In order to grasp the eutrophication potential of nonpoint sources, we performed a detailed investigation on the soils of paddy field, dry field and forest in the Koto basin of Yamaguchi Prefecture. It was found that the vertical distributions of TN, TP and IL in the soils decreased exponentially with increasing the depth of the soils. The amount of accumulated N and P in the layer from 0 to 50cm was calculated. The accumulated N was 72 times of the used N-fertilizer per year in the paddy field and 51 times in the dry field. The accumulated P was 56 times of the used P-fertilizer per year in both paddy field and dry field. The percentages of soluble N in the surface soils were 2.5% for dry field and 0.1% for paddy field and forest. The percentages of soluble P were 0.8% for paddy field and dry field, and close to zero for the forest. The fraction of soluble N increased with increasing depth of the soil, whereas soluble P was mainly remarkable in the surface soil. The present investigation gives insight to the actual situation of the eutrophication potentials of nonpoint sources in agricultural land.
