Agronomic performance of urea briquette applicator in transplanted rice

Field trials were conducted in the Philippines and India during 1989 and 1990 seasons to study comparative yield responses of transplanted rice (Oryza sativa L.) to pillow-shaped urea briquettes (UB) deep placed by an applicator (prototype developed by IFDC) and by hand immediately after transplanting. The applicator-placed UB consistently increased grain yields over the split-applied prilled urea, and the additional yields ranged from 0.23 to 1.48t ha^–1 (5 to 83%) for 25 to 63 kg N ha^–1. Agronomic responses of transplanted rice to the UB placed by the applicator and by hand were statistically equal. Modified rice hill spacing may be considered as a requirement for efficient use of the applicator. The results demonstrate that with the UB applicator it is possible to deep place UB mechanically and achieve the agronomic efficiency that is achieved by hand deep placement of the UB.     

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.     

Fate and efficiency of nitrogen fertilizers applied to wetland rice. I. The Philippines

Urea is the main form of fertilizer nitrogen applied to wetland rice. As part of an effort to evaluate the efficiency of nitrogen fertilizers, conventional urea and modified urea products such as sulfur-coated urea (SCU), urea supergranules (USG), and sulfur-coated urea supergranules (SCUSG) were compared with ammonium sulfate on an Aquic Tropudalf at the experimental farm of the International Rice Research Institute (IRRI) in the Philippines. The sulfur-coated materials were prepared in the laboratory and were not completely representative of commercial SCU. Two experiments were conducted in the wet season (1978, 1979) and one in the dry season (1979). All fertilizers were labeled with 5% or 10% excess¹⁵N so that the fertilizer-N balance at two or three sampling times during the growing season could be constructed and the magnitude of N loss assessed. The SCU, USG, and SCUSG were applied at transplanting, and the whole dose of nitrogen was¹⁵N-labeled. The urea and ammonium sulfate applications were split: two-thirds was broadcast and incorporated at transplanting, and one-third was broadcast at panicle initiation; only the initial dose was¹⁵N-labeled.Deep-point placement (10 cm) of urea supergranules (USG) between the rice hills consistently provided the highest plant recovery of¹⁵N in all experiments and at all harvest times; recoveries ranged from 48% to 75% with an average of approximately 58% at maturity. Among the fertilizers broadcast and incorporated before transplanting, average plant recoveries of¹⁵N were only approximately 34% and 26% from urea and ammonium sulfate, respectively. Plant recovery of¹⁵N from the broadcast and incorporated SCU (37%) was far inferior to that from USG. Sulfur coating of supergranules did not improve plant recovery over USG alone although sulfur coating delayed the plant uptake of¹⁵N from the USG.The¹⁵N not accounted for in the plant and soil was presumed lost. Loss of N from urea and ammonium sulfate was high (63%) in the dry season. Coating with sulfur gave a slight improvement, and deep placement of USG and SCUSG greatly reduced the losses. Losses of N were substantially lower in the wet season than in the dry season for broadcast and incorporated urea, SCU, and ammonium sulfate (9%–30%), whereas losses from deep-placed urea remained more or less the same as in the dry season. Net immobilization of¹⁵N from the broadcast fertilizers in the wet season ranged from 49% to 53% in the first experiment and from 16% to 32% in the second experiment, presumably because of aquatic weeds and green algae; immobilization was proportionally less at higher rates of fertilizer application. Deep placement reduced the extent of¹⁵N immobilization in the soil plus roots to less than 21% in all experiments.     

Nitrogen use efficiency,floodwater properties,and nitrogen-15 balance in transplanted lowland rice as affected by liquid urea band placement

Alternative N fertilizer management practices are needed to increase productivity and N use efficiency in lowland rice (Oryza sativa L.). In 1986 dry season, a field study using¹⁵N-labeled urea evaluated the effect of time and method of fertilizer N application on grain yield and N use efficiency. Conventional fertilizer application was compared with band placement of liquid urea and point placement of urea supergranules (USG). Grain yields were significantly higher with either band or point placement than with broadcast and incorporation or surface application. Partial pressure of NH₃ (NH₃) was significantly reduced when N was deep-placed.¹⁵N balance data show that fertilizer N applied basally and incorporated gave a total¹⁵N recovery of 52% and crop (grain + straw) recovery of 30%. Band placement of liquid urea N resulted in 82–90% total and 57–65% crop¹⁵N recovery. USG point placement gave 94% total and 70% crop¹⁵N recovery. Deep placement of second N application gave only slightly higher (98%)¹⁵N recovery compared with broadcast application (89%).     

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.     

