The Structure of Soils as Affecting Soil Erosion

Since soil structure undoubtedly is one of the important factors influencing soil erosion it is desirable to determine those structural properties that enhance or diminish the erosiveness of soils. In studying these properties in this investigation, two soils, one a very erosive soil — the Iredell sandy clajl loam — and a non-erosive one — the Davidson clay — were selected. These two soils are characteristic of the basic crystalline rock division of the Piedmont Plateau region of North Carolina. They have usually been considered as weathering from "the basic crystalline rocks'' without regard to any particular kind of rock. Marbut (2), however, has suggested that the Davidson comes from a more basic rock than does the Iredell. They occur in adjacent areas and, consequently, practically all of the external factors except topography which might affect erosion, are constant; the topography is such that it should favor erosion more on, the Davidson. That the Iredell is more erosive than the Davidson is a general observation which has confirmation in the work of Middleton (3). He calculated erosiveness from data on dispersion and moisture equivalent measurement?, obtaining what he calls the "erosion ratio." The value of this was found by him to be two times higher in the A' horizon and four times higher in the B horizon of the Iredell than in the corresponding horizons of the Davidson. The work of Slater and Byers (4) on the percolation of water through these soils showed that the rate of percolation was 34.2 times greater through the Davidson than through the Iredell if the samples were not disrupted in collecting, and 15.2 times greater if they were disrupted. The data presented here explain, in part at least, why there is such a difference" in the rate of percolation. First, the Davidson is highly aggregated as compared with the Iredell. This is clearly shown in Table 2, and especially in graphs 1 and 2. The degree of aggregation is used as an index of the structure. It was determined by elutriation according to the method described by Baver and Rhoades (1). The percent of the soil fractions less than .05 mm in diameter which is aggregated was computed as follows: