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.     

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.     

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.     

Nitrogen deep-placement technologies for productivity, profitability, and environmental quality of rainfed lowland rice systems

The recovery of applied nitrogen by rice is low due to several loss processes operating in the ricefields. Split application of fertilizer suggested for increasing nitrogen-use efficiency is often not practical in rainfed lowland rice due to adverse soil–water situations. Hence, the entire required amount of N has to be applied in one single application when the water regime is favorable. A single broadcast application, however, increases N loss. Deep placement of urea supergranules (USG) has been proven to improve N fertilizer efficiency. The placement technology is best suited to conditions where the predominant N loss mechanism is ammonia volatilization rather than leaching or denitrification. Deep placement of USG thus has greater benefit over surface split application on soils with moderate to heavy texture, low permeability and percolation rate, and high cation exchange capacity and pH. Environments and management factors conducive to high ammonia volatilization potential would benefit most from deep-placement technology. Improved N recovery and efficiency of USG has been well-documented for lowland rice, but its market availability and methods to achieve placement pose problems. The technology has very limited adoption because USG is not commercially available or manufactured in most countries, and labor requirement is high with hand placement. Manual application creates more difficulties in handling the granules, besides taking 36–42 more hours per hectare, than 2 split broadcast applications of prilled urea. Applicators developed so far have not worked satisfactorily under standing water conditions and in direct-seeded rice conditions due to hardness of the soil. Hence, it is necessary to develop a suitable applicator to overcome these difficulties. Alternatively, for direct-seeded rice, N-fertilizers can be subsoil-banded near seedrows. The placement technology, if adopted by the farmers of the potential lowland areas in eastern India, is expected to give an additional production of 5.6 million tons of rice.     

Use of nitrification inhibitors (neem and DCD) to increase N efficiency in maize-wheat cropping system

A 2-year field experiment was conducted to study the effects of the nitrification inhibitors dicyandiamide (DCD) and neem cake on the efficiency of applied prilled urea nitrogen in a maize-wheat cropping system. Prilled urea (PU), neem cake coated urea (NCU) and DCD blended urea (DCDU) were applied to maize at two levels (60 and 120 Kg N ha^–1) and two methods (all preplant and split) of N application along with a no-nitrogen control and their relative residual effect was studied on succeeding wheat grown with three levels of N as PU.In 1990 maize responded well to N up to 60 kg N ha^–1; at this level PU increased maize yield by 1.03 t ha^–1, whereas NCU and DCDU increased maize yield by 1.55 and 1.18 t ha^–1 over the control, which was equivalent to an application of 127 and 94 kg N ha^–1 as PU, respectively. Furthermore, when the results were averaged over two years of study, residual N from the application of NCU and DCDU at 60 kg N ha^–1 left after maize cropping increased the grain yield of the succeeding wheat crop grown with 60 kg N ha^–1 as PU by 1.97 and 1.68 t ha^–1, respectively, over a no nitrogen control or 60 kg N ha^–1 as PU applied to the maize. This was equal to an application of 96 and 82 kg N ha^–1 as PU to wheat.Thus, neem cake increased the efficiency of urea N applied to maize and benefits were also seen in the succeeding wheat yield in the maize-wheat cropping system.     

Ammonia volatilization from point-placed urea in upland,sandy soils

The upland fertilization practice in Africa of placing N fertilizer below the soil surface near the plant might be facilitated through use of urea supergranules (USG). Since little is known about N losses from point-placed urea on light-textured African soils, laboratory studies were conducted in a forced-draft system to determine (a) the influence of soil properties on ammonia loss from USG and (b) to compare N loss from USG with that from broadcast N sources. Ammonia loss from 1.1 g USG placed at a 4-cm soil depth ranged from 2.9 to 62% of the added N on six light-textured soils. Ammonia loss was correlated with soil clay content (r = –0.93^**) but not with pH. A more detailed study on a soil from Niger revealed significantly less ammonia loss from either surfaced applied urea (18%) or surface-applied calcium ammonium nitrate (7%) than from USG placed at a 4-cm depth (67%). Amendment of surface-applied urea with 1.7% phenyl phosphorodiamidate (PPD), a urease inhibitor, essentially eliminated ammonia loss (1.9%). An¹⁵N balance confirmed that ammonia volatilization was the major loss mechanism for all N sources. The results suggest that point-placed urea may be prone to ammonia volatilization loss on light-textured African soils moistened by frequent light rainfall. In such cases, broadcast application of urea, CAN, or urea amended with PPD may be less prone to N loss.     

