A review of the usefulness of relative bulk density values in studies of soil structure and compaction

The state of compactness is an important soil structure and quality attribute, and there is a need to find a parameter for its characterization that gives directly comparable values for all soils. The use of some relative bulk density value for this purpose, particularly the degree of compactness (Hakansson, 1990), is discussed in this review. The degree of compactness has been defined as the dry bulk density of a soil as a percent of a reference bulk density obtained by a standardized uniaxial compression test on large samples at a stress of 200 kPa. The bulk density should be determined at standardized moisture conditions, to prevent problems caused by water content variations in swelling/shrinking soils. The degree of compactness (D) makes results of soil compaction experiments more generally applicable. Whereas the bulk density or porosity optimal for crop growth vary greatly between soils, the optimal D-value is virtually independent of soil composition. Critical limits of penetration resistance (3 MPa) and air-filled porosity (10%, v/v) are similarly related to the D-value and matric water tension in most soils. As the D-value increases above the optimal, the tension range offering non-limiting conditions becomes increasingly limited. The D-value of the plough layer induced by a given number of passes by a certain vehicle is similar in all soils, provided the moisture conditions are comparable. The degree of compactness facilitates modelling of soil and crop responses to machinery traffic. Although this parameter was primarily introduced for use in annually disturbed soil layers, its use may be extended to undisturbed soil layers.