Penetration depth of broadcast urea granules in puddled wetland rice soils

The efficiency of urea in wetland rice cultivation is known to be increased by placement below the soil surface. The penetration of broadcast urea into puddled soil might be a way to achieve placement of urea in soil. This paper combines an analysis of the free fall of urea granules in the atmosphere and a layer of water on the soil surface with measurements of granule penetration into puddled soils. The process of free fall can be described in terms of the height of fall in air, the depth of the water layer, and the terminal velocities and characteristic distances for free fall in air and water. The penetration depth of a particular granule with a particular velocity at the water/soil interface depends on the type of soil and its physical condition. Granule mass ranged from 0.1 to 0.5 g, granule velocity from 1 to 10 m s^–1, depth of the water layer from 0 to 30 mm and penetration depth from 0 to 35 mm. There is some indication that the penetration depth is proportional to the square root of the kinetic energy at the water/soil interface.     

Nitrogen behavior in tropical wetland rice soils

Laboratory and greenhouse pot experiments show that tropical wetland rice soils are very diverse in the inherent fertility. The mineralization of native soil nitrogen differed among the soils studied. Air-drying and oven-drying of soils resulted in a large increase in nitrogen mineralized. The main source of mineralized N was the amino acid and amino sugar fractions.N uptake by rice was well correlated with N mineralized in incubation of air-dried soil which should reflect accurately the nitrogen-supplying capacities of tropical wetland rice soils during the growing season. However, plant uptake of available soil nitrogen was also correlated with total nitrogen content of soils, and it would appear that total soil nitrogen which is simple to determine is a satisfactory index of the N supplying capacities of tropical wetland rice soils. Exclusion of problem soils like acid sulfate soils improved the correlation.     

Effect of phenyl phosphorodiamidate on the fate of urea applied to wetland rice fields

The effect of phenyl phosphorodiamidate (PPD) on floodwater properties, N uptake,¹⁵N recovery, and grain yield of wetland rice (Oryza sativa L.) was evaluated in a series of field studies conducted at Muñoz and Los Baños, Philippines. Prilled urea and PPD-amended urea were applied to soil and incorporated immediately prior to transplanting or applied to floodwater after transplanting. Urea was also deep-placed or added in a coated form in two studies.The addition of PPD with urea retarded urea hydrolysis by 1–3 days, depending on the time and method of application. Significant reductions in the concentration of ammoniacal-N in floodwater resulted when PPD-amended urea was applied between 18 and 26 days after transplanting (DT). In contrast, PPD did not appreciably affect the concentration of ammoniacal-N in floodwater when applied with urea either immediately before or after transplanting of the seedlings.Plant N uptake and grain yield were not significantly affected by the addition of PPD with urea in three of the four experiments conducted, even though PPD substantially reduced the concentration of ammoniacal-N in the floodwater in several treatments in these studies. The¹⁵N balance studies conducted at both field locations showed PPD to increase total¹⁵N recovery by between 10% and 14% of the¹⁵N applied, 14 days after the application of urea. No further loss of¹⁵N occurred between the initial sampling (40 DT) and grain harvest at Los Baños. An increase in¹⁵N recovery occurred at grain harvest at Muñoz because¹⁵N-labeled urea was applied at 50 DT in the study. PPD increased the amount of¹⁵N in the plant and nonexchangeable soil N fraction at all harvests at Los Baños. In contrast, at Muñoz, PPD increased the quantity of¹⁵N in the KCL-extractable pool 14 days after urea was applied. Reasons for the discrepancies in results between experiments and the overall failure of PPD to increase grain yield are discussed.     

