Balanced phosphorus supply and foliar Zn-EDTA improve nutrient acquisition, physiological Zn function, and potential seed Zn bioavailability in blackgram grown on Zn-deficient soil

Abstract

Phosphorus (P) - zinc (Zn) interactions regulate nutrient acquisition, physiological function, productivity, and seed nutritional quality. However, their integrated effects in blackgram remain insufficiently characterized. A single-season field experiment was conducted with blackgram cv. VBN 8 on a Zn-deficient Typic Haplustalf using a 4 × 3 factorial randomized block design (FRBD) with three replications. All plots received a uniform basal application of 25 kg ZnSO₄ ha⁻¹. Treatments comprised four P₂O₅ rates (0, 25, 50, and 75 kg ha⁻¹) and three supplemental foliar Zn-EDTA concentrations (0, 0.5, and 1.0%), applied at 30 and 45 days after sowing. Soil biochemical properties, plant nutrient concentration and uptake, carbonic anhydrase activity, growth, yield, seed Zn concentration, phytic acid concentration, and phytic acid molar ratio were evaluated. Factorial ANOVA revealed significant P × Zn-EDTA interactions for seed and haulm yields. P₂Zn₁ produced the highest numerical seed yield of 9.48 q ha⁻¹, compared with 5.47 q ha⁻¹ under P₀Zn₀, representing a significant increase of 73.3%. However, its yield was statistically comparable with P₂Zn₂ at 9.27 q ha⁻¹. P₂Zn₁ also recorded the highest seed Zn concentration of 52.75 mg kg⁻¹ and a favourable phytic acid molar ratio of 22.92. Under the tested single-site and single-season conditions, 50 kg P₂O₅ ha⁻¹ combined with 0.5% supplemental foliar Zn-EDTA delivered the most favourable integrated productivity and seed-quality response. The phytic acid molar ratio was interpreted as an indirect indicator of potential seed Zn bioavailability rather than a direct measure of Zn absorption. These findings provide a strong scientific basis for multi-season and multi-environment validation before broader agronomic recommendation.