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.     

Nitrogen use efficiency of irrigated tropical rice established by broadcast wet-seeding and transplanting

Broadcast wet-seeding is gradually replacing transplanting in irrigated rice systems of Southeast Asia. Previous studies reported higher fertilizer-N use efficiency for broadcast-seeded than transplanted rice despite similar grain yields in treatments that received N fertilizer. To re-examine this issue, we compared crop performance and the recovery efficiency (_r, N uptake per unit N applied), agronomic efficiency (_a, grain yield per unit N applied), and partial factor productivity from applied N (PFP, grain yield per unit N applied) in broadcast-seeded and transplanted rice across a wide range of N fertilizer rates at research stations and in farmers' fields. Rice crop established by broadcasting had more rapid leaf area development, dry matter accumulation, and N uptake than transplanting during vegetative growth stages, but slower growth rates and N uptake after panicle initiation, particularly during the grain filling period. Without applied N, grain yield and N accumulation at maturity were significantly lower in broadcast-seeded than transplanted rice, whereas yields and N uptake were comparable for both planting methods with equivalent rates of applied N. Although both _r and _a were higher for broadcast-seeded than transplanted rice, this advantage was an artifact of lower yields and reduced N uptake by broadcasting without applied N rather than improved performance with applied N. In contrast, PFP values were similar for broadcast-seeded and transplanted rice at comparable fertilizer-N rates and in the absence of lodging. We conclude that the PFP from applied N provides a more relevant measure of N use efficiency of different crop establishment methods, and that the system-level N use efficiency of broadcast-seeded rice was not greater than that of transplanted rice.     

Interpreting fertilizer-use efficiency in relation to soil nutrient-supplying capacity, factor productivity, and agronomic efficiency

Although efficient use of N remains a critical constraint to productivity in irrigated lowland rice, a comprehensive database does not exist for the efficiency of on-farm management of N and other nutrients. In 1994, IRRI initiated its Mega Project on Reversing Trends of Declining Productivity in Intensive Irrigated Rice Systems in selected rice production domains of five tropical Asian nations to improve on-farm fertilizer-use efficiency and to monitor long-term productivity trends as related to fertilizers and other inputs. Data are reported here for the first crop cycle, the 1994–95 dry season. The indigenous soil N supply (INS) was estimated by aboveground crop N uptake and grain yield (GY) in plots without applied N established in farmers' fields under otherwise favorable growth conditions. The fertilizer N rate each farmer applied to his/her field surrounding these plots was recorded; GY was also measured in that area. In each domain, GY in unfertilized plots varied considerably among farms, as the range between maximum and minimum values within each domain was at least 2.8 t ha-1, thus of comparable magnitude to mean GY for these plots. Fertilizer N rates varied from 36–246 kg ha-1 across all domains, but their lack of relationship to INS contributed to relatively low fertilizer N efficiency and high variability in efficiency among farms. Mean agronomic efficiency (GY/applied N rate) for each domain was only 6–15 kg grain kg-1 N, while values for individual farmers ranged from 0 to 59 kg grain kg-1 N. Initial data on P and K fertilizer management also suggest highly variable applications at suboptimal efficiency. These results indicate the potential for greater fertilizer efficiency from improved congruence between the indigenous soil supply and applied fertilizer, and emphasize the need for field-specific nutrient management. Although agronomic efficiency and partial factor productivity (GY/applied N rate) can each be used to describe the efficiency of fertilizer applications, a complete analysis of nutrient management should include both terms, grain yield, fertilizer rates, and native soil fertility.     

1. The chemistry and biology of flooded soils in relation to the nitrogen economy in rice fields

Response of lowland rice to sources and methods of nitrogen fertilizer application were summarized for more than 100 experiments. In about 2/3 of the experiments, the yield increase per kg of fertilizer N was judged to be relatively poor with best split applications of urea. Based on frequency distribution, sulfur coated urea and urea briquets or urea supergranules deep placed more often produced satisfactory yield increases than best split urea, but even with these sources/methods the yield increases were judged to be relatively poor in about 1/2 of the experiments. There is an enormous potential to increase rice production with no further increases in inputs of fertilizer N if we could learn to match the best method/source of fertilizer with the soil-crop management complex.About 60% of the yields with no fertilizer N were in the range of 2 to 4 t/ha. Based on the average yield response to urea, this is equivalent to about 100 kg of urea N. It would appear worthwhile to study ways to improve utilization of soil nitrogen since it is already in place on the land and apparently in fairly abundant amounts in many soils.About 50 experiments with¹⁵N fertilizers were summarized. In almost all cases, the uptake of tagged fertilizer was less than the net increase in N in the above ground matter. In about 2/3 of the experiments, the addition of fertilizer N increased soil N uptake more than 20% and in 1/3 of the experiments the uptake of soil N was increased more than 40%. These results lead to much uncertainty about practical interpretation and use of¹⁵N data.     

