Soil Chemical Analysis

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Laboratory manual for physiological studies of rice

Publisher Summary This chapter focuses on nitrogen transformation processes in wetland rice soils. It summarizes information on the physical, physicochemical, and biochemical processes relevant to nitrogen transformations and to improving fertilizer nitrogen efficiency in wetland soils. Wetland soils differ markedly from well-drained dryland soils. The characteristics of the wetland rice soils have been studied and reported. Some dynamic aspects of the wetland rice soils that are pertinent to nitrogen transformations are summarized as retardation of gaseous exchange between flooded soil and atmosphere. In the wetland rice soil, a reduced plow layer is sandwiched between a thin oxidized surface soil layer and a partly oxidized subsurface soil layer. Within the reduced soil layer, in which most of rice roots proliferate and derive nitrogen, is a complex mosaic consisting of aerobic and anaerobic microsystems. Flooding soil results in the accumulation of ammonium, the instability of nitrate, and a lowered nitrogen requirement for organic matter decomposition. Flooding also causes inefficient utilization of applied nitrogen. Ammoniacal nitrogen is subject to fixation by clays, loss by volatilization, leaching, runoff, seepage, and nitrification followed by loss through denitrification.

Nitrogen Transformations in Wetland Rice Soils

Publisher Summary According to the International Food Policy Research Institute, between 1993 and 2020 A.D. the global demand for cereals is expected to increase by 41%. It has been projected that annual rice production must increase from 556 million tons in 2000 A.D. to 758 million tons by 2020 A.D., a 36% increase. Rice-wheat cropping system (RWCS) is a long-established grain production system in China. The wheat yield following rice was only 0.7 to 1.0 tons ha-1 until the 1940s and it increased progressively after the 1950s as a result of improved varieties, better agronomic management, and pest control. RWCS in the Indian subcontinent is quite new and started only in the late 1960s with the introduction of dwarf wheat from CIMMYT, Mexico, which required a lower temperature for good germination than that required for traditional tall Indian wheat. There could be many more variants involving vegetables and other short duration crops. Most rice in RWCS is transplanted and rice varieties grown are of 90–140 days duration (seed to seed) of which 25–45 days may be spent in nursery. The estimates of area under RWCS in the world vary considerably. In RWCS there is very little turn-around time between rice harvest and wheat sowing. Depending on the time of harvest of the rice crop, conventional tillage requires pre-sowing irrigation on well-drained soils or draining or drying of soil in lowlands followed by one or two diskings, two harrowings, and leveling. Wheat in the RWCS belt in Indo-Gangetic Plains (IGP) is an irrigated crop. Considerable research has been conducted in India on the irrigation of wheat. Currently, there is a growing concern in sustainability of RWCS as the growth rates of rice and wheat yields are either stagnant or declining. Studies on socioeconomic and policy factors on the productivity of RWCS will be effective for measuring outcome of RWCS.

Rice–Wheat Cropping Systems

About 90% of rice is produced and consumed in Asia, where the demand for rice is on the increase due to increasing population. Rice is a semiaquatic plant and grows well under lowland flooded anaerobic conditions. Most high yielding varieties yielding 6–8 t/ha have been developed to suit such conditions. However, there are large areas, where rice is grown under upland aerobic conditions with drought tolerant varieties that yield about 1 t/ha or a little more. Aerobic rice varieties are now being developed that have drought tolerance as well as high yielding ability. Aerobic rice system (ARS) is a new production system in which rice is grown under nonpuddled, nonflooded, and nonsaturated soil conditions. The expected yields in ARS are somewhat lower than those obtained under lowland flooded conditions, but double or treble of that obtained under upland conditions. However, ARS has been successful in cool temperate regions. For warm humid rice growing regions, a partial aerobic rice system (PARS) may be a more plausible alternative. PARS technologies are available. Some available results, however, show a decline in rice yield over years in ARS and suggest that ARS may not be sustainable over a long period. The factors responsible appear to be N, P, K, Fe, and Zn deficiencies, weeds and soil-borne root-knot nematodes. Some of these problems can be overcome by introducing legume such as soybean in ARS/PARS. ARS/PARS call for application of herbicides and namaticides, which are going to add additional burden on poor Asian farmers. In addition, farmers have to be trained in the careful and proper use of herbicides and nematicides. Herbicides have also been associated with environmental pollution problems.

Aerobic Rice Systems

Measurement of leaf area of Rice , Measurement of leaf area of Rice , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Measurement of Leaf Area of Rice1

Leaching Loss of Nitrogen in Submerged Rice Cultivation

Under the three levels of submergence (0, 5 and 10 cm. depths), water requirement of rice was maximum at 10 cm. but crop performance was similar to that at 5 cm. With medium and high levels of fertilization the crop had a higher water requirement than under low level, for ‘aus’ and ‘aman’ crops. Compacted soil compared well with puddled soil, but water requirement was appreciably higher with shallow puddling. In the two seasons water requirement, transpiration, evaporation and percolation losses were greater under submergence, and only transpiration was greater under higher levels of fertilization. Transpiration was maximum at flowering stage.

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Effects of Depth of Submergence, Fertilization and Cultivation on Water Requirement and Yield of Rice

Limited moisture supply caused reduction in leaf area, number of effective tillers, ratio of sound to unsound grains and the nitrogen, phosphorus, iron and manganese content of rice. Crop yield was adversely affected in the absence of standing water. Two levels of nitrogen (40 and 60 kg.N/ha.) caused no significant difference in yield in 1965, but in 1966, when 30 and 60 kg. N/ha. were used, the latter gave better yields.

Effects of Moisture and Nitrogen on Growth, Yield and Mineral Content of Rice

Four soil moisture and two nitrogen levels were combined in eight treatments to study moisture and nitrogen effects on Dular (June–September crop) and Patnai-23 (June–November crop) varieties of rice. Both the varieties when grown under natural precipitation yielded significantly less than at other moisture levels. Whereas there were no significant differences amongst the three treatments of submergence (15–10 cm), cyclic submergence (15–0 cm) and cyclic wetting-drying (saturation to field capacity) for Dular rice, the former two treatments proved better than the latter for Patnai-23 rice. Dular responded to the treatments of cyclic wetting-drying in a way similar to that of submergence because the crop completed its life cycle during the period of low evaporative demand. Among the levels of nitrogen, 40 kg N/ha proved optimum for Dular rice and 6O kg N/ha for Patnai-23 rice. The increase in yield of grain was associated with increase in the number of panicles, number of spikelets/panicle and percentage of ripened spikelets. The concentration of N, P, Fe and Mn in the plant was maximum under submergence followed by cyclic submergence, cyclic wetting-drying and natural precipitation. Higher levels of nitrogen increased only N and P contents in the plant.

H. K. Pande

Papers

Measurement of Leaf Area of Rice1

Leaching Loss of Nitrogen in Submerged Rice Cultivation

Effects of Depth of Submergence, Fertilization and Cultivation on Water Requirement and Yield of Rice

Effects of Moisture and Nitrogen on Growth, Yield and Mineral Content of Rice

Water and fertility management of rice varieties under low atmospheric evaporative demand.