Acidification and soil productivity of allophanic Andosols affected by heavy application of fertilizers

The acidification of allophanic Andosols by fertilizer application in relation to soil productivity was studied with special reference to the soil colloidal composition. Among the Japanese cultivated allophanic Andosols, in 95% of the samples, the exchange acidity y(1) was < 6, while in 5% of the samples (30 soil samples) the exchange acidity y(1) was >= 6. The strongly acidic allophanic Andosols (exchange acidity y(1) >= 6) did not significantly differ from the weakly acidic allophanic Andosols in the contents of amorphous materials. However, the mean values of the strongly acidic allophanic Andosols for the soil pH and base saturation which were 4.5 +/- 0.3 and 15 +/- 9% respectively, were relatively lower than those of weakly acidic allophanic Andosols. The content of KCI-extractable Si in the strongly acidic allophanic Andosols dominated by allophane (exchange acidity y(1) >= 6 and allophane / clay content ratio > 0.7) ranged from 51 to 141 mg kg(-1), and the values increased with increasing concentrations of KCI-extractable Al. KCI-extractable Al in the cultivated allophanic Andosols was considered to be derived from both the allophane fraction and Al-humus complexes. In the field studies of allophanic Andosols with successive application of fertilizers conducted at the Fujisaka Branch of Aomori Agricultural Experiment Station, the value of the exchange acidity y(1) of the topsoil and subsoil exceeded 6 about 40 and 50 years after the start of the experiment, respectively. The yield of flint corn (Zea mays L. var. indurata Bailey. cv. Onoa) in the fertilizer plot decreased by the strong soil acidification. The amount of nitrogen mineralized in the soil of the fertilizer plot during a 280-d incubation was 0.114 g kg(-1) in the topsoil and 0.041 g kg(-1) in the subsoil. These values were relatively low compared with those in the lime + fertilizer plot. Thus, we confirmed that the subsoil fertility of the fertilizer plot decreased by heavy application of fertilizers.