Erodibility indices for wildland soils of Oahu, Hawaii, as related to soil forming factors

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Abstract
Soil samples collected along 31 transects in the watersheds of the Koolau and Waianae ranges were analyzed in the laboratory for the size distribution of water‐stable aggregates and suspension per cent. Indices of soil erodibility were related by principal component analyses with varimax rotation to these 7 soil‐forming factors: parent material, rainfall, elevation, vegetation type, slope, aspects, and zone. Parent rock material was the most important factor in explaining variation of water‐stable aggregates of soils in Hawaii. Nevertheless, differences in water‐stable aggregates were also associated with differences in vegetation type and other soil‐forming factors. Most of the soils studied were rated as slightly to moderately erodible under full vegetative cover. The loose condition plus the dispersive and slaking characteristics of volcanic ash soils suggest that they may be twice as erodible as soils of basaltic flow or of the colluvial origin. Among the soil characteristics that serve as indices of erodibility, the suspension per cent was found to be independent of other measures; hence, it may be expected to serve well as a part of an erodibility ratio, such as suspension‐mean weight diameter or a surface‐aggregation ratio. Conversion of the native koa‐ohia scrubby forests by planting paperbark or silk‐oak trees promises to result in development of less erodible soils as well as more useful forest products.