Movement and distribution of fertilizer nitrogen as affected by depth of placement in wetland rice

Experiments were conducted to monitor the movement and distribution of ammonium-N after placement of urea and ammonium sulfate supergranules at 5, 7.5, 10, and 15 cm. By varying depths of fertilizer placement, it is possible to determine the appropriate depth for placement machines. There were no significant differences in grain yields with nitrogen placed 5 and 15 cm deep. However, grain yields were significantly higher with deep placement of nitrogen than with split application of the fertilizer. The lower yields with split-applied nitrogen were due to higher nitrogen losses from the floodwater. The floodwater with split application had 78–98µg N ml^–1 and that with deep-placed nitrogen had a negligible nitrogen concentration.Movement of NH ₄ ⁺ -N in the soil was traced for various depths after fertilizer nitrogen application. The general movement after deep-placement of the ammonium sulfate supergranules was downward > lateral > upward from the placement site. Downward movement was prevalent in the dry season: fertilizer placed at 5–7.5 cm produced a peak of NH ₄ ⁺ -N concentration at 8–12 cm soil depth; with placement at 15 cm, the fertilizer moved to 12–20 cm soil depth. Fertilizer placed at 10 cm tended to be stable. In the wet season, deep-placed N fertilizer was fairly stable and downward movement was minimal.A substantially greater percentage of plant N was derived from¹⁵N-depleted fertilizer when deep-placed in the reduced soil layer than that applied in split doses. The percent N recovery with different placement depths, however, did not vary from each other. The results suggest that nitrogen placement at a 5-cm soil depth is adequate for high rice yields in a clayey soil with good water control. In farmers' fields where soil and water conditions are often less than ideal, however, it is desirable to place nitrogen fertilizer at greater depths and minimize NH ₄ ⁺ -N concentration in floodwater.     

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.     

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.     

Fate and efficiency of nitrogen fertilizers applied to wetland rice. II. Punjab,India

Two modified urea products (urea supergranules [USG] and sulfur-coated urea [SCU]) were compared with conventional urea and ammonium sulfate as sources of nitrogen (N), applied at 58 kg N ha^–1 and 116 kg N ha^–1, for lowland rice grown in an alkaline soil of low organic matter and light texture (Typic Ustipsamment) having a water percolation rate of 109 mm day^–1. The SCU and USG were applied at transplanting, and the whole dose of nitrogen was¹⁵N-labeled; the SCU was prepared in the laboratory and was not completely representative of commercial SCU. The SCU was broadcast and incorporated, whereas the USG was point-placed at a depth of 7–8 cm. The urea and ammonium sulfate applications were split: two-thirds was broadcast and incorporated at transplanting, and one-third was broadcast at panicle initiation. All fertilizers except the last one-third of the urea and ammonium sulfate were labeled with¹⁵N so that a fertilizer-N balance at flowering and maturity stages of the crop could be constructed and the magnitude of N loss assessed.At all harvests and N rates, rice recovered more¹⁵N from SCU than from the other sources. At maturity, the crop recovered 38 to 42% of the¹⁵N from SCU and only 23 to 31% of the¹⁵N from the conventional fertilizers, urea and ammonium sulfate, whose recovery rates were not significantly different. In contrast, less than 9% of the USG-N was utilized. Fertilizer nitrogen uptake was directly related to the yield response from the different sources. Most of the fertilizer N was taken up by the time the plants were flowering although recovery did increase up to maturity in some treatments.Analysis of the soil plus roots revealed that less than 1% of the added¹⁵N was in the mineral form. Between 20 and 30% of the¹⁵N applied as urea, SCU, and ammonium sulfate was recovered in the soil plus roots, mainly in the 0–15 cm soil layer. Only 16% of the¹⁵N applied as USG was recovered in the soil, and this¹⁵N was distributed throughout the soil profile to a depth of 70 cm, which was the lowest depth of sampling.Calculations of the¹⁵N balance showed that 46 to 50% of the urea and ammonium sulfate was unaccounted for and considered lost from the system. Only 27 to 38% of the¹⁵N applied as SCU was not recovered at maturity, but 78% of the USG application was unaccounted for. The extensive losses and poor plant recovery of USG at this site are discussed in relation to the high percolation rate, which is atypical of many ricegrowing areas.     

Significance of timing and method of N fertilizer application for the N-use efficiency in flooded tropical rice

N-use efficiency in flooded tropical rice is usually low. Fertilizer N losses result mainly from losses of volatile NH₃ after broadcast application of urea into floodwater between transplanting and early tillering which is a common practice of farmers. Losses appear predominantly during the first week after urea application. With broadcast and incorporation of N into soil at transplanting losses may be reduced but are still substantial. Deep placement of urea supergranules (USG) has not been adopted by farmers because it is very laborious. A new application technique, namely injection of dissolved urea into the upper soil layer, was developed by which fertilizer N losses were effectively minimized while at the same time allowing flexible timing of application independent of crop stage and water management. It provides N-use efficiency equal to that achieved by USG point placement but is less labor-intensive.     

