Griffith, NSW, Australia. Both incorporation and deep placement of urea decreased N₂+N₂O emission compared to urea broadcast into the floodwater. Addition of ECC significantly (P < 0.05) reduced emission of N₂+N₂0 from incorporated or deep-placed urea and resulted in increased exchangeable ammonium concentrations in the soil in both treatments. Fifty percent of the applied N was lost when urea was broadcast into the floodwater. Total N loss from the applied N was significantly (P < 0.05) reduced when urea was either incorporated or deep placed. In the presence of ECC the losses were reduced further and the lowest loss (34.2% of the applied N) was noted when urea was deep-placed with ECC.     

Influence of modified forms of urea and nitrogen levels on weed growth and grain yield of lowland rice

The growth of weeds and their subsequent reduction of rice yield as affected by N source neem cake coated urea (NCU), dicyandiamide coated urea (DCU), rock phosphate coated urea (RPCU), urea supergranules (USG) and prilled urea (PU) was studied on a clay loam soil at Coimbatore, India. Experiments were conducted in northeast monsoon (NEM) 1981, summer 1982, and southwest monsoon (SWM) 1982 seasons.The crop was associated with eleven weed species, and the dominant weeds wereEchinochloa crus-galli, Cyperus difformis andMarsilea quadrifolia. The weed flora varied between seasons. Deep placement of USG reduced the dry weight of weeds in NEM and summer seasons at 60, 90 and 120 Kg N ha^–1 whereas it increased the dry weight at 60 and 90 but not 120 Kg N ha^–1 in SWM season. The dry weight of weeds decreased with increased N rates for all N sources during NEM and summer seasons. In SWM season, dry weight of weeds increased with increased N rates for all N sources except USG. The grain yield of rice was drastically reduced with the deep placement of USG at 60 but not 120 Kg N ha^–1 in SWM season. The differential effect of the N sources between seasons was due to the change of the weed flora. Dominance ofE. crus-galli during SWM season had greater influence on weed dry weight and grain yield of rice.Nitrogen uptake by weeds was frequently greater in unfertilized plots, particularly in NEM and summer seasons. In SWM season, the apparent fertilizer N recovery by weeds was high for USG. It decreased from 53% for 60 Kg USG-N ha^–1 to 4% for 120 Kg USG-N ha^–1.Contribution from the part of Ph.D. work of the first author at Department of Agronomy, Tamil Nadu Agricultural University, Coimbatore-641 003, Tamil Nadu, India.