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

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

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

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

Control of gaseous nitrogen losses from urea applied to flooded rice soils

This paper reports field experiments designed to determine whether the two main processes responsible for nitrogen (N) loss from flooded rice (ammonia volatilization and denitrification) are independent or interdependent, and glasshouse studies which investigated the effect of soil characteristics on gaseous nitrogen loss.In the first field experiment ammonia (NH₃) loss from the floodwater was controlled using algicides, biocides, frequent pH adjustment, shade or cetyl alcohol, and the effect of these treatments on total N loss and denitrification was determined. Most treatments reduced NH₃ loss through their effects on algal growth and floodwater pH. Total gaseous N loss (54% to 35%) and NH₃ loss (20% to 1.2%) were affected similarly by individual treatments, indicating that the amount lost by denitrification was not substantially changed by any of the treatments.In a subsequent field experiment NH₃ and total N loss were again affected similarly by the treatments, but denitrification losses were very low. In control treatments with different rates of urea application, NH₃ and total N loss were each a constant proportion of the urea applied (NH₃ loss was 17% and total N loss was 24%). These results indicate that techniques which reduce NH₃ loss can be expected to reduce total gaseous N loss.The glasshouse experiment showed that gaseous N losses could be reduced by draining off the floodwater, and incorporating the urea into the 0–0.05 m soil layer before reflooding. Even with this method, losses varied widely (6–27%); losses were least from a cracking clay and greatest from a coarse sand which allowed the greatest mobility of the applied N. Incorporation of applied urea can therefore be expected to prevent losses more successfully from clay soils with high ammonium retention capacity.     

Deep placement of urea supergranules in transplanted rice: Principles and practices

Rice is the most important food crop in the developing countries of Asia, where population densities are very high and overall dietary levels are not adequate. In south and southeast Asia, rainfed and irrigated transplanted rice occupies nearly two–thirds of the rice-growing area and produces more than 80% of the paddy rice. In these areas, prilled urea (PU) conventionally applied by farmers is very inefficiently used by transplanted rice largely because of serious losses (up to 60% of applied N) via NH₃ volatilization, denitrification, leaching, and/or runoff. In order to minimize N loss, especially loss due to denitrification, historically the Japanese have used different ways of deep placing fertilizer N. In 1975, IFDC proposed use of supergranules of urea (USG) in place of mudballs containing urea fertilizer to achieve the same agronomic benefits as achieved through the Japanese concept of deeppoint placement of fertilizer N in transplanted rice. USG can be prepared by melt-type processes (pan granulation, falling curtain, and fluid bed) and briquetting (a special type of compaction). The latter process seems to be the most cost-effective viable alternative. Small-scale briquetting machines have been developed to produce urea bgriquettes (UB) at village level at a rate of 200–250 kg h^?1. Basically, USG are large, discrete particles of ordinary urea [(NH₂)₂CO] containing 46% N as NH₂ (amide form); their weights may vary from 1 to 2 g per particle. USG from melt granulation process are nearly spherical with a relatively smooth surface, while UB from briquetting will be pillow-shaped with broken edges. Placement of USG can be done efficiently by handafter conventional line transplanting (e.g., researcher's method or IFDC transplanting guide method) orduring line transplanting (e.g., IFDC dispenser method) at the rate of one USG near the center of each four rice hills to a 7–10 cm soil depth. The IFDC methods have been developed mainly for economically disadvantaged small rice farmers of developing countries, especially those who transplant rice at random in rainfed areas. Other alternative manual methods such as incorporation of broadcast USG, random deep placement of USG by hand before line transplanting, or the deep placement by foot before or after transplanting may be less labor intensive; however, their agronomic efficiency has been low and highly variable, and they therefore cannot be recommended to farmers. Various continuous operation-type applicators (prototypes) have been developed in the Philippines, India, and China for mechanical deep placement of USG in line-transplanted rice. A few prototypes have been found to be labor saving and agronomically efficient when tested on research farms. However, several design-related problems associated with their metering mechanisms, placement depths, closing of furrows at the placement sites, output per workday, and/or operators' comfort, etc., need to be solved. In short, continuous operation-type applicators that are affordable and still efficient for deep placement of UB are not yet available for use on farmers' fields where floodwater and soil conditions vary substantially. The noncontinuous operation-type UB applicator prototype developed by IFDC is not as labor saving as the continuous operation-type applicators. However, its proper use with adequate practice can help to minimize the drudgery and to save up to 40% of the labor required for the hand placement method. This completely manual UB applicator, made of polyvinyl chloride (PVC) is simple to use, lightweight, and affordable as well as agronomically efficient on farmers' fields. As a result of diffusive transport and cation exchange, typically steep concentration gradients (or spatial distribution patterns) of ammonium exist at the placement sites and eventually control the rate and duration of availability of USG-N to the rice plants. USGper se is not a slow-release nitrogen fertilizer but behaves like a slowly available nitrogen fertilizer. Because the deep-placed USG-N is well protected from various N loss mechanisms (except leaching) at the placement sites in soils and the spatial ammonium concentration gradients help to improve its plant availability, (1) uptake of N by rice plants (recovery) is significantly increased, (2) relatively smaller amounts of USG-N as nonexchangeable ammonium and/or immobilized organic N stay in soil, and (3) eventually N losses (gaseous and runoff) are markedly decreased. Thus, this practice is agronomically efficient as well as environmentally safe. However, this practice should not be used in permeable soil with coarse texture and low cation exchange capacity (CEC) because the high loss of USG-N via leaching will significantly decrease N uptake by the rice plants and eventually grain yield too. Several hundred field trials conducted by national and international institutions in south and southeast Asia since 1975 have demonstrated the agronomic superiority of the deep placement of USG vis-a-vis split applications of PU in transplanted rice. In general, paddy yield responses to deep-placed USG tend to be more curvilinear than do those to split-applied PU, thus resulting in higher agronomic efficiency for deep-placed USG in the lower range of N rates (30–80 kg N ha^?1) than in the higher range of N rates (> 90 kg N ha^?1). Depending on agroclimate and N rates used, in general deep-placed USG can help to provide a saving of urea fertilizer of up to 65% with an average of 33% and can help to increase grain yields up to 50% with an average of 15% to 20% over that with the same amount of split-applied N as PU, especially in the lower range of N rates. USGper se is not an efficient nitrogen fertilizer, but the proper deep placement of USG in transplanted rice makes it agronomically efficient. In using USG, consideration of the following factors should help to ensure agronomic efficiency of deep-placed USG and increase the chances of obtaining additional yield.

