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

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

Nitrogen availability of soluble and slow release nitrogen fertilizers as assessed by electroultrafiltration

The efficiency of different nitrogenous fertilizers under forced leaching conditions was determined in pot experiments using ryegrass as an indicator plant in a sandy loam soil. Treatments were: ON (unfertilized), AN (ammonium nitrate), U (urea), DAP (diammonium phosphate), T (Triabon), FK (Floranid Komplett) and CDU (crotonylidendiurea).N source effects on N uptake, and N leached (as % of N applied) were evaluated. Results show that the fertilizers which behaved best with regard to N uptake were FK followed by Triabon and AN. The most soluble fertilizers, AN and urea, gave the highest N leaching losses.Significant correlations between EUF-NI (rapidly soluble fraction) and leaching, as well as between EUF-N (I + II) and N uptake by the plant (where EUF-N II relates to the slowly soluble fraction) were obtained.     

Crop response to sulfur-coated urea (SCU) and other N-carriers in sandy soils of Saudi Arabia

Nitrogen losses due to leaching, volatilization and denitrification result in low fertilizer efficiency. Slow-release N fertilizers are proposed to minimize these losses, and sulfur-coated urea (SCU) has been examined. A greenhouse study was conducted using two coarse textured: loamy sand and sandy loam soils from Al-Kharj, Saudi Arabia to compare sulfur-coated urea (SCU) with urea, ammonium nitrate (AN) and ammonium sulfate (AS) and to determine the effectiveness of single appliction of SCU over split application of soluble N sources. Tomato (Lycopersicon esculentum L.) and sorghum (Sorghum bicolor L.) were grown as indicator crops. Dry matter yields showed a significant increase with SCU fertilizers over other sources applied single or split, particularly in loamy sand soil. SCU-22 resulted in a consistent increase of dry matter yield and N-uptake compared to SCU-30 and SCU-40. The potential benefit of using SCU in coarse textured soils appers to be promising for increasing N fertilizer efficiency. Producing these slow release N fertilizers seems feasible if a coating step is added to the manufacturing process of urea produced from some fertilizer plants in Saudi Arabia.Contribution from the Regional Agriculture and Water Research Center (RAWRC), Ministry of Agriculture and Water, Riyadh, Saudi Arabia. Presented in Div. S-8, Am. Soc. Agron. Meeting, Atlanta, GA, November 1981.     

NPK-fertilizer efficiency — a situation analysis for the tropics

This paper examines the efficiency of applied N, P, and K fertilizers under tropical conditions. To meet their food demands, tropical countries are importing large quantities of fertilizers at an enormous cost. There is a need for improving crop yields at a reduced cost and a better understanding of the factors that contribute to the overall efficiency of applied fertilizers. It is estimated that under tropical condition, the efficiency of applied N is less than 50%, less than 10% for P and for K it is somewhere around 40%. Losses of N are mainly due to leaching, runoff and volatile losses of ammonia. Under flooding and in alternate wetting and drying conditions of rice lands and low lands, dentrification and volatile ammonia losses are considerable. The N losses from these soil could be minimized by proper management such as rate, methods and time of application. The coating of urea with S has shown some improvement in increasing efficiency. Nitrification and urea hydrolysis inhibitors can improve fertilizer efficiency in certain situations provided they are properly used. The efficiencies of these inhibitors depend on the nature of the chemical compounds, soil properties, and method of application. Low efficiency of applied P fertilizer is mainly due to retention of P by soil clay fractions and iron and aluminum hydroxides. Even though retained P is not available to the first crop, it is made available to a certain extent to the succeeding crops. The rate and methods of P applications and forms of P determine the efficiency of applied P fertilizers. The use of native rock phosphate along with P fertilizers on acid soils appears to be an attractive alternative in reducing the fertilizer cost. The loss of K in tropical soils is largely attributed to leaching and runoff. To reduce K loss by leaching, it is more advisible to apply K in split doses than a single dose. Liming has a beneficial effect in retention of K and reducing P fixation in acid soils.Senior author formerly was a Research Advisor to EMBRAPA/IICA/World Bank program at National Corn and Sorghum Research Center, Sete Lagos, MG, Brazil.     

