FACTORS AFFECTING ZINC UPTAKE IN CROPPING SYSTEMS

Zinc availabilities can change with different cropping management prac tices. The objective of this study was to identify some of the causati ve factors associated with previous crops contributing to Zn uptake di fferences in a subsequent crop. Field studies over 3 yr evaluated the Zn availability after four precropping treatments: bean (Phaseolus vul garis L.), corn (Zea mays L.), wheat (Triticum aestivum L.), and fallo w, 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 b efore 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 sample s estimated nutrient concentration and uptake. Soil Zn extracted by di ethylenetriaminepentaacetic acid (DTPA) was increased by Zn fertilizat ion but not affected by precropping treatments. Zinc uptake by bean wa s significantly higher after precropping with corn and lower after fal low regardless of Zn fertilization. Uptake differences were most prono unced during early plant growth. Phosphorus and Cu uptake varied with treatment in a similar pattern as Zn uptake, and were positively corre lated with each other. Zinc up. take was also positively correlated wi th soil organic matter and negatively correlated with soil P. Soil res piration rate was significantly lower after the fallow treatment compa red with other precropping treatments. Vesicular-arbuscular mycorrhiza (VAM) colonization in the test crop roots was higher after corn and l ower after fallow regardless of soil Zn concentrations. Colonization w as 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 v ariables affecting Zn uptake during early plant growth. These results emphasize the importance of the soil's biological activities on Zn ava ilability and may help explain some field observations where chemical soil tests appear to fail.