A Comparison of Electric Resistance Units for Making a Continuous Measurement of Soil Moisture under Field Conditions.

Since its introduction in 1940, the plaster of Paris block method of measuring soil moisture by means of electrical resistance (4) has been widely employed in agricultural fields. Hydrologic research2 (16), war production problems involving guayule culture (11, 12), and many other investigations ranging from field irrigation to greenhouse studies have made use of this technique. Briefly, the method consists of imbedding within the soil a plaster of Paris block containing two electrodes. At a constant temperature, the electrical resistance of this absorbent material varies with its moisture content, which, in turn, varies according to the magnitude of the forces exerted by the surrounding soil. A determination of block resistance thus gives a measure of these soil moisture forces, a measurement which can be made with a high degree of accuracy and reproducibility within the moisture range that is of significant importance to growing plants (12). Fundamental principles