Nitrogen leaching and plant uptake from controlled-release fertilizers

Controlled-release N fertilizers are commonly used in the production of container-grown ornamental crops, yet the relative effects of various nutrient sources on N leaching are not well known. A 27-week experiment was conducted to evaluate N leaching loss and plant growth following two applications of six controlled-release N fertilizers and one soluble N fertilizer to container-grownEuonymus patens Rehd. The controlled-release fertilizers evaluated were (noncoated) isobutylidene diurea, oxamide, urea formaldehyde, and (coated) Osmocote, Prokote Plus, and sulfur-coated urea. Of the fertilizers tested, the coated fertilizers generally out-performed the noncoated fertilizers in reducing N leaching losses, stimulating plant growth, and increasing tissue N concentrations. Low N concentrations in the leachate of some treatments indicated efficient nutrient use by the plant. In other treatments, low N concentrations in the leachate merely reflected incomplete N release from the fertilizer. A daily application of NH₄NO₃ resulted in a constant rate of N loss but was not the most effective in promoting growth. Plant growth, tissue N concentrations, and N leaching losses were all increased by doubling the fertilizer application rate from 1 kg N m^–3 to 2 kg N m^–3.     

Alleviating soil fertility constraints to food production in West Africa: Efficiency of nitrogen fertilizers applied to food crops

An overview is provided of the N efficiency research conducted within the West African Fertilizer Management and Evaluation Network (WAFMEN). Factors such as N rate, mode of N fertilizer application and choice of N sources for different agroecological zones of West Africa are discussed in relation to crop yield response. The interactive effects of cropping density and rainfall on N efficiency and yield are examined with particular emphasis on production of millet in Niger. The potential role of new, slow-release fertilizers as well as urea amended with urease inhibitors is mentioned in relation to present and future fertilizer N requirements in West Africa.     

Use of urease inhibitors and urea fertilizers on winter wheat

Phosphoroamide urease inhibitors were evaluated for their ability to increase grain protein and yield of winter wheat (Triticum aestivum L.) when added to surfaceapplied urea-based fertilizers. Six urease inhibitors [trichloroethyl phosphorodiamidate, diethyl phosphoric triamide, dimethyl phosphoric triamide, N-(diaminophosphinyl)-cyclohexylamine, N-benzyl-N-methyl phosphoric triamide, and phenylphosphorodiamide] were evaluated. Nitrogen treatments were urea prills, urea solution, and ureaammonium nitrate (UAN) solution broadcast and UAN solution band applied. Ammonium sulfate and no N treatments were included as controls. Fertilizer treatments were applied in the fall and spring. Soils were Ryker silt loam (Typic Paleudalf), Rensselaer loam (Typic Argiaquoll), and Avonburg silt loam (Aeric Fragiaqualf).Grain yield was a more responsive indicator of N addition than was grain N content. Urea prills and ammonium sulfate were more effective fertilizers than was UAN solution. UAN was not more effective applied in a band than broadcast. Urease inhibitors did not improve the efficiency of urea fertilizers since NH₃ volatilization did not appear to be a problem following addition of urea fertilizers in spring or fall.Journal Paper No. 10528. This work was supported in part by a grant from Allied Chemical, Solvay, NY 13209.     

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

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

缓释化肥概况（上）

在查阅了大量有关缓释化肥文献的基础上,对缓释化肥的发展、现状及生产工艺作了回顾,对缓释化肥的分类,缓释尿素（包括涂硫尿素,大颗粒尿素,包膜尿素、添加阻溶类物质和抑制剂缓释尿素）、长效碳铵、缓释复混肥料及含农药缓释肥料的研究和生产情况作了介绍,并对我国缓释化肥的发展前景进行展望。     

Evaluation of compacted urea fertilizers prepared with acid and non-acid producing chemical additives in three soils varying in pH and cation exchange capacity; II. Yield and N use efficiency by rice

With a view to evaluate the effect of different chemical additives on the efficacy of urea, a green house study was conducted with wet season rice (Oryza sativa L.) as test crop. The results showed that compaction of phosphogypsum (PG), diammonium phosphate (DAP), ZnSO₄, NH₄Cl or KCl with urea increased the rice yield and N use efficiency as compared to that obtained with straight urea. A significant positive correlation between potential N loss (measured by soluble N in floodwater) and NH₃ volatilization (measured by forced air draft system) was observed. The results clearly indicate that N use efficiency can be increased through suitable modification of urea by compacting it with acid and non-acid producing fertilizers such as NH₄Cl, KCl, ZnSO₄ and DAP or industrial byproduct like phosphogypsum.     

