Factors Associated with Zn Chlorosis in Dryland Beans

Dryland pinto beans (Phaseolus vulgaris L. ‘San Juan’) are grown in more than 40,000 ha in the San Juan Basin of Colorado. Chlorosis and root rot are two major problems encountered by the bean growers. This research was initiated to investigate the cause of chlorosis of beans. Paired plots, one chlorotic and the other relatively green, were selected on 18 farms in 1975 and 1976. The soils of the experimental sites belong to Ackmen, Witt, and Pulpit series of Fine-silty, mixed, mesic family. The Ackmen series belong to the Cumulic Haplustolls. The Witt and Pulpit series belong to the Ustollic Haplargids. Yield, root rot, soil factors, and plant composition were compared for the chlorotic and the green plots. In 1976 the chlorotic plots were divided into two subplots, one receiving zinc spray to confirm the results of regression analysis that suggested zinc deficiency was the cause of chlorosis. The chlorotic plots had significantly higher mean levels of soil moisture and pH, and lower organic matter than the green plots. The chlorotic leaves had higher mean levels of P and P/Zn, and lower Zn than the green leaves. The chlorotic plants had a higher incidence of root rot than the green plants. The mean soil P and soil Zn availability indexes were not different in the chlorotic and the green plots. Soil Zn levels in all plots except one were lower than 0.5 ppm. It was observed that the chlorotic areas were situated lower on the slope than the green areas, with a marked demarcation between them. The regression analysis suggested that 72% of the variability in yield was due to leaf P/Zn ratio, root rot and their positive interaction. Other measured variables did not significantly contribute to R². Spraying the chlorotic plants with a solution containing 1% zinc sulfate removed chlorosis and increased the yield by 18 to 92%. In general, the yield of the chlorotic plants sprayed with zinc solution did not increase to the level of the green plants. This difference was attributed to the higher root rot index of the chlorotic plants. The data suggested that higher soil moisture in low lying areas increased the P availability and thus caused a P-induced Zn deficiency in the bean plants growing in these soils of marginal Zn availability.