The Fate of Fertilizer Nitrogen Applied to the Paddy Field and its Absorption by Rice Plant : IX. The fate of soil ammonium nitrogen and absorption of nitrogen by rice plants in four different soil types with special reference to the accumulated effective thermal index

Abstract

The comparative study was conducted on the behavior of soil ammonium nitrogen and absorption of nitrogen by rice plants in four experimental fields with special reference to the accumulated effective thermal index (AETI). The soils of these fields were classified into Gray-Brown Soil, Peat Soil, Strong Gley Soil and Wet Andosol. The decrease of soil ammonium nitrogen after transplanting in all the fields was shown as an exponential equation of AETI. The amounts of nitrogen per unit area absorbed by rice plants (y) in each field were also indicated to be closely related to AETl (x). An exponential equation (y=cdx) was obtained for the early growth stage, and linear one (y=ax+b), for the middle and late growth stages. However, it was noticed that the parameters of these equations differed considerably with difference of the soil type. Since the soil ammonium nitrogen was observed to be enough for rice growing in the early stage, it was assumed that the ability of nitrogen absorption of rice plant was the limiting factor for the exponential part. On the contrary, the soil ammonium nltrogen disappeared in the middle and late stages, the rate of mineralization of soil organic nitrogen was considered to be the limiting factor for the linear part. The recoveries of basal nitrogen in the rice plants ranged from about 40% in Gray Brown Soil to 20% in Strong Gley Soil. The maximum number of tillers stage came in all the experimental fields at the time when almost all of soil ammonium nitrogen disappeared. It was also recognized that the crossing point of the exponential and linear equations was approximately coincident with the stage. Therefore, the maximum number of tillers stage is presumably determined mainly by the nitrogen supply to the rice plants under the common cultural condition.