Abstract
Tillage experiments were conducted on volcanic ash soil. Treatments were as follows : I. shallow tillage, 12 cm, standard treatment. II. deep tillage, 45 cm cultivated soil mixed with subsoil. III. deep tillage, 45 cm cultivated soil. The yield of upland rice plant and soil pysical properties and nitrate formation in soil were studied. The yield of upland rice plant was the highest at treatment III and the lowest at treatment I, but the increment of straw weight was larger than that of grain weight at deep tillage plots. Some effects of deep tillage on crop yield are attributed to the fact that deep tillage gives influence on soil porosity and nitrate content. Deep tillage have been shown to increase the aeration porosity and decrease water content in deeper layer and consequently have increased the entry of air into soil, accelerated nitrate formation and allowed for greater root development.