Coated and modified urea materials for increasing nitrogen use efficiency of lowland rice in heavy clay soils

Conventional as well as modified nitrogen sources and application methods were evaluated under rainfed lowland conditions in heavy clay soils of Bihar, India for 4 years. Modified nitrogen sources, viz. sulfur-coated urea (SCU) and urea super-granules (USG) were tested against prilled urea (PU) under four levels of N (0, 29, 58 and 87 kg N/ha) in the wet season. A high yielding nonphotoperiod sensitive, long duration variety Pankaj was grown in all the four years.Point placement of USG and basal incorporation of SCU resulted in significantly higher panicle numbers per square meter, 100 grain weight and grain yield at all the levels of N tested. The unfilled grain percentage was lower in USG and SCU treatments.Regression analysis using a multifertilizer response model (MRM) showed that rice responded significantly to PU in three years out of four years, to SCU in four years and USG in three years.Economic analysis viz. input and output analysis based on the price of fertilizer (1 kg N as PU at $0.5; USG and SCU costing 10% more than PU), rough rice (ranging from 18.0 to 20.0 $ per ton) and labour wages at 1.0 $ per man day unit, also showed that USG and SCU are more input efficient than PU.     

Environmental and economic opportunities of applications of different types and application methods of chemical fertilizer in rice paddy

Mitigating greenhouse gas (GHG: Methane and nitrous oxide) emission from the rice cropland vis-à-vis increasing rice yield is one of the important challenges to the food security and climate change research. N-fertilizer input to the crop land is the key to rice productivity and GHG emission from soil. The sustainability of different types and application methods of N-fertilizers in rice cropland was studies based on the net annual C-equivalent GHG emission (CE) and total financial profit to the farmers’. The study was conducted in a low lying experimental rice field of eastern India during two consecutive years. The experiment was laid down with five replicates of the following treatments: (1) control (no N-fertilizer); (2) broadcasting ammonium sulphate (AS); (3) prilled urea (PU) and (4) deep placement of urea briquette (UB). Compared to other treatments, significantly higher GHG emission and grain yield (5–20% higher over other fertilizer applied plots) were recorded from the PU and UB applied plots respectively. Net CE was calculated using the GHG emission and secondary CE of different processes used in each treatment. The net CE followed the order: PU > UB > Control > AS. The ratio of total grain-C to net CE was significantly higher from the AS (15–51%) and UB (8–34%) plots compared to the PU applied plots. Net financial benefit ($ ha^?1) to the farmers’ followed the order: UB > AS > Control > PU. Study indicates that UB may be a climatically sustainable mitigation option in the tropical rice paddy.     

Urea briquettes containing diammonium phosphate: a potential new NP fertilizer for transplanted rice

The rapid rise in fertilizer prices over the past 2 years coupled with the notoriously low nutrient recovery of fertilizer by lowland rice as managed by farmers of most developing countries has prompted a re-examination of urea briquette agrotechnology that improves fertilizer use efficiency.Urea briquettes containing diammonium phosphate (UB-DAP) can be cost effectively produced using a portable fertilizer briquetter on a small scale (200 kg^-1 h^-1) at the village level and at a price affordable by small rice farmers. Their improved management consists of hand placement of properly sized (weight) UB-DAP (N:P = 4:1) per briquette for every four rice hills, and at 7–10 cm soil depth, on the day of or the day after transplanting using modified 20 × 20 cm spacing (25 hills m^-2). This management is simple to adopt, saves up to 50% of the labor normally required for its conventional hand placement, and helps to reduce the lag period of spatial nonavailability of DAP-P to the rice plants. Results of several farmer-managed field trials conducted during the 1990–95 wet seasons in India demonstrate that the UB-DAP management makes the fertilizer agronomically more efficient, economically more attractive with less risk, and reduced losses of nutrients as compared with conventional use of prilled urea and single superphosphate. The fertilizer use offers women farmers a unique opportunity to play an important role in increasing rice productivity. The management of UB-DAP can be integrated with plant nutrient recycling and limited Gliricidia green manuring (an agroforestry approach). This integrated use of UB-DAP has the potential to increase rice production of small resource-poor rice farmers with less fertilizer and in sustainable manner in rainfed as well as irrigated transplanted rice ecoregions of developing countries, while protecting the environment. Therefore, the UB-DAP fertilizer can be an important NP source for transplanted rice in the 21^st century.     

