Formation, management and consequences of soil crusting in leafy packet salad production
Creators: name: Almajmaie, AHT
Publisher: name: University Of Tasmania
Year: 2023

description: The south east of Tasmania produces a large proportion of Australian packet salad. Intensive production with frequent cultivation on sandy clay loam soils has resulted in severe erosion, with soil crusting and low irrigation efficiency. Crust formation is common on cultivated soils, occurring in a wide variety of soil types, climates and agricultural practices. Soil crusts form as a thin, dense, low hydraulic conductivity layer on the soil surface that reduces infiltration, increases runoff and restricts seedling emergence. The objective of this study was to: (i) evaluate different approaches for measuring aggregate stability and the severity of soil crusting, (ii) identify mechanisms responsible for aggregate breakdown and crust formation, (iii) identify which soil properties contribute to aggregate stability/instability, (iv) evaluate options for reducing soil crusting, and (v) quantify the physical properties of soil crusts. Soil samples were collected from 20 sites over five farms all managed for packet salad production. Aggregate stability was measured by rainfall simulation, wet sieving, ultrasonic vibration (dry and moist aggregates) and clay dispersion (dry aggregates). The mechanisms responsible for aggregate breakdown were investigated by comparing the effect of different wetting fluids on dry aggregate stability. Chemical, mineralogical and physical soil characteristics were measured to identify the soil properties related to aggregate stability/instability. The different methods by which aggregate stability were measured were poorly correlated with each other. This suggested that each approach applied a different type of disruptive energy to the aggregates such that the method of assessment needs to be matched to the type by field soils. Aggregate breakdown largely resulted from slaking by clay swelling and raindrop impact, and to a lesser extent clay dispersion and air compression. Aggregate stability determined by rainfall simulation was highly correlated with soil properties that promote aggregation such as the effective cation exchange capacity (ECEC) and the proportion of polyvalent cations, whilst aggregate stability determined by wet sieving was highly correlated with soil properties that promote disaggregation such as sand content and to lesser extent, the proportion monovalent cations especially exchangeable K. Two experiments were undertaken to evaluate the potential of commercially available products to reduce soil crusting. A preliminary experiment evaluated 11 products, with the four most effective products included in a second experiment. The second experiment compared application of paper waste, gypsum, phosphoric acid and wire mesh, at three different rates and various combinations. Soil chemical and physical properties were monitored over 71 days to understanding how soil crust developed and what changes in soil physical properties occurred over time. Paper waste and gypsum significantly reduced the severity of soil crusting relative to the control. However, these reductions in crusting were minor, as no product was able to adequately prevent crusting. Physical and hydrological properties indicated that about 80 % of soil crust formation was found after 9 rainfall or irrigation events. In summary, management options for improving aggregation appear limited as aggregate stability was mostly related to inherent soil properties such as sand/quartz content. Application of paper waste and gypsum reduced the severity of crust formation, however no product was able to adequately prevent crusting. Data collected in this study are expected to enable development of soil crusting routines to improve soil water models.