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.     

A comparative study of urea hydrolysis rate and ammonia volatilization from urea and urea calcium nitrate

A comparing of urea hydrolysis and NH₃ volatilization from urea supergranules and urea calcium nitrate (UCN, a new fertilizer produced by Norsk Hydro A/S, Norway) was made on two different flooded soil types, a high-CEC clay loam (Ås) and an intermediate-CEC clay loam (Kinn).Nitrogen loss by ammonia volatilization was reduced from 17% by surface application of urea supergranules (USG) on flooded Ås soil to 3% and 6% by UCN briquettes at either the same urea or nitrogen concentration as USG. A significant reduction was even found with the surface application of prilled UCN, 12% and 18% N-loss for prilled UCN and urea, respectively. The floodwater pH and NH ₄ ⁺ content was lower with UCN than urea, which reduced the potential for ammonia volatilization.NH₃-loss (5%) was significantly less when USG was surface applied on Kinn soil, while NH₃-loss from UCN briquettes was independent of soil type. The reduction in NH₃-loss from USG on Kinn soil was due to a decrease in the pH and NH ₄ ⁺ content of the floodwater caused by a reduced rate of urea hydrolysis.The rate of urea hydrolysis was lower with UCN than USG in both soils, but the difference between UCN and USG was greater in the Ås soil than in the Kinn soil. Three days after deep placement (10 cm), 18% of UCN urea and 52% of USG urea were hydrolyzed in Ås soil, while only 12% UCN and 17% USG were hydrolyzed in the Kinn soil.The surface application of USG on flooded soil reduced the rate of urea hydrolysis as compared to deep placement. 30% and 17% of USG urea was hydrolyzed after four days on Ås and Kinn soil, respectively. During the first few days the rate of hydrolysis of UCN was more affected by the soil type than the application method. Four days after surface application 32% and 13% UCN urea was hydrolyzed on Ås and Kinn soil, respectively. The rate of urea hydrolysis exhibited a zero-order reaction when USG and UCN-briquettes were point placed in flooded soils.     

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.     

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.     

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.     

Ammonia volatilization from a flooded rice field fertilized with amended urea materials

The extent of ammonia volatilization from prilled urea, urea supergranule and urea amended with neem seed cake, shell-lac and dicyandiamide was studied in a field experiment on flooded rice. The ammonia loss was measured by the closed acid trap method. The collected ammonia was highest from unamended prilled urea, accounting for 19 to 20 per cent of the applied N in 1983 and 20 to 24 per cent of the applied N in 1984. Coating of urea prills was either coaltar soaked neem seed cake or shell-lac was more effective than coating with dicyandiamide in reducing ammonia loss. Deep placement of urea as a supergranule was the most effective method of reducing ammonia volatilization. A diurnal variation in the pH and temperature of floodwater was observed. The quantity of ammonia collected in the acid trap was closely related to ammoniacal-N concentration and pH of the floodwater.     

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.     

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.     

Effect of nitrification inhibitors on herb and essential oil yield of Japanese mint on sandy soil

Pyrethrum (Chrysanthemum cinerariefolium) flowers have been observed to have insecticidal properties and could be used as an indigenous nitrification inhibitor for increasing N-use efficiency. A field experiment was conducted at the Central Institute of Medicinal and Aromatic Plants, Lucknow, India during 1988 and 1989 to evaluate the relative performance of pyrethrum flower waste and Dicyandiamide (DCD) as nitrification inhibitors applied with prilled urea (PU) to Japanese mint (Mentha arvensis L.). The results revealed that application of the nitrification inhibitors with prilled urea significantly increased the herb and essential oil yield of the crop compared to that of prilled urea alone. Addition of Dicyandiamide and pyrethrum flower waste gave 30 and 23% more herb yield than prilled urea alone, the corresponding increase in oil yield being 27 and 22%, respectively. Application of nitrogen at 200 kg ha^–1 in dicayndiamide or pyrethrum flower waste treated soil significantly enhanced the herb and essential oil yields and N-uptake by the crop to more than that for 300 kg N ha^–1 with prilled urea. Both the materials improved the N use efficiency by one and half time as compared to that with PU at 100 kg N ha^–1. The results indicate pyrethrum flower dust can be effectively used as a potential nitrification inhibitor.     

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%).     

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.     

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.     

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.     

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.