Effects of phosphorus levels on soybean growth and nutrient uptake in acidic soils.

We explored how phosphorus (P) availability affects the growth and nutrient uptake of six soybean geno-types in acidic red soils in South China, with a long-term field experiment involving normal (13.68 kg·hm^-2, P₂O₅) and low (6.84 kg·hm^-2) P levels. We measured plant biomass, root traits, and mineral uptake at early flowering stage, and genotypic differences in soybean growth and the patterns of nutrient uptake. The results showed that compared with normal P level, the low P level significantly reduced biomass, total root length, and nutrient contents of all tested genotypes. Under the low P level, the average aboveground dry weight and root dry weight of soybean plants significantly decreased by 55.4% and 45.6%, the average total root length and root surface area significantly decreased by 24.8% and 29.4%, the shoot average phosphorus and potassium concentrations in plants significantly decreased by 25.6% and 9.0%, while the average iron and manganese concentrations significantly increased by 83.5% and 17.8%, respectively. However, there were no significant changes in the concentrations of nitrogen, calcium, and magnesium. Under the normal P level, plant dry weight was negatively correlated with root nitrogen, iron, and manganese concentrations. Under low P level, it was negatively correlated with aboveground iron/manganese and root manganese concentrations. We recommended that when growing soybeans in acidic low-phosphorus soils, to supplement potassium fertilizer while applying phosphorus, avoiding the toxic effects caused by excessive absorption of iron and manganese.