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Hydraulic Conductivity as Related to Certain Soil Properties in a Number of Great Soil Groups—Sampling Errors Involved¹

ABSTRACT

This study was undertaken to provide information about hydraulic conductivity, percent large pores, and bulk density, their interrelationships, and the magnitude of the sampling error components involved over a wide range of soil conditions. Data from approximately 10,000 individual core samples from about 900 sites in 7 states, furnished in a cooperative project with the Soil Conservation Service, United States Department of Agriculture, were analyzed. Each sample was classified according to management, genetic unit (great soil group, high family, low family, and series), texture, and horizon.

Mean values for hydraulic conductivity, percent large pores and bulk density, when classified by textural groups, showed a consistent decrease in magnitude with increases in silt and clay content. Mean values for the three properties, when classified by great soil groups, showed consistent differences between groups. Correlations between hydraulic conductivity and percent large pores was positive and comparatively consistent; correlations between hydraulic conductivity and bulk density were negative and generally of a low absolute value. The indicated conclusion was that bulk density, in itself, is a poor indicator of soil permeability.

Variance components for among cores within sites and horizons, and between sites, after removal of all other classifiable variation, indicated that the between site component of the sampling error was 2 to 3 times larger than the within site component. These components are used to estimate "distinguishable permeability classes" for various sampling schemes, and by assuming various cost ratios, optimum sampling rates are estimated.

¹ Contribution from Departments of Experimental Statistics and Soils, North Carolina Agr. Exp. Sta., Raleigh, North Carolina. Published with approval of the Director as No. 763 of the Journal Series. Presented before Div. VI, Soil Science Society of America, Nov. 15, 1956. Received Nov. 21, 1956. Approved May 28, 1957. This investigation was initiated at the suggestion of Dr. L. T. Alexander, Chief, Soil Survey Laboratory, Soil Conservation Service, U.S.D.A., and the computations and preliminary reports were made possible by funds from that organization. Grateful acknowledgement is made to the State Conservationists, S.C.S., U.S.D.A., from the seven states that supplied the raw data for the study. The assistance of David C. Hurst, Research Assistant, Dept. of Exp. Statistics, North Carolina State College in organizing, coding, and in initiating the computations, and the aid of Dr. S. B. McCaleb in the genetic classification of the soils is gratefully acknowledged.

² Professor of Statistics, Professor of Soils, and formerly Assistant Statistician, respectively, North Carolina State College, Raleigh, North Carolina. The latter now is Exp. Sta. Statistician, Oregon State College, Corvallis, Oregon.