FACTORS AFFECTING ZINC UPTAKE IN CROPPING SYSTEMS

Zinc availabilities can change with different cropping management practices. The objective of this study was to identify some of the causative factors associated with previous crops contributing to Zn uptake differences in a subsequent crop. Field studies over 3 yr evaluated the Zn availability after four precropping treatments: bean (Phaseolus vulgaris L.), corn (Zea mays L.), wheat (Triticum aestivum L.), and fallow, across two Zn fertilization rates (with and without 11 kg Zn ha(-l) as ZnSO4), using the 'Viva' bean as a test crop. Soil samples taken before and after the test crop were analyzed for extractable P, Zn, Cu, Mn, and Fe, and organic matter. Soil respiration during the test crop was periodically estimated the last cropping year. Whole plant samples estimated nutrient concentration and uptake. Soil Zn extracted by diethylenetriaminepentaacetic acid (DTPA) was increased by Zn fertilization but not affected by precropping treatments. Zinc uptake by bean was significantly higher after precropping with corn and lower after fallow regardless of Zn fertilization. Uptake differences were most pronounced during early plant growth. Phosphorus and Cu uptake varied with treatment in a similar pattern as Zn uptake, and were positively correlated with each other. Zinc up. take was also positively correlated with soil organic matter and negatively correlated with soil P. Soil respiration rate was significantly lower after the fallow treatment compared with other precropping treatments. Vesicular-arbuscular mycorrhiza (VAM) colonization in the test crop roots was higher after corn and lower after fallow regardless of soil Zn concentrations. Colonization was positively cor-related with Zn, P, and Cu uptake during early plant growth. The VAM colonization, soil respiration, and DTPA-extractable Zn were selected by a stepwise regression procedure as the important variables affecting Zn uptake during early plant growth. These results emphasize the importance of the soil's biological activities on Zn availability and may help explain some field observations where chemical soil tests appear to fail.