ABSTRACT
A better understanding of the nutrient economy in relation to productivity is immensely important to develop a sustainable rice-wheat cropping system in the Indo-Gangetic Plain (IGP). Fertilizer (N, P and K) use pattern for the rice-wheat system in the IGP varies greatly at the country, state, village, and farmer field level. In general, fertilizer use is conspicuously more in the western compared to the eastern IGP. Continuous government subsidies on nitrogenous fertilizers have helped create imbalanced N, P, and K fertilizer use by farmers. Removal of P and K by the rice-wheat system far exceeds its additions through fertilizers and recycling.
Nitrogen from rice-wheat systems can often be lost via ammonia volatilization, denitrification and leaching. Nitrification acts as a key process in determining fertilizer-use-efficiency by crops as well as N losses from soils. Loss of N via ammonia volatilization can be substantial when urea is top-dressed. Placement of fertilizer N beneath the soil surface or transportation of N to subsoil layers along with irrigation water can be a useful management option to reduce NH3 volatilization losses. Water management in rice fields influences the extent of N losses due to nitrification-denitrification. Up to 50% of the applied N can be lost through denitrification when alternating aerobic-anaerobic conditions prevail in rice fields. Leaching losses of N as NO3 are minimal under wheat but can be substantial under rice grown in readily percolating coarse textured soils found in northwestern India. Irrespective 24of its source, application of fertilizer N at 120 kg N ha−1 has been the recommended level for rice and wheat in most of the Indo-Gangetic Plain. However, recent trend indicates the use of more than 150 kg N ha−1, particularly to rice grown in northwestern Indian states, where more than 10 t ha−1 grain yield per annum is being harvested from the rice-wheat system. Efforts to increase the notoriously low fertilizer N use efficiency in rice through modifications in N sources have been promising for slow release N fertilizers. In spite of the distinct advantage of using supergranules of urea in fine textured soils, its use is not popular amongst farmers due to the lack of a suitable mechanical applicator. Application of fertilizer N in three and two equal split doses to rice and wheat, respectively, has proved to be an efficient management option. Need based fertilizer N applications to rice using chlorophyll meter is promising for increasing the fertilizer use efficiency. In northwestern India, nitrate content in ground water bodies is continuously increasing and it has been linked to the inefficient fertilizer N use in the rice-wheat system.
In the rice-wheat system, response of wheat to fertilizer P application is more common than its application to rice. Moreover, the residual response of P applied to wheat on rice is greater than that applied to rice on wheat. The availability of soil and fertilizer P increases under submergence and at high temperatures prevailing during the rice season. Thus rice could meet its P requirement from the soil and the residual fertilizer P, when the recommended P fertilizer has been applied to the preceding wheat crop. Improved chemical tests for predicting indigenous soil P supply need to be developed to predict response of rice and wheat to fertilizer P in the cropping system. In neutral and alkaline soils, fertilizers containing P in water-soluble forms are more effective than those containing P soluble in citric acid. Nitrophosphates containing 60% or more of total P in water-soluble forms have been found to be as efficient as diammonium phosphate and superphophates in wheat. In some neutral and acidic soils, a part of the total fertilizer P supplied through diammonium phosphate and superphosphates can be replaced by rock phosphate.
Most of the soils in the Indo-Gangetic Plain contain illite as dominant clay mineral and are medium to high in ammonium acetate (1 M, pH 7.0) extractable K. Therefore, response of rice and wheat to applied K are generally small. Farmers apply very small quantities of K fertilizers to rice and wheat. Total annual K removal by the rice-wheat system is quite large causing depletion of soil K supply. The suitability of ammonium acetate extractable K as an index of plants available K for different soils varying in texture and clay mineralogy remains controversial. Dynamic soil test using resin capsule, which integrates intensity, quantity and delivery rate measures of P and K to rice and wheat can overcome 25many of the theoretical limitations associated with rapid chemical extraction. [Article copies available for a fee from The Ha worth Document Delivery Service: 1-800-342-9678. E-mail address: <getinfo@haworthpressinc.com> Website: <https://www.HaworthPress.com> © 2001 by The Haworth Press, Inc. All rights reserved.]
