Soil solution at the flowering stage as an indicator of phosphorus and zinc interaction and their biofortification in soybean grains

Soil solution at the flowering stage is an adequate compartment to evaluate the interaction between phosphorus (P) and zinc (Zn) in soil and their relationship with soybean (Glycine max) nutrition. This study aimed to evaluate the interaction between P and Zn as assessed through soil solution analysis and its relationship with nutrient uptake by soybean. Soybean plants were grown in a greenhouse and were evaluated for the interaction between two application rates of P (200 and 400 mg kg− 1) and four application rates of Zn (0, 25, 50, and 100 mg kg− 1). Soil solution samples were collected at the beginning of the flowering stage of soybean. Dry matter production, shoot and grain nutrient accumulation, and the concentrations of P, Zn, and Fe in the grains were determined. In the soil solution, P and Zn exhibited antagonism. The soil solution evaluated at the beginning of flowering showed antagonism between P and Zn, where the lowest applied P rate, combined with the highest Zn rate, favored Zn concentration while hindering P concentration. However, this increase was not proportional, resulting in a reduction in Zn translocation from shoot to grain, reducing the percentage of Zn translocation from 37.6–46.7% at a rate of 0 mg kg− 1 to around 4.6–9.9% at the higher rate of Zn applied (100 mg kg-1), leading to a saturation of this element in the grain. The application of P and Zn was associated with increased soybean production and the concentration of P, Zn, and Fe in the grains. Therefore, Zn concentration in the soil solution can be used to predict grain Zn concentration, with high regression coefficient value for prediction model (R2: 0.81).