A Preliminary Study of the Role of Cover Crops in Improving Soil Fertility and Yield for Potato Production

This study examined the potential of cover crops (fodder rape, oats, and white lupin) to improve phosphorus (P) availability in a high P-fixing Red Ferrosol soil at Robertson, New South Wales. Of particular interest was a fourth cover crop that consisted of white lupin which has P releasing characteristics, grown in combination with fodder rape, which scavenges P with its extensive fine root system. The cover crops were grown for 17 weeks and then mulched at 30 cm above ground to simulate grazing. The cultivar of white lupin used (Kiev Mutant) made poor regrowth after mulching. At 27 weeks after sowing, the cover crops were slashed at soil level and incorporated into the soil. After 7 weeks of decomposition, each cover crop plot was split, and seed potatoes were sown with and without fertilizer. Levels of Olsen-extractable P in the fertilized subplots previously sown to lupin and the lupin/fodder rape combination were significantly higher (16 and 38%, respectively) than in the unfertilized plots at 8 weeks after sowing the potatoes. However, Olsen-extractable P levels in the fertilized oat and fodder rape subplots were similar to those in the unfertilized subplots. Fertilizer increased the yield of potato tubers, but prior cover crop had no effect on tuber yield. The mean apparent recoveries of fertilizer P and N in tubers from the fertilized plants were 4 and 52%, respectively. Although tuber yield did not reflect differences in extractable P among the fertilized subplots, the results suggest that cover crops containing white lupin have the potential to improve P availability to the following potato crop. However, the cultivar of white lupin must be capable of recovering after grazing and match the growth of a companion cover crop. The cover crops containing fodder rape significantly reduced the incidence of Rhizoctonia, presumably due to the biofumigation effect of the rape. The high biomass of the fodder rape and lupin/fodder rape cover crops provided excellent protection for the soil surface against erosion during high intensity rainstorms, as well as providing grazing for stock.