Effect of green manuring with Sesbania aculeata and nitrogen fertilization on the performance of direct-seeded flood-prone lowland rice

Green manuring of rice with dhaincha (Sesbania aculeata) is widely practised under irrigated puddle-transplanted conditions. In flood-prone lowlands, the rice is established through direct seeding early in the season and flooding occurs after 1–2 months of crop growth following regular rains. The low yields are due to poor crop stands and difficulty in nitrogen management under higher depths of water. The effect of green manuring with dhaincha intercropped with direct-seeded rice vis-à-vis the conventional practice of incorporating pure dhaincha before transplanting was investigated under flood-prone lowland conditions (up to 50–80 cm water depth) at Cuttack, India. Treatment variables studied in different years (1992, 1994 and 1995) were: rice varieties of different plant heights, crop establishment through direct seeding and transplanting, varying length of periods before dhaincha incorporation, and urea N fertilizer levels. Dhaincha accumulated 80–86 kg N ha^-1 in pure stand and 58–79 kg N ha^-1 when intercropped with direct-seeded rice in alternate rows at 50 days of growth. The growth of rice improved after dhaincha was uprooted manually and buried in situ between the rice rows when water depth was 10–20 cm in the field. The panicle number was lower but the panicle weight was higher with dhaincha green manuring than with recommended level of 40 kg N ha^-1 applied as urea. The grain yield was significantly higher with direct seeding than with transplanting due to high water levels (>60 cm) immediately after transplanting. Dhaincha manuring was at par with 40 kg N ha^-1 as urea in increasing the yield of direct-seeded and transplanted crops. The highest yield of direct-seeded crop was obtained when 20 kg N ha^-1 was applied at sowing and dhaincha was incorporated at 50 days of growth. The results indicate that green manuring of direct-seeded rice with intercropped dhaincha is beneficial for substituting urea fertilizer up to 40 kg N ha^-1 and augmenting crop productivity under flood-prone lowland conditions.     

Nitrogen application in dry-seeded delayed-flooded rice in Italy

Dry-seeded delayed-flooded rice in Italy is important in some areas with high sand content. The effects of N application timing and rates on grain yield, milling yield, plant height, total biomass, harvest index (HI) and crop N content were evaluated for dry-seeded delayed-flooded rice in Italy to increase the N use efficiency and to study N fertilizer recommendations. Two widely grown non-semi-dwarf varieties were studied: Drago and Loto. Three N rates were used: 60, 120 and 180 kg N ha^–1. For each rate the fertilizer was applied in seven splits across three growth stages: pre-sowing, pre-flooding and panicle differentiation. Drago produced higher yields and N application at all stages increased yield. Late N application caused the development of secondary sterile tillers, decreasing harvest index. Loto responded to earlier N applications but not to late N applications. In contrast with other trials outside Italy, pre-plant N fertilization was effective in increasing yield. Brown rice and total milled rice yields were higher for Loto. Increasing the applied N rate increased the head rice yield range. With low N no low values were recorded and variation between splits was small. With high N the highest head yield was observed with split application. Height was significantly affected by N rate and split, with a range of 65 to 92 cm and 54 to 86 cm for Drago and Loto, respectively. N application at tillering was more effective. N rate and split affected biomass and decreased HI. Crop N content at maturity was affected by late N application and by total applied fertilizer. N content in the panicle showed lower variation with N split and rate and was negatively correlated with yield. At 120 and 180 kg N ha^–1 all nitrogen applied pre-flooding gave good results in both sites, but the best results were obtained with N split at pre-sowing and pre-flooding, with low yield increase moving from 120 to 180 kg N ha^–1. We recommend late N applications for Drago (medium-late variety), but not for Loto (early variety).     

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.     

