Soil physical quality, S, as a basis for relationships between some key physical properties of arable soils

Structure is the most important soil physical characteristic for all aspects of soil use and management due to its great impact on other soil physical properties. A new measure of soil physical quality, S, is used. The S index is a measure of soil structure that controls other soil physical properties. S is defined in terms of the van Genuchten equation as the slope of water retention curve at its inflection point. In this paper the plant available water (PAW) and the content of readily-dispersible clay (RDC) were measured on two silt loam and three sandy loam soils from Poland, and the relationships between these properties and the S index were investigated. It was found that good physical condition of the soils as quantified by the S index is associated with increased water storage capacity of the arable layer of Polish soils. A relationship was found also between S and RDC. Soil physical quality index, S, decreases sharply up to a certain value of RDC (2.0 NTU/(g L-1)), beyond which any increase in the amount of RDC results in low values of S. The value of RDC of 2.0 NTU/(g L-1) is considered as a threshold value for soil physical quality of arable layer of Polish soils.