STRUCTURAL DEGRADATION OF 2 VERTISOLS UNDER CONTINUOUS CULTIVATION

Changes in soil physico-chemical properties as a result of continuous cereal cropping for up to 64 years of two southern Queensland Vertisols were investigated. The soils were the Waco black earth and the Langlands grey clay. For both soils, cultivation increased bulk densities relative to those of respective virgin soils, but soil density varied little among cultivated sites. The size and strength of dry seedbed aggregates decreased owing to cultivation, but remained suitable for wheat establishment. The stability of wet aggregates decreased with cultivation as indicated by dispersion and slaking measurements. The increased dispersion was associated with decreases in hydraulic conductivity which may limit infiltration and storage of soil water during the summer fallow, thereby reducing crop yields. Organic carbon contents decreased with cultivation while exchangeable sodium percentage (ESP) and pH values increased. Increased dispersibility was correlated with increased sodicity, although ESP values remained below accepted critical values. The relationship was strongest when both dispersibility and exchangeable sodium were expressed on an oven-dry soil basis rather than as the relative parameters, dispersion ratio and ESP. Organic carbon content accounted for less than 40% of the temporal variation in aggregate stability. The changes in physical and chemical properties with cultivation were consistent with exposure of subsurface material following soil erosion at rates typical for the region. We concluded that erosion control would limit further changes in levels of exchangeable sodium and thus soil dispersibility and hydraulic conductivities. The rate of loss of organic carbon would also be reduced and soil physical properties and chemical fertility would be maintained.