Some Limitations in the Use of the Air Picnometer for Soil-Porosity, Soil-Moisture, and Particle-Density Determinations

Abstract

The air picnometer does not appear to be suitable for accurately determining either the particle density, the porosity, or the moisture content of soil samples. However, it appears well suited to making rapid, approximate field estimates of soil porosity and moisture status. In dry soils, gaseous adsorption on the surface of dry colloids introduces a serious error in picnometric determinations. In moist soils gaseous solution in soil moisture while under pressure in the air picnometer and "packing" of water molecules on colloidal surfaces introduce a similar error. Structure also exerts a marked influence on picnometric determinations either by modifying the area of air-water interface, thus affecting the amount of air dissolved in the soil moisture, or through the effect of structure on the quantity of air which may be trapped and thus not be subject to pressure changes during the determinations. The air picnometer has various innate weaknesses. It must be checked constantly for leakage. Since changes in atmospheric pressure affect results, calibration curves must be prepared for use at different pressures. The air picnometer must also be recalibrated periodically as variations occur with usage. Data are presented showing the results of particle-density, soil-porosity, and soil-moisture determinations made by various methods on several soils of Puerto Rico. The relative merits of the various methods are discussed.