Improved water control and crop management effects on lowland rice productivity in West Africa

Much of the rapidly-growing demand for rice in West Africa will be met from production in inland valley swamps which are abundant and relatively robust with regard to cropping intensification. Rice yields in the traditional, predominating non-bunded systems are low (about 1.5 Mg ha^?1). Informal development of traditional rainfed lowlands is currently occurring across the region, mainly with low-cost interventions such as the building of field bunds. Relating lowland rice yield to quantitative information on resource base quality and agronomic practices will improve future research priorities and guide site and system-specific targeting of available technical options. Diagnostic on-farm trials were conducted in 62 traditional and 40 improved (bunded) rainfed lowland fields in three agro-ecological zones of Côte d'Ivoire (Guinea savanna, bimodal forest, and monomodal forest zones) to quantify effects of improved water control on productivity. Weed biomass and rice grain yield were determined in farmers' fields and in super-imposed researcher-managed subplots (clean weeding, zero N control treatment, and mineral fertilizer N application). Grain yield variability was attributed to agronomic practices using multiple regression analysis. Bunding significantly increased rice grain yield across sites by about 40% and controlled weeds, with approximately 25% less weed biomass in bunded than in open plots. Mineral fertilizer N application significantly increased rice yields (18% on average across sites) only in bunded fields, where N use efficiency was 12 compared to 4 kg of rice grain per kg of applied N in open fields. Across environments, about 60% of the observed variability in rice grain yields was explained by water control and the timing of agronomic management interventions (weeding, N application). The increased development of lowland rice areas through the construction of field bunds has the potential to significantly increase rice production in West Africa, while also possibly reducing labor requirements for hand weeding and allowing for a more efficient use of mineral N fertilizers. The benefits from construction of bunds are likely to be largest in the relatively well-drained inland valleys of the savanna and bimodal rainfall forest, compared to the high-rainfall monomodal forest zones.     

6. Ammonium dynamics of puddled soils in relation to growth and yield of lowland rice

The release of non-exchangeable (fixed) NH ₄ ⁺ and the importance of exchangeable NH ₄ ⁺ at transplanting (initial exchangeable NH ₄ ⁺ ) for rice (Oryza sativa L.) growth was studied in representative lowland rice soils of the Philippines.The experiments showed that initial exchangeable ammonium behaved like fertilizer N and thus may serve as a valuable guideline for nitrogen fertilizer application rates when calculated on a hectare basis. By using the¹⁵N tracer technique it was found that nonexchangeable ammonium in soil may contribute to the nitrogen supplying capacity of lowland rice soils. Fixation and release of NH ₄ ⁺ seem to be more dependent on the form of clay minerals than on clay content. In soils rich in vermiculite non-exchangeable ammonium should be considered together with other available N sources such as exchangeable ammonium for N fertilizer recommendations for lowland rice.     

Yield variation of rainfed rice as affected by field water availability and N fertilizer use in central Benin

Rice is mainly grown under rainfed conditions in West Africa. Unpredictable and variable rainfall, poor soil quality, and suboptimal crop management practices are the main determinants of low productivity. We assessed the effects of soil water availability and fertilizer application, and their interaction on the yield of rainfed rice in Glazoué, Department of Zou-Collines, central Benin between 2010 and 2013. On-farm fertilizer management trials and field surveys were conducted in 13–39 farmers’ fields per year. Field water conditions were visually assessed three times per week during the rice-growing season and flood and drought indices were calculated on the basis of number of days with ponded water and dry surface soil relative to the total number of days for the vegetative, the reproductive and whole rice-growing period. Variations in flood and drought indices were related to the sand content of the soil. While nitrogen was the most limiting nutrient, average response to N fertilizer application was low with an agronomic N use efficiency of only 7–9 kg grain per kg of N applied. Year-to-year variation in rainfall and spatial variation in field water status affected both rice yield and response to N fertilizer. Some 47% of the observed yield variation was explained by field water status and the amounts of N fertilizer applied, with rice response to N fertilizer being less when water was limited. We conclude that the prevailing blanket fertilizer recommendations are unlikely to contribute to yield increases in rainfed systems of West Africa. There is a need for field-specific recommendations that consider soil texture and the spatial–temporal dynamics of water availability.     

Soil and crop management technologies for enhancing rice production under micronutrient constraints

