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In Situ Determination of Physical Properties of the Surface Layer of Field Soils¹

ABSTRACT

A set of gamma ray attenuation vs. soil water matric potential data was obtained in situ by saturating an undisturbed block of soil, and by lowering the water level in steps and measuring the count rate through the soil and the corresponding water matric potential at each step. Relative water contents were calculated from the count rates by use of a rational calibration equation. The corresponding matric potential was measured with a low impedance tensionmeter. A partial water retention curve was plotted from these data, from which in turn the air intrusion value and the aeration porosity were inferred. It was shown that collimation is essential for measurements within the 1-cm thick surface layer, but it is not required for measurements within the bulk of a layered system. With a 5-mc cesium-137 source, a collimated beam and a 4-min counting time, the precision was ±0.6% water content due only to errors associated with random emission. There was good agreement between a partial water retention curve obtained in situ and the one obtained with an outflow method in the laboratory.

Contribution from the Department of Soil Science, University of British Columbia. This work was supported in part by National Research Council (Canada) grant no. A2556.

² Assistant Professor, University of British Columbia, Vancouver, Canada.

Received for publication September 26, 1968. Accepted for publication January 10, 1969.