黒ボク土分類のための土壌の特徴的性質

抄録

The soils formed from volcanic origin which are widely distributed in Japan, are one of the important arable soils in the country. The soils have been called as different names such as Volcanic Ash soils, Ando soils, Andosols, Kuroboku soils, Humic Allophane soils, Humified Allophanitic soils, Black soils recognized as an independent genetic soil type. The parent materials of kuroboku soils are originally from volcanic mateiials, however, all the soils from volcanic origin are not grouped into the Kuroboku soils. It is a basic principle that the kuroboku soils should not be classified only based on the parent materials, but on the soil characteristics. The most important features to be Kuroboku soils are that the soils are dominated by amorphous materials including allophane and allophane-like silicate minerals of alminium in the clay fraction. Amorphous materials in higher amounts reflect to the physico-chemical properties of the Kuroboku soils. At first, the bulk density of Kuroboku soils is lower than 0.9g/cm^3, exept for coarse textured Regosolic Kuroboku soils. Secondly, there are two types in dispersibility of Kuroboku clay : one is dispersible in a medium of hydrochloric acid, and the other is dispersible in a medium of hexamethaphosphate. The former is called as Typical Kuroboku soils, and the latter is Para-Kuroboku soils. The amounts of amorphous material of Typical Kuroboku soils are more than 20 to 30 percent, highr than those of Para-Kuroboku soils. The forms of alminium in Kuroboku soils are basic hydroxyl alminium at the pH between 7 and 4 where diverts from Al(OH)^+_2 to Al(OH)^<+2> according to the pH values of the bulk solution. Basic hydroxyl alminium is not exchangeable on the site of pH-dependent charge, resulting in low exchange acidity and low exchangeable aluminium contents for Typical Kuroboku soils. Trivalent alminium, stable in the pH lower than around 3.8, prevents clay dispersion, because the concentration of trivalent aluminium in the solution increases by the dissolution of aluminium from soil perticles. The values of exchange acidity and exchangeable aluminium of Para-Kuroboku soils are higher than those of Typical Kuroboku soils. It can be suggested that Alludands may be included in the Para-Kuroboku soils which are of low phosphate retention and low amorphous material contents. Typical Kuroboku soils have phosphate retention values (Blakemore method) of higher than 85 percent, and phosphate absorption capacity of higher than 3,500 mg P_2O_5 per 100 g soil. On the other hand, Para-Kuroboku soils have phosphate absorption capacity of 2,000 to 3,500 mg P_2O_5 per 100 g soils. It means that Para-Kuroboku soils are excluded from Andisols. The high humic horizons are denned as the horizons having more than 13 percent of organic carbon, and show a color value of 1.7 or less and a chroma of 1 in the field. The horizons are different from the melanic horizons in Andisols. According to these differentiation criteria, Kuroboku soils are classified into seven soil groups : Typical Kuroboku soils, Para-Kuroboku soils, Regosolic Kuroboku soils, Wet Kuroboku soils, Wet Para-Kurobok soils, Wet Regosolic Kuroboku soils and Kuroboku Gley soils. Typical Kuroboku soils have five subgroups : Thick High-humic, High-Humic, Thick Humic, Humic and Light-colored. The subgroups are subdivided into Active Aluminous, Allophanic, and Crystallitic, depending upon the composition of clay fraction. Para-Kuroboku soils, consist of two soils, Redeposited Ccrystallitic and Wind-blown Crystalliitc at a soil family level. Regosolic Kuroboku soils are defined as the soils which have more than 2.9 percent organic carbon (5 percent organic matter) in a weighted average of 25 cm from the surface, 2,000 mg P_2O_5 or less of phosphate absorption capacity and less than 10 percent of clay.