Micronutrient deficiency is considered as one of the major causes of the declining productivity trends observed in ricegrowing countries. The submergence created for rice cultivation influences electrochemical and biochemical reactions, and alters pH, pCO2 and the concentration of certain ions. This environment increases the availability of Fe and Mn with concomitant decrease in Zn and Cu. It is well known that Zn deficiency is predominant in lowland ecosystems. Sodic and upland soils and calcareous coarse-textured soils with low organic matter content suffer from Fe deficiency, besides Zn and Cu deficiencies. Rice cultivars do not experience deficiency of B and Mo. The acid soils and the lowlying, poorly drained alluvial and colluvial soils are prone to Fe toxicity. Experiments in different agroecological zones all over India showed that Zn doses to correct Zn deficiency varied from 2.5 to 22 kg ha-1; 5.3 kg Zn ha-1 proved optimum and economical, with a maximum rice yield increase of 4.8 t ha-1. In the lowland ecosystem, amending the soil with the required amount of Zn before transplanting was effective and easy to adopt, compared with repeated foliar sprays of 0.5% ZnSO4 or use of Zn-enriched seedlings through seed soaking in 2–4% ZnSO4 solution, fertilizing the nursery with Zn, or seedling root dipping in 2% ZnO slurry. Hepta as well as monohydrated ZnSO4 were better than other sources of Zn (ZnO, ZnCl2 and Zn frits). The Zn-blended diammonium phosphate (Zn-DAP), superphosphate, and nitrophosphates also proved effective. The Zn-enriched organic manures (farmyard manure, green leaf manure, and coir pith compost) were found advantageous for the direct and residual crops. Zinc fertilization with an optimal dose of 25 kg ZnSO4 ha-1 once a year yielded high economic return. A differential response of rice up to a maximum increase in yield of 4.8 t ha-1 was observed with the foliar spray (1–2% FeSO4 solution) or soil incorporation of Fe (50 kg FeSO4 ha-1) with bulky organic manure (12.5 t ha-1). The application of 12.5 kg CuSO4 ha-1 ameliorated Cu deficiency and significantly enhanced rice production. Management strategies such as liming and additional multinutrient supply (P. K, Mg, Zn, Cu, and B), besides improving drainage, enhanced the rice productivity of soils prone to Fe toxicity by correcting the multinutrient deficiency syndrome.     

Effects of fertilizer price on feasibility of efficiency improvement: Case study for an urea injector for lowland rice

Efficiency improving techniques, such as the introduction of a urea injector for lowland rice production, appear to lead to higher yields, lower fertilizer use and less environmental pollution at the same time. If farmers are free to decide on the amount of fertilizer they use, economic rationality leads to a choice between using the improved technique for saving fertilizer while obtaining the same yield, for increasing yield (at the same fertilizer rate) or for a mixed strategy (a slightly higher yield and a different fertilizer rate). The economic optimum fertilizer rate was calculated with a simple yield model for a low and a high fertilizer application efficiency to predict which strategy would be best for the farmer.Calculations for a standard data set for lowland rice show that the greatest benefit from an increase in application efficiency by urea deep placement instead of broadcast application can be expected when a marginal efficiency of about 9 kg rice per kg fertilizer N is used for determining the fertilizer rate. For a marginal efficiency of less than 6, savings on fertilizer are the main benefit of efficiency improvement; for higher marginal efficiencies yield increases become the main component of total benefit; for marginal efficiencies above 9, fertilizer use will increase when a more efficient technique is used, but increased yields compensate for their costs. In the four countries where a manually operated pneumatic urea injector was tested (Togo, Bangladesh, Indonesia and Ivory Coast) the price ratio of rice and fertilizer N ranged from 1.1 to 2.5. Even when a risk-avoidance multiplier of 2 is used, we may conclude that fertilizer prices were too low relative to rice to make optimum use of the existing techniques for efficiency improvement. An equation is derived for estimating the price ratio at which the probability of farmer acceptance of techniques for improving fertilizer use efficiency is highest.     

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.     

Nitrogen-15 balances and nitrogen fertilizer use efficiency in upland rice

Field experiments were conducted in the 1984 and 1985 wet seasons to determine the effect of N fertilizer application method on¹⁵N balances and yield for upland rice (Oryza sativa L.) on an Udic Arguistoll in the Philippines. The test cultivars were IR43 and UPLRi-5 in 1984 and IR43 in 1985. Unrecovered¹⁵N in¹⁵N balances for 70 kg applied urea-N ha^–1, which represented N fertilizer losses as gases and movement below 0.5 m soil depth, ranged from 11–58% of the applied N. It was lowest (11–13%) for urea split applied at 30 days after seeding (DS) and at panicle initiation (PI), and highest (27–58%) for treatments receiving basal urea in the seed furrows. In all treatments with basal-applied urea, most N losses occurred before 50 DS.Heavy rainfall in 1985 before rice emergence resulted in large losses of native soil N and fertilizer N by leaching and possibly by denitrification. During the week of seeding, when rainfall was 492 mm, 91 kg nitrate-N ha^–1 disappeared from the 0.3-m soil layer in unfertilized plots. Although rainfall following the basal N application was less in 1984 than in 1985, the losses from basal applied urea-N were comparable in the two years. Daily rainfall of 20–25 mm on 3 of the 6 days following basal N application in 1984 may have created a moist soil environment favorable for ammonia volatilization.In both years, highest grain yield was obtained for urea split-applied at 30 DS and at PI. Delayed rather than basal application of N reduced losses of fertilizer N and minimized uptake of fertilizer N by weeds.     

