Comparing irrigated biodynamic and conventionally managed dairy farms. 1. Soil and pasture properties

Ten paired irrigated dairy farms under biodynamic ( BD) and conventional ( CV) management were compared over a 4-year period (1991-94). The paired farms were located in the irrigation districts of northern Victoria and southern New South Wales and were matched for soil type, climate, cattle breed and farm area. Farms had been practising BD principles for an average of 16 years before the commencement of the study and had not received phosphorus ( P) fertiliser for an average of 17 years. The effects of farm management on soil chemical and biological properties and the nutritive properties and botanical composition of pasture were examined at varying sampling times during the study. Soil Olsen extractable P concentrations were consistently 2-3 times higher under CV management at various sampling depths ( mean = 22 mg/ kg, 0-10 cm), and were generally marginal under BD management in the surface 10 cm ( mean = 8.5 mg/ kg). Low soil extractable P concentrations were also reflected in consistently lower mean pasture P concentrations under BD management (0.25 compared with 0.35% on CV farms). Lower soil and pasture P concentrations under BD management were the result of a large negative P balance across BD farms (- 17 kg P/ha. year). A mean negative P balance under BD management was a result of low P imports (2 kg P/ha. year) in comparison with large quantities of P (19 kg P/ha. year) effectively lost from the farming system through animal products, estimated losses in water runoff and slowly reversible soil P reactions. These results suggest that greater P imports are required to ensure the future sustainability of BD dairy pasture farming systems. There were few differences in soil biological properties, with earthworm weights significantly higher under CV management, but no difference in soil organic carbon, humus concentration, the weight of the organic mat or microbial biomass, between the two management systems.