SOIL CHARACTERISTICS SIGNIFICANT IN EVALUATING PERMEABILITY

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Abstract
A study of 96 soil horizons of 75 different soil profiles in widely separated locations in the North Central and Southeastern states shows that the 7 classes of soil permeability, tentatively established, can be fairly accurately and consistently identified in field examinations of soils through the use of soil characteristics including structure, ease and direction of natural fracture when lumps are broken, size and number of visible pores, texture, and intensity of mottling. The investigations included a study of profiles at sites where permeability measurements had been made; the correlation of soil characteristics or clues with the 7 permeability classes; and testing the reliability of these clues for evaluating and predicting permeability classes. In order to estimate permeability each significantly different horizon of a profile must be appraised in terms of type of structure, that is, whether it is dominantly prismatic, cubical blocky, fragmental, nuciform, platy, or single grain. Type of structure alone is not sufficient; more precise information is needed on the relationship between length of horizontal and vertical axes of structural aggregates, amt. and direction (whether horizontal, oblique, or vertical) in which structural aggregates overlap, shingle fashion, one with the other, and grade or durability of the aggregates. In some sections, heavy textures go along with slow permeability and light textures with rapid permeability. A soil may be mottled regardless of its permeability if water is held by a barrier or is the result of seepage. In some soils the durability of the aggregates is correlated with permeability; in others the size and number of visible pores or the direction of natural fracture, when lumps are broken, are significant clues. Seldom can permeability be evaluated on the basis of one characteristic alone. Usually all factors mentioned must be considered singly and in relation to one another.

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