Relationship of strength properties to bulk density and macroporosity in cultivated loamy sand to loam soils

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Field studies on soil tillage are dependent on relatively simple predictive methods for critical soil properties, and their relationships, to assess changes in soil structure. Regression equations were used to relate both shear strength and penetration resistance, at a controlled water potential (-6 kPa), to the volume of macropores (> 50 μm) in a range of loamy sand to loam soils (Orthic Podzols). In undisturbed soil cores from four different soil types, 66% of the variation in vane shear strength was related to bulk density and macropore volume. Addition of fine sand, clay, and organic carbon to the stepwise regression model slightly improved the relationship ( r 2 = 0.74). Regression equations between vane shear strength and macropore volume, in undisturbed soil cores, accounted for 59 – 67% of the variation in different soil types and tillage situations. Field measurements of soil penetration resistance, taken over a 2-year period in a mouldboard ploughed and direct drilling study, were significantly related to macropore volume in soil cores ( r 2 = 0.505). Overall, the critical range of 8 – 14% (v/v) for macropores in these specific soil types was interrelated to concomitant ranges for bulk density (1.44 – 1.29 Mg m −3), vane shear strength (32-18 kPa), and penetration resistance (1.50 – 0.90 MPa). Empirically determined relationships between soil behavioural properties provide a cumulative description of soil physical quality and allow the development of a soil structure-rootability index for specific soil types.