The importance of initial exchangeable ammonium in the nitrogen nutrition of lowland rice soils

Abstract

The importance of initial exchangeable soil NH 4 + in nitrogen nutrition and grain yield of rice was studied in a number of representative lowland rice soils in the Philippines. The initial exchangeable soil NH 4 + +fertilizer N plotted against nitrogen uptake by the crop resulted in a highly significant linear relationship (R2=0.91), suggesting that the presence of exchangeable NH 4 + in the soil at transplanting behaved like fertilizer nitrogen. The correlation between N fertilizer rate and N uptake by the rice crop was relatively poor (R2=0.73). On the other hand, relative grain yield was more closely correlated with the initial exchangeable soil NH 4 + +fertilizer N than with fertilizer nitrogen applied alone. These results indicate that the initial exchangeable NH 4 + in the soil contributed substantially to the nitrogen uptake of the crop. Critical nitrogen levels in the soil defined as the initial exchangeable soil NH 4 + +fertilizer N at which the optimum grain yield (95% of the maximum yield) is obtained, varied from 60 to 100 kg N/ha in the wet season and from 100 to 120 kg N/ha in the dry season for the different fertilizer treatments. The results further suggest that the initial exchangeable soil NH 4 + should serve as a guide in selecting an optimum nitrogen fertilizer rate for high grain yields.