UBC Theses and Dissertations
Study of measurement of soil moisture by the plaster of Paris block method Day, John Howarth
The Plaster of Paris Block Method for measuring soil moisture is apparently affected by three factors, (l). Calibration of the blocks in terms of block resistance-soil moisture, (2) Soil temperature, (3) Soluble salts in the soil solution. Experiments designed to evaluate the method and to study these factors have revealed that (l) the Plaster of Paris Block Method is suitable for measurement of soil moisture. It operates satisfactorily over the critical moisture range and shows its greatest sensitivity in the vicinity of the permanent wilting percentage, (2) a major source of variation in the method is due to the Plaster of Paris blocks. In selecting blocks for use their performance should be checked at both low and high resistances. The most satisfactory calibration and performance of the Plaster of Paris blocks is dependent upon the presence of roots of transpiring plants in the immediate vicinity of the blocks, (3) soil temperatures may be estimated in situ with the same resistance bridge used with the Plaster of Paris blocks. Resistance thermometers found satisfactory for this purpose are Thermistors type 14A, manufactured by the Western Electric Company, (4) Electrolytes in the soil solution affect moisture estimates made with the Plaster of Paris Block Method . The magnitude of the effect produced with Na₂S0₄ over the critical moisture range is 0.3 to 0.4 percent decrease in moisture estimate with each 500 p.p.m. increase in salt. The magnitude of the effect is apparently the same in soils of differing texture.