Glossary:Saturated Paste

The mixture of soil and water which occurs when all soil pores are just filled with water. In undersaturated soil, deionized water is added to a soil sample with minimal mixing until all soil pores are filled with water and there is negligible air in the pores. Mixing is minimized to retain natural pore size distribution as much as possible and to minimize expansion of expandable clay minerals such as smectite and vermiculite. The percent of soil-pore water weight to dry soil weight at this precise moisture content is called the saturation percentage, and this is unique for each different soil due to different types, sizes, and shapes of solid constituents. The saturated paste moisture content is useful as a reference because it represents the actual concentrations and ratios of dissolved constituents which are available for uptake by plant roots.