Impact of Practice Change on Runoff Water Quality and Vegetable Yield—An On-Farm Case Study

Intensive agricultural practices in farming systems in eastern Australia have been identified as a contributor to the poor runoff water quality entering the Great Barrier Reef (GBR). A field investigation was carried out to measure the off-farm water quality and productivity in a coastal farming system in northeastern Australia. Two vegetable crops (capsicum and zucchini) were grown in summer 2010–2011 and winter 2011 respectively using four different management practices (Conventional—plastic mulch, bare inter-row conventional tillage and commercial fertilizer inputs; Improved—improved practice with plastic mulch, inter-row vegetative mulch, zonal tillage and reduced fertilizer rates; Trash mulch—improved practice with cane-trash or forage-sorghum mulch with reduced fertilizer rates, minimum or zero tillage; and Vegetable only—improved practice with Rhodes grass or forage-sorghum mulch, minimum or zero tillage, reduced fertilizer rates). Results suggest improved and trash mulch systems reduced sediment and nutrient loads by at least 50% compared to conventional systems. The residual nitrate nitrogen in soil accumulated at the end-of-break crop cycle was lost by deep drainage before the subsequent sugarcane crop could utilize it. These results suggest that future research into establishing the linkages between deep drainage, groundwater quality and lateral movement into adjacent streams is needed. The improvement in runoff water quality was accompanied by yield reductions of up to 55% in capsicum and 57% in zucchini under trash mulch systems, suggesting a commercially unacceptable trade-off between water quality and productivity for a practice change. The current study has shown that variations around improved practice (modified nutrient application strategies under plastic mulch, but with an inter-space mulch to minimize runoff and sediment loss) may be the most practical solution to improve water quality and maintain productivity. However, more work is required to optimize this approach and thus reduce the size of any potential productivity and profitability gap that would necessitate an expensive policy intervention to implement.