Crop yields and soil properties on eroded slopes of red ferrosols in north-west Tasmania

The differences in soil properties and crop yield on slopes of varying steepness in intensively cropped paddocks with red ferrosols (Humic Eutrodox), which had evidence of erosion over many years, were determined at 5 sites in north-west Tasmania. We found that soils on the steeper slopes, which have had greater soil loss, had significantly lower topsoil carbon concentrations than soils on flatter slopes. We found that significant redistribution of soil had occurred on the sites in this study. Areas with concave slope positions had over-thickened topsoils. However, the effect of erosion on the topsoil thickness of eroded slope positions was masked by the uniform depth of cultivation across paddocks. Penetration resistance increased gradually with depth on all transects, indicating no compacted layers or plough pans on these intensively cropped sites. Soil physical properties did not significantly change from non-eroded, level areas to steep, eroded parts of the paddocks, even although considerable soil loss had occurred from the steep slopes. We attribute this to the gradational nature of these red ferrosols, which characteristically have strongly structured subsoils. Crop yield was significantly correlated with slope but was not significantly different on land up to 18% slope. We anticipate that continued erosion at current rates on this less steep land will inevitably reduce crop yields. We found a significant correlation between organic carbon, total exchangeable bases, extractable K, and crop yield, but no correlation between available water content and crop yield. We conclude that soil organic matter contributes to productivity through its effect on nutrient concentrations. Total exchangeable bases appeared to provide the best and simplest explanatory model to predict crop yield. We propose a set of 4 tests for future assessment of soil condition on these red ferrosols.