Degree of soil compactness is highly correlated with the soil physical quality index S

The soil physical parameter S (S-value or S-index) has been proposed as an index of soil physical quality. Soil physical quality is negatively affected by soil compaction (e.g. caused by agricultural field traffic). It has previously been shown that S decreases with increasing soil bulk density. This study investigated whether the relationship between S and soil compactness can be described by a single function that is valid across soil textures when soil compactness is expressed in terms of the degree of compactness (DC), which is relative density expressed as the ratio of bulk density to a reference density. This would provide an alternative measurement for soil physical quality that is more easily obtained than S. We also evaluated different methods for deriving reference density and tested whether reference values for S suggested in previous studies correspond to critical levels of DC reported in the literature. The relationships between S and DC were investigated for the 12 FAO/USDA soil textural classes based on pedo-transfer functions, and compared with data reported in the literature. A strong positive correlation was found between DC and In (1/S), and a unique function was found between S and DC that is valid across soil textures, with the possible exception of poorly sorted soils with a high sand or silt concentration. Experimental data on S obtained from the literature supported these findings. The reference value of S (0.035) previously proposed as a boundary between good and poor soil physical conditions was found to agree well with the level of DC (87%) reported in the literature as critical with respect to plant growth. Proctor density was found to be the most useful measure of reference density, better than Hakansson reference density, which introduced some texture dependency into the relationship between S and DC. Our findings indicate that 1/S is a good measure of soil compactness and support the usefulness of S as a soil physical quality index. However, our findings suggest that DC can also be used as an index of soil physical quality, and is much easier to obtain than S. (C) 2016 Elsevier B.V. All rights reserved.