Abstract
From a result of soil analytical data, it was ascertained that no suppression soil of potato scab occurrence had had a condition of low exchange acidity y_1. And potato scab was suppressed with a high level of exchange acidity y_1 by decreasing soil pH at an open-air field experiment on no suppressible potato scab soil. The results were as follows: 1) The severity of potato scab occurrence varies with different soil groups at the same soil pH. Soil groups can be separated to easily controllable areas and incontrollable areas of potato scab by exchange acidity y_1. The exchange acidity y_1 reaches 7-8 at soil pH 5.3 in the high level soil groups of Yotei and Kuriyama areas in which potato scab is easily controlled by simply decreasing soil pH. Few to no cases of potato scab are observed below soil pH 5.3 in the soil groups. 2) On the other hand, exchange acidity y_1 in the low level soil group is only 3.8 soil pH 5.3; here, potato scab is incontrollable. The soil pH of about 4.5 is needed for exchange acidity y_1 7-8 in the low level soil group of Abashiri-Shari area. 3) The exchange acidity y_1 is a practical measure of exchangeable aluminum (ex. Al) which is analyzed from same 1N KCl extractions with exchange acidity y_1 in soil samples. The correlation coefficient was found high level value of 0.99 between exchange acidity y_1 and ex. Al. Exchange acidity y_1 0.6 is almost 0 mg kg^<-1> of ex. Al, and exchange acidity y_1 7=ex. Al 44 mg kg^<-1>, exchange acidity y_1 10=ex. Al 65 mg kg^<-1>. The equation is ex. Al=-4.31+6.9 exchange acidity y_1. 4) The averages of Si/Al mol. ratio are 0.23 in all soil types in Niseko, Kuriyama areas, and 0.48 in ordinary Andosols, 0.54 in Acid Brown Forest soils, 0.67 in Brown Andosols of Abashiri area where there is the worst area for potato scab suppression. 5) At the open-air field in Brown Andosols, the y_1 reaches 7-3 at soil pH4.5~4.6 during potato tubering with aluminum sulfate and ferrous sulfate of 4g kg^<-1> added to the soil, and potato scab could be suppressed in Abashiri-Shari area. an exchange acidity y_1 that is too high hinders the growth of potato plants.
