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 (Alf isols), Grey Clays, and Black Earths (Vertisols) in northern New South Wales. The degradation of soil physical properties between the croppe d 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 labil e carbon (C-L) was usually greater than the loss of total carbon (C-T) A Grey Clay which had been cropped for >40 years had lost 63% and 51% of its C-L and C-T, respectively. An adjacent Grey Clay which had bee n cleared and cropped for only 2 years had lost 43% and 26% of its C-L and C-T, 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 lo ss of total and available nutrients after cropping was also found, wit h the magnitude of the losses modified by fertiliser history. A highly significant correlation was found between C-T or C-L and the proporti on of water-stable aggregates >250 mu m for the Red Earth and Grey Cla y soils, but this correlation was poor for the Black Earth. The import ance of organic matter in maintaining aggregation in low clay soils su ch as the Red Earth was highlighted.