P. L. G. Vlek

4. Reappraisal of the significance of ammonia volatilization as an N loss mechanism in flooded rice fields

The role of ammonia volatilization as a nitrogen loss mechanism in lowland rice (Oryza sativa L.) has recently been extensively reevaluated using techniques that do not disturb the field environment. This paper summarizes methodologies used in this research and discusses findings from recently conducted micrometeorological studies on ammonia volatilization. Factors affecting ammonia loss and the contribution of this process to the overall nitrogen loss from lowland rice systems are also outlined. Suggestions for future research are included.

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

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

Urease activity and inhibition in flooded soil systems.

Ammonia volatilization is an important mechanism of N loss from flooded rice soils. Inhibition of urease may delay the formation of conditions favorable to NH3 volatilization in the floodwater, thus giving the soil and plant a better chance to compete with the atmosphere as a sink for N. The experiments reported here were designed to identify the site of urea hydrolysis in flooded soils and to attempt selective urease inhibition with some of the inhibitors reported in the literature.Studies with three flooded soils using15N-labeled urea showed that 50–60% of the urea was found in the floodwater, despite incorporation. This floodwater urea is hydrolyzed largely at the soil—floodwater interface and subsequently returns to the floodwater (> 80%) or is retained by the soil (< 20%). Of the following urease inhibitors (K-ethyl-xanthate; 3 amino-1-H-1, 2, 4-triazole; phenylphosphorodiamidate) added at 2% (w/w of urea), only the latter was able to delay the appearance of NH3 (aq) in the flood-water and thus delay NH3 volatilization. Use of an algicide addition to the floodwater depressed NH3 (aq) levels during the entire period studied, but in the presence of PPD the algicide had little additional effect.

6. Micronutrients and the agroecology of tropical and Mediterranean regions

The status of micronutrients in soils and their availability to plants are the result of a complex of factors related to parent material, soil type, and climate. Some progress has been made in understanding the effects of several individual factors on the availability of micronutrients; however, the possibility of identifying areas of potential micronutrient problems on the basis of existing information on soils and climate is still remote. Such areas, therefore, are identified through site characterization based on surveys of micronutrient deficiency symptoms, extensive micronutrient soil testing, or selective micronutrient trials. Attempts to compile maps of areas with micronutrient deficiencies in the tropics have been virtually nonexistent. The value of such maps is, in fact, questionable since their scale would not allow delineation of actual areas of deficiency. Moreover, new information being generated would rapidly render such maps obsolete [11]. Finally, huge areas of the tropics remain entirely unexplored while existing information is heavily biased toward commercially produced, high-value crops. As is evident from the previous chapters, making an inventory of the micronutrient problem areas in the tropics would not provide a data base large enough to allow extrapolation to regions yet unexplored. Except in India, micronutrient studies in tropical countries have been sparse, and even in India most studies have been conducted in the subtropical region in the north. In an attempt to collect a broad data base on micronutrients, FAO initiated a global study, involving 30 countries, to produce fresh information on the problems of a number of micronutrients under different soil, climatic, and cultural conditions. Twenty-four of these countries are situated partly or totally in the tropics or Near East. The results of this study appeared as FAO Soils Bulletin, No. 48 [22], and they form to a large extent the basis of this chapter. It is tempting to assume that areas with potential for micronutrient problems can be recognized solely through information on the agroecology of

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.

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.

1. The chemistry of micronutrients in soil

Of the 17 elements known to be essential for plants, 7 are required in such small amounts as to be called micronutrients. Micronutrient elements include boron (B), manganese (Mn), iron (Fe), copper (Cu), Zinc (Zn), molybdenum (Mo), and chlorine (Cl). This text will be limited to B, Mn, Fe, Cu, Zn, and Mo since natural deficiencies of Cl are essentially nonexistent.

Fate of fertilizer nitrogen applied to wheat under simulated Mediterranean environmental conditions.

Triticum aestivumThe fate of fertilizer nitrogen applied to dryland wheat was studied in the greenhouse under simulated Mediterranian-type climatic conditions. Wheat, L., was grown in 76-cm-deep pots, each containing 50–70 kg of soil, and subjected to different watering regimes. Two calcareous clay soils were used in the experiments, Uvalde clay (Aridic Calciustoll) and Vernon clay (Typic Ustochrept). Fertilizer nitrogen balance studies were conducted using various15N-labeled nitrogen sources, including ammonium nitrate, urea, and urea amended with urea phosphate, phenyl phosphorodiamidate (a urease inhibitor), and dicyandiamide (a nitrification inhibitor). Wheat yields were most significantly affected by available water. With additional water during the growing period, the recovery of fertilizer nitrogen by wheat increased and the fraction of fertilizer nitrogen remaining in the soil decreased. In the driest regimes, from 40 to 65% of the fertilizer nitrogen remained in the soils. In most experiments the gaseous loss of fertilizer nitrogen, as estimated from unaccounted for15N, was not significantly affected by water regime. The15N not accounted for in the plant and the soil at harvest ranged from 12 to 25% for ammonium nitrate and from 12 to 38% for regular urea. Direct measurement of labeled ammonia loss from soil indicated that ammonia volatilization probably was the main N loss mechanism. Low unaccounted-for15N from nitrate-labeled ammonium nitrate, 4 to 10%, indicated that N losses due to denitrification, gaseous loss from plants, or shedding of anthers and pollen were small or negligible. Amendment of urea with urea phosphate to form a 36% N and 7.3% P product was ineffective in reducing N loss. Dicyandiamide did not reduce N loss from urea presumably because N was not leached from the sealed pots and denitrification was insignificant. Amendment of urea with 2% phenyl phosphorodiamidate reduced N loss significantly. However, band placement of urea at as 2-cm soil depth was more effective in reducing N loss than was amendment of broadcast urea with phenyl phosphorodiamidate.

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.

Path coefficient analysis of N nutrition on yield and yield components for rice in a highly percolating soil

Abstract Data pertaining to grain yield, yield parameters, and N uptake during different periods of rice growth, in three field experiments, were subjected to the statistical procedure of path coefficient analysis. The observed grain yield response to the applied fertilizer N was predominantly reflected in an increased panicle density and spikelet number. The 1,000‐grain weight was only slightly influenced by N fertilization. Since both panicle density and spikelet number are known to be largely determined within 70 days after transplanting (DAT), N uptake during this ‐period was found to be critical to achieve a maximum grain yield response to applied N. The K uptake during 0–40 DAT correlated positively with panicle density, whereas absorption of N during 40–70 DAT determined both panicle density and spikelet number. Since N uptake during 0–20 DAT exerted a significant positive influence on grain yield through increased panicle density, the basal application of a part of fertilizer at the time of transp...