Influence of soil properties on the aggregate stability of cultivated sandy clay loams

Frequent cultivation and overhead irrigation have led to severe surface crusting, erosion and poor irrigation performance on sandy clay loam soils in the Coal River Valley, Tasmania, Australia. This study was established to identify the key soil properties related to aggregate breakdown determined by different methods, and explore options for reducing soil crusting. Soil aggregates were collected from 0 to 5 cm depth from 20 sites managed for packet salad and lettuce production. The stability of air-dried 2.00-4.75 mm aggregates was determined by rainfall simulation, wet sieving and clay dispersion. Soil aggregates were analysed for particle size, mineralogy, soluble and exchangeable cations, pH, EC, labile carbon and total carbon. The association between aggregate stability and the measured soil properties was explored using Spearman correlation, linear regression and regression tree analysis. Aggregate stability determined by rainfall simulation was closely associated with soil properties that promote aggregation, including effective cation exchangeable capacity (ECEC) and the proportion of polyvalent cations (Ca2+, Al3+). In contrast, aggregate stability determined by wet sieving was associated with soil properties that promote disaggregation, including quartz and sand content, and to lesser extent, the proportion of monovalent cations (especially K+). Clay dispersion was closely associated with pH, quartz content, soil texture and the sodium adsorption ratio. Soil carbon appeared to have only moderate influence on aggregate stability, but not clay dispersion, while labile carbon was not significantly related to any measure of aggregate stability or clay dispersion. Similarly, the proportion of Na+ ions was not related to either measure of aggregate stability and was only moderately related to clay dispersion. Options for improving aggregate stability appear limited as aggregate stability was strongly related to the content of inherent soil properties such as sand/quartz and smectite contents. However, high correlation between exchangeable Ca2+ and aggregate stability determined by rainfall simulation indicates that soil crusting may be reduced through application of products that rich in Ca2+ such as gypsum.