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Estimating Permanent Wilting Point in the Field by a Direct Laboratory Calibration¹

ABSTRACT

A method is described whereby soils may be calibrated with fiberglass soil moisture units at 15-atm. tension using the pressure membrane apparatus.

Moisture units to be installed in the field were first calibrated in a shallow pan with a bulk soil sample taken from the site of the proposed field installation. Ten units were selected for further laboratory calibration and the remaining ones installed in the field. Soil samples were spooned into rubber rings, the moisture units imbedded in the soil, and the assemblies soaked overnight in distilled water. The soils and units were subjected to 15-atm. tension for 8 days, after which resistances were determined. Moisture content of the soils was determined in order to validate the run. This procedure was repeated with moisture units rearranged and placed in different soils. Statistical analysis indicate whether or not separate means are required for all soils.

Field moisture curves indicate that under the conditions studied, resistance readings in the surface soil (2-inch depth) may rapidly pass through the wilting range (mean 15 atm., ± 1 standard deviation). Deeper installations in vegetated areas indicate leveling-off in the wilting range.

¹ Contribution from the Department of Soils and Forest Exp. Sta., Oregon State College, Corvallis, Oregon. Presented before Div. V-A, Soil Science Society of America, Cincinnati, Ohio, Nov. 13, 1956.

² Instructor, Forestry Dept., Ohio Agr. Exp. Sta. formerly Graduate Research Assistant, Soils Dept., Oregon State College. The author wishes to acknowledge the aid rendered by L. D. Calvin, Oregon State College, and C. R. Weaver, Ohio Agr. Exp. Sta., for assistance in statistical analysis. The author expresses his appreciation to C. T. Youngberg for his help and guidance. This study was made possible through funds provided by the Oregon State Board of Forestry and the Forest Protection and Conservation Committee.

Received for publication December 3, 1956. Accepted for publication January 9, 1958.