IMPORTANCE OF EXCHANGEABLE AND NONEXCHANGEABLE SOIL NH4+ IN NITROGEN NUTRITION OF LOWLAND RICE

Abstract

In lowland rice (Oryza sativa L.) soils, NH4+-N is considered the most important inorganic nitrogen form. Therefore, we examined the influence of NH4+ fertilizer application and N uptake of the crop on the level of exchangeable and nonexchangeable soil NH4+ Field experiments at three sites in major rice-growing areas in the Philippines showed that NH4+ fertilizer application was clearly reflected by an increase of exchangeable and nonexchangeable soil NH4+, while N uptake resulted in a decrease of both NH4+ fractions. We found major NH4+ contributions from nonexchangeable soil NH4+ only in Maligaya silty clay loam, which is rich in vermiculite. On all soils in the wet and dry seasons the exchangeable soil NH4+ was depleted during the first 8 wk of crop growth, indicating that this fraction is easily available to the crop. The net release of exchangeable and nonexchangeable NH4+ throughout the growth period ranged from 0 to 100% of the N uptake of rice and depended greatly on soil and weather conditions. In the wet season most of the N taken up by the crop could be accounted for by the net release of NH4+, whereas in the dry season most of the N taken up by the crop originated from soil organic N. © Williams & Wilkins 1985. All Rights Reserved.