Relative ammonia loss from urea-based fertilizers applied to rice under different hydrological situations

Relative ammonia volatilization loss from prilled urea, urea supergranule (USG), neem cake-coated urea (NCU), rock phosphate-coated urea (RPCU), gypsum-coated urea (GCU), and prilled urea supplemented with dhaincha (Sesbania aculeata) green manure (Dh + PU) was measured in the fields under different hydrological situations of rice growing. Ammoniacal-N and pH of flood water were less with point placement of USG and Dh + PU treatments than with single basal broadcast applications of urea-based fertilizers. Ammonia collected with an acid trap in an enclosed chamber ranged from 1.47–3.07, 0.24–3.74, 0.80–3.50 and 0.50–1.20% of the applied N in upland, alternate wetting and drying, shallow submergence and intermediate deep water situations, respectively. The collected ammonia was less with point placement of USG at 5 cm depth in all situations and with Dh + PU treatment in shallow submergence than with other sources of N. Single basal broadcast applications of RPCU or NCU resulted in relatively higher loss. The loss from top-dressed urea was less than that from basally applied urea because of larger crop canopy at later stages of crop growth.     

Herbage biomass and uptake under low-input grazing as affected by cattle and sheep excrement patches

Urea deep placement (UDP) has demonstrated its benefits of saving N fertilizer and increasing nitrogen use efficiency (NUE) and grain yields. However, studies on its environmental impacts, particularly on nitrous oxide (N₂O) and nitric oxide (NO), are limited. We conducted multi-location field experiments in Bangladesh to determine the effects of UDP versus broadcast prilled urea (PU) on N₂O and NO emissions, NUE, and rice yields. N₂O and NO emissions were measured from three N fertilizer treatments—no N, UDP, and PU—using automated gas sampling and analysis systems continuously for two rice-growing seasons—Aus (May–August) and Aman (August–December). Fertilizer-induced peaks in N₂O emissions were observed after broadcast application of PU but were rarely observed after UDP. Total seasonal N₂O and NO emissions, yield-scaled emissions, and fertilizer-induced emissions were affected by fertilizer treatments and sites. Though nitrogen fertilizer increased emissions significantly over the control, emissions resulting from UDP and PU were similar. Effects of N placement on grain yields and NUE were site- and season-specific. Of the N placement methods, UDP increased grain yields by 13% (p < 0.05) during the Aman season and gave similar yields in spite of lower N application during the Aus season. UDP increased N recovery from 25 and 16% of broadcast PU to 61 and 73% during the Aus and the Aman seasons, respectively in one site, but was similar in another site. On the other hand, alternate wetting and drying irrigation reduced grain yield and N recovery at the BRRI site during the Aman season.     

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.     

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.     

Effect of granule size and the placement geometry on the efficiency of urea supergranules for wetland rice grown on a permeable soil

In a laboratory experiment 5 cm depth of water was allowed to percolate daily down through a 15 cm thick soil (Typic Ustipsamment) layer. It was observed that leaching losses of urea supergranules (USG)-N could be decreased by about 20% by the placement of four 0.25 g granules at four points instead of one 1 g granule at one point. In field microplots, the placement of approximately 30 granules of 0.30 g size instead of 9 granules of 1.00 g size resulted in reduced leaching of USG-N and, in turn, increased rice yield. In a follow-up field study, the advantage of more frequently placed USG was confirmed. As compared with 1 g USG placed in the usual manner in the center of four rice hills, increasing the density of placement in soil produced 15% more rice grain. Further increase in rice yield could be obtained by increasing the number of USG placed in the soil and decreasing the size of the granule from 1.00 g to 0.70 or 0.35 g. With USG of 0.35 and 0.70 g yields were equal or sometimes even slightly higher than with split application of prilled urea on a heavily percolating, low-CEC, light-textured soil.     

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.     

Nitrogen movement and uptake by rice fertilized with urea supergranules in two contrasting Mollisols

In a glasshouse experiment, the periodic movement, loss and uptake of N by lowland rice fertilized with point-placed urea supergranule (USG) was studied in two soils differing in texture. Movement of urea-N, NH ₄ ⁺ -N and NO ₃ ^- -N was significantly faster in Patharchatta sandy loam (Typic Hapludoll) than in Beni silty clay loam (Aquic Hapludoll) and was mostly downward with peak concentration near the placement site.Nitrogen in leachate was higher in Patharchatta sandy loam than in Beni silty clay loam. About 60–70% of leaching of urea-N took place within 2 days of USG placement. The leaching of NH ₄ ⁺ -N and NO ₃ ^- -N increased till 14 and 21 days of USG placement in Patharchatta sandy loam and Beni silty clay loam, respectively. Nitrogen leached through urea, NH ₄ ⁺ and NO ₃ ^- forms was, respectively, 64, 25 and 25% higher from sandy loam. During 49 days, 49 and 32% of the applied N was recovered by rice plants from silty clay loam and sandy loam, respectively.