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Measurement of Field Capacity with a Neutron Meter¹

ABSTRACT

An experiment to determine the moisture vs time curve, and hence the field capacity, was carried out at 6-inch increments to a depth of 5 feet on 4 soils. Plots of soil were soaked with water and a soil moisture meter employing neutron scattering used to determine the moisture content of the soil profile at different times following water entry into the soil. The data were plotted as curves of soil moisture content on a volume basis vs time in hours following wetting. The general shape of the curves was about as expected for 2 silt loam soils and 1 sandy loam soil, but certain layers in these 3 soils deviated from the normal pattern. The deviations are explained on the basis of physical conditions of the soil existing some distance from the soil layer in question. A clay loam soil showed extreme variability, with little movement of appreciable quantities of water; a field capacity for it could not be defined. The neutron meter used was found to be an excellent device for this type of study; confounding of soil variation in sampling errors was, through its use, eliminated.

¹ Journal paper No. J-3255 of the Iowa Agr. and Home Economics Exp. Sta., Ames, Iowa. Project 1276. Contribution from the Department of Agronomy (Soils). Presented before Div. I, Soil Science Society of America, Atlanta, Ga., Nov. 21, 1957.

² Graduate Assistant, and Professor of Agronomy (Soils), respectively. The senior author is now with the Eastern Soil and Water Management Research Branch of the Agricultural Research Service, Ames, Iowa.

Received for publication September 16, 1957. Accepted for publication October 10, 1957.