Methylene urea as a slow-release nitrogen source for processing tomatoes

The potential for improved fertilizer N use efficiency was tested using a slow release N fertilizer, methylene urea (MU), on processing tomato (Lycopersicon esculentum Mill.) in a 2-year field study in the Sacramento Valley, California. Fertilizer N use efficiency of urea and a (50:50, w:w) mixture of urea and MU (uMU) was determined in direct-seeded and transplanted tomato plots with winter cover crop (CC) or winter fallow (F) using ¹⁵N labeled fertilizers. Residual MU-N was estimated from tomato N uptake in the ¹⁵N microplots, and from residual ¹⁵N uptake of wheat grown after two tomato crops. No significant differences were found in the quantity and quality of tomato yields among fertilizer and management treatments during the first year. Total yields in transplanted FuMU plots were significantly lower in the second test year, suggesting slow mineralization of MU-N in the F treatment. On average, about 40% of added fertilizer N was taken up in both fertilizer treatments, and the recovery of ¹⁵N in plant biomass and soil was 75–96 and 50–74% in seeded and transplanted blocks, respectively. In the laboratory, mineralization of MU started faster in soils with past MU use, but the enhanced mineralization did not affect the plant N uptake in the field. MU is potentially an environmentally attractive fertilizer, but without an immediate increase in yield and N use efficiency compared to conventional fertilizers, its use on row crops may not be economically feasible unless the positive environmental factors like decreased leaching of N are considered.     

Effect of lime nitrogen on the efficiency of urea and other ammonium nitrogen fertilizers

Five pot experiments were conducted with wheat and rice in a net house to study the effect of lime nitrogen (LN, contains about 55% calcium cyanamide) amendment rates on the efficiency of urea, the recovery urea-¹⁵N, the efficiency of the three nitrogen fertilizers(NF), on the efficiency of urea in the three soils, and on NO ₃ ^- -N leaching from a flooded soil. A rate of LN-N of 5–8% of applied fertilizer N increased the recovery of labeled urea-N by 9.42%. The effect of LN on the efficiency of NF was urea > ammonium sulfate > ammonium chloride. Under flooded conditions, LN decreased NO ₃ ^- formation and leaching.Responses of several crops to LN amended fertilizers were also studied in field experiments. At equal NPK applications, the efficiency of basal applications to rice, wheat, corn, potatoes, soybean, peanut, grapes, peaches, melon and watermelon were bette r with LN than without. Efficiency with a basal fertilizer for rice or wheat with LN were the same as with the same fertilizer without LN applied in split applications.     

控释肥料基施及普通尿素不同基追比对夏玉米产量及氮肥利用率的影响

研究了控释尿素和控释BB肥一次性基施后的肥料效果,以及普通尿素不同基追比对玉米产量和氮肥利用率的影响.研究结果表明:以33.3％普通尿素基施+33.3％大喇叭口期追施+33.3％抽雄期追施的处理最佳,产量最高达8 700 kg/hm2,氮肥利用率为34.3％.控释尿素基施处理和控释BB肥基施处理的氮肥利用率分别为41....     

农用BB肥缓释增效的研究与应用

涂层缓释一次肥以其独特的缓释增效功能,成为农用BB肥创新的亮点,其克服了一般BB肥简单掺混的先天不足,应用膜反应与团絮结构原理,针对氮、磷、钾肥料不同特性开发出三大系列涂层涂液,分别涂布于不同颗粒表面,形成一层或多层包膜,成为具有“控氮缓释、促磷增效、防钾淋失”功能,粒度均匀、色彩各异的单质改性肥料颗粒,再采用掺混工艺根据测土施肥需要科学组配与作物全生育期养分需求变化相协调的友好环境、缓释高效的肥料创新产品。几年来的示范推广实践证明,涂层缓释一次肥增产、提质、增效显著。     

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.     

国内三种基础复合肥料近期生产技术进展

利用尿素、硝酸铵、磷酸铵、氯化钾等基础肥料进行二次加工制成复合肥料,是近代化肥生产的必然趋势,也符合科学发展观。介绍了国内目前生产的硝基复合肥、尿基复合肥和硫基复合肥三种主要复合肥料的最新生产方法、近期装置建设方面的一些情况。     

Recent research on problems in the use of urea as a nitrogen fertilizer

Recent research on the NH₃ volatilization, NO ₂ ^- accumulation, and phytotoxicity problems encountered in the use of urea fertilizer is reviewed. This research has shown that the adverse effects of urea fertilizers on seed germination and seedling growth in soil are due to NH₃ produced through hydrolysis of urea by soil urease and can be eliminated by addition of a urease inhibitor to these fertilizers. It also has shown that the leaf burn commonly observed after foliar fertilization of soybean with urea results from accumulation of toxic amounts of urea in soybean leaves rather than formation of toxic amounts of NH3 through hydrolysis of urea by leaf urease. It further showed that this leaf burn is accordingly increased rather than decreased by addition of a urease inhibitor to the urea fertilizer applied. N-(n-butyl)thiophosphoric triamide (NBPT) is the most effective compound currently available for retarding hydrolysis of urea fertilizer in soil, decreasing NH3 volatilization and NO ₂ ^- accumulation in soils treated with urea, and eliminating the adverse effects of urea fertilizer on seed germination and seedling growth in soil. NBPT is a poor inhibitor of plant or microbial urease, but it decomposes quite rapidly in soil with formation of its oxon analog N-(n-butyl) phosphoric triamide, which is a potent inhibitor of urease activity. It is not as effective as phenylphosphorodiamidate (PPD) for retarding urea hydrolysis and ammonia volatilization in soils under waterlogged conditions, presumably because these conditions retard formation of its oxon analog. PPD is a potent inhibitor of urease activity but it decomposes quite rapidly in soils with formation of phenol, which is a relatively weak inhibitor of urease activity. Recent studies of the effects of pesticides on transformations of urea N in soil indicate that fungicides have greater potential than herbicides or insecticides for retarding hydrolysis of urea and nitrification of urea N in soil.     

