Identifying the causes of soil aggregate breakdown under intensive packet salad production

Frequent cultivation, irrigation and bed forming associated with packet salad production have led to a loss in soil carbon and aggregate stability at six farms operated by Houston’s farms, Richmond, Tasmania. Soil disaggregation has resulted in poor seedling establishment, soil crusting, reduced infiltration, poor irrigation performance, and increased runoff and erosion. This paper reports initial findings from detailed assessment of aggregate stability, and soil properties across the six farms. Aggregate stability was determined by rainfall simulation, wet sieving and ultrasonic disruption of 2.0-4.75 mm aggregates pre-treated to be at either air dry moisture content (40 Co for 24 h) or field capacity (-10 kPa). Aggregate were also analysed by CSBP laboratories for a range of chemical properties and indices of structural stability. Initial results demonstrated that aggregate stability at air dried moisture content was most susceptible to disaggregation by rainfall simulation, whilst aggregates at field capacity had similar levels of aggregate stability for all three disaggregation techniques. The stability of air dried aggregates was most highly correlated with CEC-cation exchangeable capacity (positive), EPP-exchangeable potassium percentage (negative), then reactive aluminium (positive), and organic carbon (positive). Aggregate stability was however poorly correlated with cation ratio of soil structural stability (CROSS) and exchangeable sodium percentage (ESP).