The authors proposed the "Coefficient of structural development (Csd)" as the index to evaluate numerically the structural development of clayey soils in the previous papers. In this paper, the significance of Csd from a viewpoint of soil physical properties was investigated for thiry-five clayey soils in which eighteen were nonvolcanic ash soils and seventeen volcanic ash soils. The relations between the soil structural development and factors such as clay content, humus content etc were

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... igated, and the methods to improve soil structure were discussed. Results obtained are summarized as follows. 1) Csd showed positive correlations (significant at 0.01 or 0.05 level) with saturated hydraulic conductivity, water content under pF 3, gravitational water content (under pF 1.8) and available water content (pF 1.8-,3). (Figs. 1-3, Table 2) 2) On the basis of desirable soil physical conditions for upland field, the following relations between Csd values and the degree of soil structural development were obtained : for Csd value <1. 5, ill-developed: for Csd value 1.5-1.8, normal; and for Csd value >1. 8, well-developed.