A RAPID METHOD OF CALIBRATING VARIOUS INSTRUMENTS FOR MEASURING SOIL MOISTURE IN SITU

Abstract

A method is described for the laboratory calibration of various instruments (Bouyoucos plaster of Paris blocks, Shaw-Baver thermal units, and sorption block units of Davis and Slater), used in measuring soil moisture in situ. The unit to be calibrated is surrounded with 1/2-inch of soil contained in a wire basket lined with cloth. The soil is wet by capillarity and placed in a humidity chamber to drain. It is then removed and subjected alternately to 5 hrs.'' evaporation under laboratory conditions and 19 hrs. in a humidity chamber. This procedure is continued until the soil no longer loses moisture when exposed to the air. The wts. are recorded and the readings for the various instruments are taken each time the soil is removed from the chamber. When a wetted block of soil of dimensions 11/2 by 23/8 by 31/2 inches was subjected alternately to 5 hrs.'' evaporation under laboratory conditions and a 19-hr. period in a humidity chamber, it was found that there was a small variation in moisture content between the outside layers and the center of the block. This variation had little effect upon the shape or location of the calibration curves. When the described procedure is used, the av. moisture content of the whole soil block very closely approximates that of the soil immediately surrounding the unit being calibrated.