This method involves saturating the soil with water and subsequent extraction under partial vacuum of the liquid phase for the determination of dissolved salts. Soil moisture at the point of complete saturation is the maximum amount of water held when all the soil pore space is occupied by water and when no free water has collected on the surface of the paste. Over a wide soil textural range, the saturation percentage (SP) is approximately twice the Field Capacity (FC) or -33kPa soil water potential and is four times the Permanent Wilting Point (PWP) or -1500 kPa soil water potential for soils of loam, to clay loam texture. The soil pH may be determined directly on the paste. From the saturated paste extract, estimates of ECe, and solution concentrations of Ca2+, Mg2+, K +, Na +, Cl -, B, HCO3-, CO32-, SO4 estimate (actual measurement is total S in extracts), SAR and ESP can be made. The method is generally reproducible within 8%.
Rhoades, J. D. 1982. Soluble salts. p. 167-179. In: A. L. Page et al. (ed.) Methods of soil analysis: Part 2: Chemical and microbiological properties. Monograph Number 9 (Second Edition). ASA, Madison, WI.