Determinations of bulk density and water retention by soil cores, fragments, and sieved samples

Characterization of the water holding properties of soils has increased in importance. Water in soil is held against gravity by soil suction forces that can be simulated in the laboratory and measured in terms of equivalent negative pressure. The suction force, and measurements of the suction force, have been called the soil moisture tension. When free drainage from a wet soil ceases, moisture tension is usually within the range of 0.06 to 0.33 bar and the water content is called field capacity. Water held at tensions between field capacity and 15 bars is generally considered available for plant growth. For most purposes water holding properties of soils are usually expressed in terms of the volume of water held per unit volume of soil at specified energy levels. Water retention measurements of sieved samples are on a weight basis and expression on a volume basis is possible only when the bulk density (dry weight per unit volume of undisturbed soil) is known. Predictions of water holding capacity of soils at specified tensions from measurements with sieved samples and soil cores equilibrated at similar tensions may be erroneous. Crushing and sieving soil may alter the pore size distribution, hence the amount of water held by a sieved sample may differ significantly from the amount held by an undisturbed sample of equal weight. Predictions of water content on a volume basis from bulk density of undisturbed cores and water content on a weight basis (bulk density x per cent water by weight) may also be inaccurate. The bulk density of cores at the water content at sampling may not be equal to the bulk density of soil at a different water content, i.e. at the predicted tension. This study was initiated to (1) develop a method to measure water content and volume of undisturbed soil fragments after equilibration to a moisture tension in the field capacity range, (2) to study the relationship between change in the bulk density of soil with a change in water content, and (3) to study the effect of alternation of pore size on water retention by soil samples equilibrated to a moisture tension in the field capacity range.