Nitrogen fertilizer application is a management practice that can be easily modified by farmers in terms of time and method of application. The method of application would considerably affect N fertilizer use efficiency (NFUE), which indicates how much proportion of N applied as fertilizer is utilized by the crop. To minimize the amount of N fertilizer which is not utilized by crops, in other words to increase NFUE, timing of application should be well synchronized with patterns of N supply from the soil and with crop requirement. In regions where intercropping is commonly practiced, most farmers do not apply, or apply very low doses (less than 25 kg N ha-1), of N fertilizer, because of economic, logistic and social reasons. When N is applied, the farmers prefer basal to delayed applica tion because they believe the crops require N for early growth. As shown in Fig. 8.3, however, an appreciable amount of N is available to the crops at the time of planting. Obviously a small dose of N at planting will be diluted by the soil N pool, leading to low efficiency for crop utilization. Thus, timing of nitrogen application becomes very important in low input farming systems.12
Delayed urea-N application results in a higher NFUE in sorghum than a basal application (Fig 8.4). The NFUE of sole crop pigeonpea is higher (14.6) than that of intercrop pigeonpea (1.8-3.9), because fertilizer is usually applied only to the sorghum rows in the case of intercrop treatment. Delayed N fertilization also enhances the dependency of pigeonpea on atmospheric N2. Grain yields and total N of sorghum in sole crop and intercrop are increased by delayed N application.8
Water and soil nutrients, especially nitrogen, are the two major natural resources whose internal and external supply are limited in SAT. For stabilization and increase in crop productivity, it is important to improve the utilization efficiency of these limited resources. In the case of nitrogen, approaches to increase nitrogen use efficiency may be:
1. To improve nitrogen uptake efficiency of crops through conventional and molecular breeding;
2. To increase the gain from biological nitrogen fixation; and
3. To alter the balance between uptake and other components of nitrogen loss by denitrification, volatilization, runoff and leaching.
The crop and resource management options for improvement of nitrogen use efficiency in SAT will include:
1. Changing fertilizer application methods such as timing and placement;
2. Selecting varieties with deeper roots; and
3. Developing a cropping system best suited to the local environments.
It is an extremely difficult task to improve productivity of the dry-land agriculture, but reconsideration of crop and resource
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Fig. 8.4. Effect of basal (BAS) and delayed (DEL) applications of urea on nitrogen use efficiency of pigeonpea and sorghum in monocropping and intercropping.
management in relation to the cropping system may provide resource-poor farmers with a new option, which may at least stabilize crop production and hopefully increase return on the investment.
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