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Japanese Andisols are volcanic ash soils that have characteristic physical properties such as low bulk density, high natural water content, high water content at 15 bars, and high liquid limit at the natural water content. They are classified as allophanic Andisols or crystalline Andisols according to the major clay minerals: chiefly allophane and imogolite for allophanic Andisols and Al-vermiculite, chlorite, small amounts of halloysite, and traces of gibbsite for crystalline Andisols.

The physical properties of Andisols are changed markedly and irreversibly on air-drying. Andisols can be classified into two types on the basis of the decrease in liquid limit on drying. A-type Andisols exhibit a large decrease in flow index, which is the slope of the shear strength-water content curve in the liquid limit test. B-type Andisols show no change in flow index with decreasing liquid limit on air-drying. Moreover, the decrease in liquid limit of A-type Andisols is larger than that of B-type Andisols.

Allophanic Andisols without a past history of drying and wetting, and a few crystalline Andisols with high organic matter content, belong to the A-type. Many crystalline Andisols and allophanic Andisols with a past history of drying and wetting belong to type B. Drying and wetting occur under natural conditions at the soil surface, but some subsoils may remain permanently wet.

The differences in physical properties between A- and B-type Andisols also show in the relationship between dry bulk density and natural water content, dry bulk density and water content at 15 bars, and plasticity index and liquid limit.

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