EVALUATION OF CRUSTING SUSCEPTIBILITY OF SOME JAPANESE SOILS BASED ONWATER PERMEABILITY AND MORPHOLOGY OF CRUSTS

Crusting susceptibility of 19 soils collected from upland fields in Ho kkaido, Kinki, and Shikoku districts was evaluated based on microscopi c observations, morphological parameters using the thin section-image analyzing technique (Tanaka and Kyuma 1992: Soil Sci. Plant Nutr., 38, 369-373) and water permeability after subjecting the samples to simul ated rainfall. The morphology of the soil surface after the simulated rainfall treatment was classified into four types: type A with less di sintegrated aggregates, thus, less crusted; type B vith a relatively d ense band, so-called ''washed-in layer'' (McIntyre 1958: Soil Sci., 85 , 185-189), composed of disintegrated small aggregates at a few millim eters below the surface; type C with densely compacted layer and a ''s eal skin'' (McIntyre 1958: Soil Sci., 85, 185-189) composed of disinte grated fine particles at the surface; type D with morphological featur es similar to those of type C but lacking a ''seal skin.'' Water perme ability parameter, -log K value, showed a high correlation with aggreg ate instability, r=0.815* (n=19), in accordance with the morphologica l types of crust. Morphological parameter (Tt/APt ratio) was also corr elated with the -log K value (r=0.656*, n=19) and aggregate instabili ty (r=0.517*, n=19). Among the parameters, the water permeability (-l og K value) was considered to be a suitable and practical measure for determining the degree of crusting, and, therefore, for comparing the crusting susceptibility among the soils. Based on the -log K value, th e potential crusting susceptibility of soils studied decreased in the following order: type C>type D >> type B>type A.