Study on measures to prevent fine red soil particles from runoff using chemical fertilizers

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This study was done with the Grant-in-Aid for Scientific Research.It became clear that chemical fertilizers dispersed up soil particles and prevented soil particles from settling. Because chemical fertilizers make soil particles fine, soil runoff from farmlands is accelerated, and sedimentation in settling ponds is prevented. As a result, fine soil particles flow out from farmlands and settling ponds, and then seacoast is polluted by red soil. In the experiment of the dispersion of soil by chemical fertilizers, soil particles were sifted in the pure water and the water with chemical fertilizers, and the size distributions of the waterproof aggregation of two conditions were compared.As a result, it was recognized that fine particles smaller than 0.25mm increased by adding chemical fertilizers. In the experiment that chemical fertilizer was added to water with Kunigami maji soil, it was confirmed that soil particles were dispersed by adding little chemical fertilizers and cohered by adding much.The reason of this phenomenon was supposed that electric charges changed from positive to negative by the specific absorption of the phosphoric acid, and negative electric charges increased, and fine soil particles were dispersed. However, by adding more phosphoric acid, the specific absorption of the phosphoric acid was saturated, and negative electric charges stopped to increase, and positive electric charges increased by adding more phosphoric acid, and negative charges neutralized with positive charges. This process is supposed the reason of the cohesion.