Study on Soil Moisture Measurement by Electrical Resistance Method (I)

Abstract

In this paper, a report is given on some experimental results of practical application of the electrical resistance method with gypsum blocks to measurement of soil moisture in fields and in pots. BOUYOUCOS moisture meter was used to measure the block resistance. A simple sectional wooden mold was deviced for making the gypsum blocks. Blocks made by mixturing 40 gm gypsum with 23 ml of distilled water were found most suitable for measuring wide ranges of soil moisture content (Fig. 1). The uniformity-test or grouping of the blocks was carried out by means of the calibration curves showing the relation between the electric resistance and the moisture content of them during the course of drying under a definite temperature (Fig. 2). When dry blocks were installed into soils of moderate wetness, their resistance reached an equilibrium at 24 hours after installation (Fig. 3). And after irrigation of 6 mm water, blocks installed 3 cm below the soil surface reached the resistance equilibrium after 150 minutes. Even in the blocks of which 36 % of their original weight had diminished their other characters did not show any considerable changes compared with the unused blocks (Fig. 4). It was confirmed that the block resistance changed well in accordance with its moisture content on the basis of oven - dry weight. Calibration tests on the relation between block resistance and soil moisture content on the basis of oven-dry weight were carried out on field, in concrete-pots (2 ft × 2 ft × 1 ft) and in wire-baskets (12 cm × 8 cm × 6 cm). The results proved generally very close agreements between the resistance-moisture relation on field and that in pots. Between those in pots and in wire-baskets, the agreement was found also well, except 4∼5 % difference in soil moisture content at maximum being observed.