1. Soil factors: Only use in soils having a low water percolation rate and a CEC ? 10 meq 100 g^?1 soil.
2. Plant factors: Give preference to short- to medium-duration dwarf rice varieties. For the longduration variety, basal deep-placed USG with a suitable topdressing of N as PU at panicle initiation stage would be helpful.
3. Management factors: Apply basally 30 to 60 kg USG-N ha^?1 using only USG of the right weight (1–2 g urea granule^?1). Place one supergranule for each four hills at 7–10 cm soil depth using the right plant population and modified spacing. Use modified 20 cm × 15 cm or 20 cm × 20 cm spacing to facilitate efficient placement of USG by hand or machine. Workers should always use the so-called traffic lane of the modified spacing for performing all post-transplanting field operations. When deep placement of USG is delayed after transplanting, extra care is necessary to close the holes left at the placement sites. When puddling is inadequate or improper and deep placement is done during transplanting, some care may be required to close the holes.

A scheme of small-scale production of UB at village level, using briquetting machines and locally available PU as a feedstock, looks promising for developing countries. The estimated production cost of UB is likely to be up to 10% higher than that of PU. In general, the estimated incremental benefit/cost ratios of hand deep-placed USG in line-transplanted rice are quite attractive, usually ?5 for small rice farmers of developing Asia. Technological and agroeconomic considerations suggest that the practice of hand deep placement of USGduring or after line transplanting appears to be a right agrotechnology for the resource-scarce small rice farmers of developing countries for efficiently using affordable doses of nitrogen (30–60 kg UB-N ha^?1) to significantly increase grain yields of transplanted rice. For other rice farmers who are not economically handicapped, who have access to irrigation, and who transplant rice in line and can afford to use high rates of N (> 90 kg N ha^?1), it can be an attractive practice, if appropriate machines for deep placement of USG have been developed. Therefore, research and development work is needed to develop affordable, labor-saving, and agronomically efficient continuous operation-type applicators for mechanical deep placement of UB. The use of USG as a source of N for transplanted rice has potential in developing countries. What is now required is to first develop practical stepwise and region-specific agrotechnologies consisting of appropriate UB supply schemes and rice farming systems based on hand or machine deep placement of UB in line-transplanted rice for different regions in a given country. Then it will be necessary to adopt an appropriate diffusion strategy for transfer of the region-specific agrotechnologies to the rice farmers. In this extension activity, long-term commitment and integrated efforts are required by national government organizations as well as by nongovernment organizations and the fertilizer industry.     

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%).     

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.     

