HYDROGEN-ION EFFLUX DIFFERENCES IN SOYBEAN ROOTS ASSOCIATED WITH YIELDS

Significant differences in hydrogen ion (H+) efflux from soybean roots among soybean cultivars were found ranging from 0.5 to 2.4 pH units i n 24 h. The hydrogen ion efflux was positively correlated with the cul tivar's seed yield in three different soil series. On highly leached, low cation exchange capacity (CEC) Oxisol (less than 10 cmol/kg), the cultivars with high H+ efflux capacity yielded highest and accumulated the highest amount of nutrients at flowering. All macronutrients, exc ept for Ca and Mg, decreased at the beginning of the seed stage. Hydro gen ion efflux was positively correlated to leaf K and the K/(Ca + Mg) ratio.The K/(Ca + Mg) ratio was positively correlated to biomass and yields, thus suggesting nutrient accumulation up to seed production. R esults from the medium CEC Ultisol (10 to 20 cmol/kg) were similar to those from the low CEC soil. On the high CEC Vertisol (above 35 cmol/k g), the cultivars with high H+ efflux depleted the rhizosphere soil zo ne for K faster than those with low H+ efflux, giving positive correla tions for seed and biomass yields but negative correlations with leaf K and the K/(Ca+Mg) ratio.