QUANTIFYING THE RESIDUAL VALUE OF ZINC FOR CANOLA PRODUCTION DUE TO REMOVAL OF ZINC IN SHOOTS AND GRAIN AND CONTINUED REACTION OF ZINC WITH SOIL

About half of the almost 18 million ha used for agriculture in southwestern Australia were initially acutely zinc (Zn) deficient. Canola (oilseed rape, Brassica napus L.) is a recent crop species grown in the region and there is only limited information on its Zn requirements. In the glasshouse pot study reported here five levels of Zn (0, 0.8, 1.6, 3.2, and 6.4 mg Zn pot^?1) were applied before sowing the first canola crop on an acid sandy loam, and shoot and grain yield responses to applied Zn, and removal of Zn in the shoots and grain, were measured for five successive crops grown to maturity. Before sowing each crop, canola seed was treated with fluquinconazole to successfully control blackleg disease Leptosphaeria maculans (Desm.) Ces. et de Not.]. Soil samples were collected from each pot before sowing each crop, and after harvesting the last crop, to measure the Zn extracted from soil by diethylenetriaminepentaacetic acid (DTPA) (soil test Zn). Removal of Zn in shoots and grain, and continued reaction of applied Zn by soil, both decreased the effectiveness of applied Zn for successive crops. For all five crops about 71% of the low levels of Zn applied (0.8, 1.6 mg Zn pot^?1) were removed in shoots and grain of the five crops, compared with about 46% for the larger levels of Zn applied (3.2 and 6.4 mg Zn pot^?1). Corresponding values for Zn removed in the grain was about 56 and 30%. Soil test Zn decreased with time since Zn application. The decrease could not all be explained by Zn removed in shoots and grain and was attributed to continued reaction of Zn with soil. For the soil type used, the relationship between either grain yield or total yield (shoots plus grain at maturity), and soil test Zn, was similar for each crop. Therefore, the soil test Zn that was related to 90% of the maximum grain or total yield (critical soil test Zn) was about 0.35 mg Zn kg^?1 soil for each crop.