WEATHER, CROPPING PRACTICES AND SAMPLING DEPTH EFFECTS ON TENSILE-STRENGTH AND AGGREGATE STABILITY

The stability in water and the tensile strength of aggregates reflect the ease of fragmentation of soil by different processes. The objectiv es of this study were to determine if these two parameters respond to weather and soil management in a similar manner and how this response changes with depth in the Ap horizon. Studies were conducted on plots of a permanent rotation trial established in 1925 on a red-brown earth (a calcic Rhodoxeralf) in South Australia. The stability and tensile strength of aggregates declined in a similar manner with the number of wetting events prior to sampling and the variation in these character istics due to weather was often much greater than the variation due to management practices. The influence of sampling depth and management practices on the two parameters was, however, very different. Tensile strength increased with depth in the Ap horizon whereas aggregate stab ility decreased. The increase in tensile strength appeared to be due t o the less severe wetting events experienced at depth whereas the decr ease in stability was correlated with the decrease in organic carbon c ontent with depth. The tensile strength of this soil was less sensitiv e to management than was aggregate stability. It is concluded that the re are only limited circumstances under which changes in tensile stren gth can be predicted from changes in aggregate stability.