A novel effectual soil diagnosis method for accumulated cation availabilities was examined with a pot experiment. Two kinds of soil with different cation accumulation levels were used for four fertilizing treatments conducted in this experiment: -K, -Ca, -Mg, and without any of them. Komatsuna (Brassica cawipestris L.) was cultivated five times continuously in four replications. Growth and absorption of cations were measured. Exchangeable Ca and Mg concentrations in applied soils were very high, but that of K was relatively lower. As a result, for low potassium level, K application was effective in dry matter weight. Exchangeable K level and Mg/K ratio at the point of K deficiency symptom in first layer and second layer soils were compared. Exchangeable K level was higher in first layer soils than in second layer soils, while the Mg/K ratio was opposite. Because the values are different in each soil, we determined that K concentration and Mg/K ratio were not suitable for diagnostics index indices of K availability. Dry matter weight changes indicated that Mg/K and Ca/Mg ratios in soil solution at seeding were more effective as indices than exchangeable cations. The proper ranges of Ca/Mg and Mg/K ratios in soil solutions were found to be 1.7-2.5 and 30-200, respectively. In this proper range, it is inferred that we can stop dressing a field. Thus the cation ratio in soil solution is considered to be a useful index for a soil diagnosis method in the field.