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A Key for Evaluating Soil Permeability by Means of Certain Field Clues¹

ABSTRACT

Field clues to soil permeability were investigated at 182 locations from 1947 through 1951. Estimates of permeability were made in the field without prior knowledge of measured percolation rates. Then the estimated permeability class was compared with the measured rate. In case of discrepancy, the profile was re-examined to see if any signfiicant clues had been overlooked. It was found that permeability could be estimated, with fair precision, from information obtained in the field, in terms of seven classes that are defined in terms of saturated percolation rates.

The first step is determination of the type of structure. Then the class of permeability is estimated from four principal clues and one or more of eight secondary clues. Types of structure and structureless conditions found significant are fragmental, platy, nuciform, cubical blocky, prismatic, single grain, and massive. Principal clues are relative dimensions, horizontally and vertically, of structural aggregates; amount and direction of over-lap of the aggregates; number of visible pores; and texture. Important secondary clues are compaction, direction of natural breakage, silt content, cementation, type of clay minerals, character of coatings on aggregates, degree of mottling and certain features of climate. None of these clues, when taken singly, is a reliable indicator of permeability, but each must be considered with reference to the others.

¹ Presented before Section VI, Soil Science Society of America, State College, Pa., August 28, 1951.

² Soil Scientist, Soil Conservation Surveys Division, S.C.S., U.S.D.A., Washington, D. C. The author gratefully acknowledges the assistance of many persons in the Soil Conservation Service who made measurements of percolation rates, participated in the development of field clues, and offered valuable suggestions relative to assembling of data.