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

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

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

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

Fate of urea nitrogen applied to a banana crop in the wet tropics of Queensland

This paper reports a study in the wet tropics of Queensland on the fate of urea applied to a dry or wet soil surface under banana plants. The transformations of urea were followed in cylindrical microplots (10.3 cm diameter × 23 cm long), a nitrogen (N) balance was conducted in macroplots (3.85 m × 2.0 m) with ¹⁵N labelled urea, and ammonia volatilization was determined with a mass balance micrometeorological method. Most of the urea was hydrolysed within 4 days irrespective of whether the urea was applied onto dry or wet soil. The nitrification rate was slow at the beginning when the soil was dry, but increased greatly after small amounts of rain; in the 9 days after rain 20% of the N applied was converted to nitrate. In the 40 days between urea application and harvesting, the macroplots the banana plants absorbed only 15% of the applied N; at harvest the largest amounts were found in the leaves (3.4%), pseudostem (3.3%) and fruit (2.8%). Only 1% of the applied N was present in the roots. Sixty percent of the applied N was recovered in the soil and 25% was lost from the plant-soil system by either ammonia volatilization, leaching or denitrification. Direct measurements of ammonia volatilization showed that when urea was applied to dry soil, and only small amounts of rain were received, little ammonia was lost (3.2% of applied N). In contrast, when urea was applied onto wet soil, urea hydrolysis occurred immediately, ammonia was volatilized on day zero, and 17.2% of the applied N was lost by the ninth day after that application. In the latter study, although rain fell every day, the extensive canopy of banana plants reduced the rainfall reaching the fertilized area under the bananas to less than half. Thus even though 90 mm of rain fell during the volatilization study, the fertilized area did not receive sufficient water to wash the urea into the soil and prevent ammonia loss. Losses by leaching and denitrification combined amounted to 5% of the applied N.     

尿素溶液喷浆造粒工艺各种改进技术特点的比较

分析尿素溶液喷浆造粒生产复合肥工艺改进的必要性,对引进的TVA、K-T、Hydro、Incro等公司的尿基复合肥生产工艺,国内改良氨酸法,台湾改良TVA法以及国内开发的硫酸铵管式反应器工艺、转鼓氨化硫基复合肥工艺等尿基复合肥生产工艺的技术特点进行了比较,为国内现有尿液喷浆造粒或蒸汽团粒法生产复合肥的厂家技术改造提供参考.     

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.     

Relationships between soil urease activity and other properties of some tropical wetland rice soils

Ten Philippine wetland rice soils differing widely in pH, texture and organic matter were studied to determine relationships between urease activity and other soil properties. Simple correlation analyses of urease activity with soil properties indicated that urease activity was correlated highly significantly with total N (r = 0.91)^**, and organic C (r = 0.89^**) but was not significantly correlated with CEC, Clay, pH active Fe or active Mn content. From multiple regression analyses it was observed that organic matter content of soils measured by organic C and total N accounted for most of the variation in urease activity.     

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.     

如何提高尿素溶液分析的质量控制

针对提高尿素溶液分析的质量控制问题，从样品的采集、仪器的使用要求、操作技能的标准化等几方面作了细致的分析，认为在抓好上述环节同时，还要建立健全各项管理制度和数据资料的监督、审核规定，是确保分析数据准确的重要步骤之一。     

Growth and yield of rice as affected by spacing,time and depth of placement of urea briquettes

A field study was conducted on a sandy clay soil (Entisol) in India to examine urea briquettes (UB) for lowland rice during 1986 and 1987. Grain yield was significantly greater for UB than a split application of prilled urea. At equal rates of N application, a spacing of 30 cm between two UBs was significantly better than 60 to 90 cm spacing. Two applications (10 DAT and at panicle initiation) of UB was no better than a single application (10 DAT). Placement of UB at 3–4 cm depth was significantly better than its surface application or placement at 0–1 cm depth.     

Critical levels of Mn in coarse textured rice soils in India for predicting response of barley to Mn application

Green house studies of 20 soils, having a range in DTPA extractable Mn, were made to determine the critical deficiency level of Mn for predicting response of barley to Mn application. Soil Mn was significantly related with both Bray's per cent dry matter yeild (r = 0.70^**) and Mn uptake (r = 0.65^**). Soil application of 25 mg Mn kg^–1 soil significantly increased yield. Both graphical and statistical models of Cate and Nelson indicated the critical level to be 2.05 mg kg^–1 soil of DTPA extractable Mn. The critical Mn deficiency level in 45 day barley plants was 18.6 mg kg^–1 dry matter. The predictability of soil and plant critical Mn level was 91 and 80 per cent respectively.     

Time and mode of nitrogen fertilizer application to tropical wetland rice

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