Effects of agricultural management on sodosols in northern Tasmania

Attributes of 25 Tasmanian sodosols were assessed using field and laboratory techniques to determine changes associated with 4 typical forms of agricultural management [long-term pasture, cropping with shallow tillage using discs and tines, cropping (including potatoes) with more rigorous and deeper tillage including deep ripping and powered implements, and cropping (including potatoes) where the potatoes were harvested when the soil was wet]. Soil organic carbon in the top 150 mm was 2.7% under long-term pasture compared with 1.8% in rigorously tilled cropping paddocks, and microbial biomass C values were 194 and 129 mg/kg, respectively. Readily oxidisable organic C concentrations were 1.8 mg/g and 1.3 mg/g, respectively. Infiltration rate was greater in paddocks with shallow tillage cropping than longterm pasture but was 43% less in paddocks which had grown potatoes and 70% less after a wet potato harvest. Dry aggregate-size showed no change under shallow tillage cropping compared with long-term pasture but decreased significantly in more rigorously tilled potato cropping paddocks. Aggregate stability in all cropped paddocks was nearly 50% less than in long-term pasture paddocks, with values in intensively tilled potato cropping paddocks approaching relatively low levels. Colwell extractable phosphorus (P) increased with all cropping, particularly after potatoes. Lower organic carbon and poorer physical properties were associated with paddocks which had grown potatoes, which adds weight to the view that cropping rotation and associated soil management practices are critical for sustainable management of Tasmanian sodosols. Farmers were surveyed about their views of the condition of their paddocks. They identified more healthy than unhealthy soil attributes under all management histories but reported more unhealthy soil attributes when potatoes were included in their rotation.