2011 Volume 80 Issue 1 Pages 96-102
We analyzed the quality of the groundwater for irrigation at the Mibu plateau which is the most active region for cultivation of upland crops in Tochigi Prefecture. Our results showed that although the concentration of nitrate nitrogen in the groundwater did not exceed the environmental standard (10 mg L-1), the concentration was higher than that in other areas in Tochigi prefecture. The results obtained from the experiments using the δ15N value method and the land use history suggested that the chemical fertilizer greatly influenced the water quality of the groundwater. In a different experiment, the yield of bottle gourd and the leaching of nitrogen from the soil in the plots that had been treated with standard fertilizer were compared with those in the plots that had been treated with heavy fertilizer. The yield of bottle gourd in the heavy fertilizer-treated plots was 10% higher than that in the standard fertilizer-treated plots. From the viewpoint of cost performance, heavy fertilization had no economic advantage because the increase in income from the high yield was equivalent to the increase in cost for fertilizer. The percentage of leached fertilizer nitrogen was 19.6% in the standard fertilizer-treated plot and 39.6% in the heavy fertilizer-treated plot. Crop cultivation using heavy fertilizer is considered to be the cause of the pollution of groundwater with nitrogen.
