(1) Applying the plate model to soil system, equations were deduced which express the leaching and migration of an ion through soil layer by downward movement of water.
(2) The leaching and migration of an ion applied to the soil surface are expressed by the equation of Poisson distribution, and those of an ion uniformly distributed in the whole layer, by cumulative Poisson distribution. On the other hand, when a solution containing an ion in a constant concentration is supplied continuously to the soil, accumullation and migration of the ion in soil is expressed by the equation showing the difference between unity and cumulative Poisson distrbution.
⑶ Attempts were made to test the applicabity of the equations to leaching and migration of ammonium ion placed initially at the soil surface. The results showed that the plate model is applicable to soil system in the first approximation. The limitations of plate model were briefly discussed.