Study on the potential of farmland soils as non-point sources of nitrogen and phosphorus in Japan

The amounts of N and P accumulated in farmland soils of 50 cm depth were equivalent to the amount of chemical fertilizer supplied for 50–70 years. The values of N/P of surface soils in farmlands were 1.0–4.3, lower than expected. The median diameter of soil particles in run-off waters was generally less than 10 μm. The mean values of particulate fractions over 1 μm and over 0.22 μm were 19% for N, 27% for P, and 39% for N, 64% for P respectively. Fine particles of soil containing concentrated phosphorus should be carefully monitored as potential sources related to eutrophication.