Leaching losses of urea-N applied to permeable soils under lowland rice

Application of 120 kg urea-N ha^–1 to lowland rice grown in a highly percolating soil in 10 equal split doses at weekly intervals rather than in 3 equal split doses at 7, 21 and 42 days after transplanting did not significantly increase rice grain yield and N uptake. Results suggest that leaching losses of N were not substantial. In lysimeters planted with rice, leaching losses of N as urea, NH ₄ ⁺ , and NO ₃ ^- beyond 30 cm depth of a sandy loam soil for 60 days were about 6% of the total urea-N and 3% of the total ammonium sulphate-N applied in three equal split doses. Application of urea even in a single dose at transplanting did not result in more N leaching losses (13%) compared to those observed from potassium nitrate (38%) applied in three split doses. Nitrogen contained in potassium nitrate was readily leached during the first week of its application. More N was lost from the first dose of N applied at transplanting than from the second or third dose. Data pertaining to yield, N uptake and per cent N recovery by rice revealed that the performance of different fertilizer treatments was inversely related to susceptibility of N to leaching.     

Urea management for lowland transplanted rice as affected by application of farmyard manure

Farmyard manure (FYM) applied to rice-growing soils can substitute for industrial fertilizers, but little is known about the influence of FYM on the effectiveness and optimal management for industrial N fertilizers. A field experiment was conducted in northern Vietnam on a degraded soil in the spring season (February to June) and summer season (July to November) to determine the effect of FYM on optimal timing for the first application of urea. The experimental design was a randomized complete block with two rates of basal incorporated FYM (0 or 6 Mg ha^–1) in factorial combination with two timings of the first application of 30 kg urea-N ha^–1 (basal incorporated before transplanting or delayed until 14 to 16 d after transplanting). The FYM was formed by composting pig manure with rice straw for 3 months. Basal incorporation of FYM, containing 23 kg N ha^–1, increased rice grain yield in both seasons. The yield increase cannot be attributed to reduced ammonia loss of applied urea-N, because FYM did not reduce partial pressure of ammonia (pNH₃) following urea application in either season. Basal and delayed applications of urea were equally effective in the absence of FYM, but when FYM was applied rice yields in both seasons were higher for delayed (mean = 3.2 Mg ha^–1) than basal (mean = 2.9 Mg ha^–1) application of urea. Results suggest that recommendations for urea timing in irrigated lowland rice should consider whether farmers apply FYM.     

Quantifying the effect of soil organic matter on indigenous soil N supply and wheat productivity in semiarid sub-tropical India

A field experiment was conducted on a loamy sand soil for six years to quantify the effect of soil organic matter on indigenous soil N supply and productivity of irrigated wheat in semiarid sub-tropical India. The experiment was conducted by applying different combinations of fertilizer N (0–180 kg N ha⁻¹), P (0–39 kg P ha⁻¹) and K (0–60 kg K ha⁻¹) to wheat each year. For the data pooled over years, fertilizer N together with soil organic carbon (SOC) and their interaction accounted for 75% variation in wheat yield. The amount of fertilizer N required to attain a yield goal decreased as the SOC concentration increased indicating enhanced indigenous soil N supply with an increase in SOC concentration. Besides SOC concentration, the soil N supply also depended on yield goal. For a yield goal of 4 tons ha⁻¹, each ton of SOC in the 15 cm plough layer contributed 4.75 kg N ha⁻¹ towards indigenous soil N supply. An increase in the soil N supply with increase in SOC resulted in enhanced wheat productivity. The contribution of 1 ton SOC ha⁻¹ to wheat productivity ranged from 15 to 33 kg ha⁻¹ across SOC concentration ranging from 3 to 9 g kg^-1 soil. The wheat productivity per ton of organic carbon declined curvilinearly as the native SOC concentration increased. The change in wheat productivity with SOC concentration shows that the effect of additional C sequestration on wheat productivity will depend on the existing SOC concentration, being higher in low SOC soils. Therefore, it will be more beneficial to sequester C in soils with low SOC than with relatively greater SOC concentration. In situations where the availability of organic resources for recycling is limited, their application may be preferred in soils with low SOC concentration. The results show that an increase in C sequestration will result in enhanced wheat productivity but the increase will depend on the amount of fertilizer applied and the existing fertility level of the soil.     

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%).     

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

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