A survey of the impact of cropping on soil physical and chemical properties in north-western New South Wales

The soil survey was conducted on cropped and uncropped Red Earths (Alfisols), Grey Clays, and Black Earths (Vertisols) in northern New South Wales. The degradation of soil physical properties between the cropped and uncropped reference sites was reflected in declines of 29–86% in hydraulic conductivity (K) and 33–71% in aggregation. Generally there was a substantial loss of carbon with cropping, and the loss of labile carbon (CL) was usually greater than the loss of total carbon (CT). A Grey Clay which had been cropped for >40 years had lost 63% and 51% of its CL and CT, respectively. An adjacent Grey Clay which had been cleared and cropped for only 2 years had lost 43% and 26% of its CL and CT, respectively, resulting in a C management index (CMI) of 55, indicating that a large proportion of soil C was lost soon after the commencement of cropping. Where well-managed legume leys had recently been grown, the loss of C was reduced, resulting in a higher CMI. A loss of total and available nutrients after cropping was also found, with the magnitude of the losses modified by fertiliser history. A highly significant correlation was found between CT or CL and the proportion of water-stable aggregates >250 µm for the Red Earth and Grey Clay soils, but this correlation was poor for the Black Earth. The importance of organic matter in maintaining aggregation in low clay soils such as the Red Earth was highlighted.