Recovery of nitrogen by spring barley from ammonium nitrate,urea and sulphur-coated urea as affected by time and method of application

The objective of this study was to increase the efficiency of fall-applied N either by placement in bands or by using a slow-release fertilizer. Four field experiments were conducted in north-central Alberta to determine the influence of N source, time of application and method of placement on the recovery of fall-applied N as soil mineral N in May, and on yield and recovery of N in grain of spring-sown barley. The recovery in soil of mineral N by May from the fall-applied fertilizers varied among treatments. More specifically, the recovery was lowest with topdressed application, highest with banding, and tended to be less with incorporation application as compared to banding. Recovery of mineral N was least for sulphur-coated urea (SCU) compared with A.N. and urea, regardless of method of application. The loss of fall-applied N was substantial, but leaching did not go beyond 60 cm deep.Yield and recovery of N in barley grain were much greater with spring application than with fall application at the 4 sites for ammonium nitrate (A.N.) and at 3 sites for urea. The SCU treatments were inferior. The A.N. and urea had greatest yield and N recovery with banding, followed by incorporation and then with topdressing for both fall- and spring-applied N. Method of application had little effect on yield and N uptake with SCU. In all, the greatest yield or crop N uptake was obtained with spring banding of A.N. or urea, while SCU did not function well as a fall- or spring-applied N fertilizer.(Contribution No. 680)     

Field evaluation of agronomic effectiveness of multi-nutrient fertilizer briquettes for upland crop production

Low nutrient recovery in upland crop production systems has prompted studies to improve the current nutrient management practices to increase fertilizer efficiency. Field studies were conducted in two growing seasons (2012 and 2013) under two land management systems (till and no-till) to evaluate agronomic effectiveness of a multi-nutrient fertilizer briquette (fertilizer briquettes) for upland crop production, using corn as test crop. The fertilizer briquettes were produced through a simple physical compaction of ordinary granular fertilizers with a final nutrient composition of 23.9% N, 19.2% P₂O₅, 19.1% K₂O, 0.9% Zn, and 2.5% S. The agronomic efficiency of the fertilizer briquettes were compared with commercial N sources, urea and ammonium sulfate supplied separately with phosphorus (P), potassium (K), zinc (Zn), and sulfur (S; for urea alone). During the wetter (2013) weather conditions, the fertilizer briquette treatment consistently produced the highest yields in both locations. At Ames Plantation, the fertilizer briquette treatment increased grain yields by ~ 16 and ~ 23% over the treatments having ammonium sulfate and urea granular fertilizers, respectively, and, in Jackson, by 16 and 34% respectively. Nutrient recovery efficiency was also greatest with the fertilizer briquettes treatment. However, during the drier weather conditions (2012), the fertilizer briquettes treatment was the least effective among the three treatments in terms of biomass and grain yields, and nutrient recovery efficiency. We conclude, with adequate rainfall conditions, the fertilizer briquettes could be an efficient fertilizer for upland crop production. However, under drier weather conditions where soil moisture is limited, the fertilizer briquettes may not be an ideal fertilizer source for upland crop production.     

7. The efficacy and loss of fertilizer N in lowland rice

Nitrogen fertilization is a key input in increasing rice production in East, South, and Southeast Asia. The introduction of high-yielding varieties has greatly increased the prospect of increasing yields, but this goal will not be reached without great increases in the use and efficiency of N on rice. Nitrogen enters a unique environment in flooded soils, in which losses of fertilizer N and mechanisms of losses vary greatly from those in upland situations. Whereas upland crops frequently use 40–60% of the applied N, flooded rice crops typically use only 20–40%. There is a great potential for increasing the efficiency of N uptake on this very responsive crop to help alleviate food deficits in the developing world.This article reviews current use of N fertilizers (particularly urea) on rice, the problems associated with rice fertilization, and recent research results that aid understanding of problems associated with N fertilization of rice and possible avenues to increase the efficiency of N use by rice.