The Fate of Fertilizer Nitrogen Applied to the Paddy Field and its Absorption by Rice Plant : VI. Influence of a thermal factor on the soil ammonium nitrogen and the absorption of nitrogen by rice plant

Abstract

The influence of a thermal factor on the soil ammonium nitrogen and its absorption by rice plants grown under the field and controlled conditions were studied. It was shown that the absorption of nitrogen by the rice plants was closely related to the accumulated effective thermal index (AETI) given by T. Hanyu and T. Uchijima (1962). The amount of nitrogen per unit area absorbed by the plants (y) grown in the field was closely related to the AETI consisting of an exponential equation (y=cdx) for the early growth stage and linear one (y=ax+b) for the middle and late growth stages. The crossing point of these equations was the AETI of about 400. The similar equations excepting the b parameter were obtained for both low and high dosages of basal nitrogen. But the crossing point of these equation was varied by the cultural conditions, especially plant density and placement of fertilizer. Soil ammonium nitrogen decreased exponentially after transplanting. Almost all of the nitrogen disappeared at the AETI of about 400, or at the commencement of ear-primodia formation. Therefore, it was assumed that the rate of nitrogen absorption of the rice plants was the limiting factor in the exponential part. On the other hand, the rate of supply of soil ammonium nitrogen, or the rate of mineralization of soil organic nitrogen was assumed to be the limiting in the linear part.