A GROSS MARGINS ANALYSIS COMPARING TWO SOIL ORGANIC MATTER MANAGEMENT SCENARIOS IN COMMERCIAL VEGETABLE PRODUCTION

A gross margins analysis was conducted based on one example of a potentially cost-effective scenario of compost use in vegetable production. This scenario was developed in consultation with a leading Melbourne vegetable grower with the aim of identifying potential opportunities for the use of compost as a soil amendment based on the grower’s particular farming context. Soil organic matter management in the standard grower practice (SGP) treatment involved biennial green manure cropping and twice yearly applications of poultry litter at 70-90 m³/ha. The compost treatment consisted of compost applications (~10 t/ha dry weight) before each crop and no green manure crop. A green manure crop was not grown at all in the compost treatments. Composts were applied at a rate of approximately 10 t/ha dry weight before each crop. The SGP cropping cycle began with green manure in summer followed by six cash crops; the compost cropping cycle was the same except a lettuce (Lactuca sativa L.) crop was grown in place of the green manure crop. Gross margins were found to be most sensitive to the market price for vegetable produce, which fluctuated widely during the period of assessment. At high summer prices a very high yielding lettuce crop could be worth about $A 62,000/ha. However, at mean market prices, a typical-yielding lettuce crop could return $A 8,000/ha and a 6-8% yield reduction in the compost treatment could eliminate the advantage of growing an additional lettuce crop. The main effect of compost application on gross margins was the potential impact on yield since it represented only about 3% of the total variable costs of production. We suggest that reducing the price of compost to near zero would be unlikely to increase its rate of adoption particularly if there is a real or perceived risk that yields could be adversely affected.