Moderate Phosphorus Plus Foliar Zinc Spray Boosts Blackgram Yield and Seed Nutrition on Zinc-Poor Soil

Zinc deficiency is one of the most widespread micronutrient problems in the world’s cultivated soils, and in India it quietly undermines both the harvests and the nutritional value of staple pulse crops. A new field study from Tamil Nadu Agricultural University in Coimbatore, published in Discover Soil, suggests that the answer to this hidden hunger is not more fertilizer, but smarter fertilizer. Working with blackgram, a short-duration pulse central to South Asian diets, researchers found that a moderate dose of phosphorus combined with a modest foliar spray of zinc-EDTA outperformed every higher-input alternative, lifting seed yield by 73.3 percent while simultaneously enriching the grain with zinc and improving its potential bioavailability to human consumers.

The experiment took place during the kharif season of 2023 on a Typic Haplustalf of the Noyyal series at the university’s Wetland Farm, a clay loam soil with a moderately alkaline pH of 8.2 and a diethylenetriaminepentaacetic acid extractable zinc level of just 0.63 milligrams per kilogram, well into deficient territory. The team, led by Tasso Chama with co-authors Meeniga Venkateswarlu and R. Shanmugasundaram, grew the blackgram cultivar VBN 8 across a 4 by 3 factorial randomized block design with three replications. Every plot received a uniform basal dressing of 25 kilograms of zinc sulfate per hectare, so the zinc treatments tested the incremental benefit of spraying chelated zinc directly onto leaves rather than comparing fertilized plants against completely zinc-starved ones. Phosphorus pentoxide was applied at 0, 25, 50, and 75 kilograms per hectare, while foliar zinc-EDTA was sprayed at 0, 0.5, and 1.0 percent concentrations at 30 and 45 days after sowing, timed to coincide with peak vegetative and early reproductive demand.

The central tension the researchers set out to resolve is a classic antagonism in soil chemistry. Phosphorus fertilization drives biomass, root proliferation, and nodule function, but phosphate ions can also adsorb zinc onto iron and aluminum oxides, promote its precipitation, and lock the metal away from plant roots. When phosphorus supply outruns zinc availability, plants can develop what agronomists call phosphorus-induced zinc deficiency, with tissue zinc concentrations falling even as the crop grows vigorously. Because zinc governs enzyme activation, protein synthesis, auxin metabolism, chlorophyll formation, and reproductive development, this imbalance ripples through the entire physiology of the plant and ultimately into the seed that people eat.

The results were striking. The treatment combining 50 kilograms of phosphorus pentoxide per hectare with 0.5 percent foliar zinc-EDTA, designated P2Zn1, produced the highest numerical seed yield of 9.48 quintals per hectare, compared with 5.47 quintals under the control that received basal zinc sulfate but no added phosphorus or foliar spray. Haulm yield rose by 62.4 percent under the same regime. Factorial analysis of variance revealed significant phosphorus by zinc-EDTA interactions for pods per plant, hundred-seed weight, seed yield, and haulm yield, meaning that productivity depended on the combined nutrient regime rather than on either input alone. Notably, doubling the foliar spray to 1.0 percent did not improve on the 0.5 percent rate, and pushing phosphorus to 75 kilograms per hectore offered no further advantage, a pattern the authors interpret as evidence that nutrient balance, not nutrient intensity, governs the system.

Beneath the yield figures lies a coherent physiological story. Carbonic anhydrase, a zinc-dependent metalloenzyme that catalyzes the reversible hydration of carbon dioxide, responded sharply to the balanced regime, and its activity correlated strongly with dry matter production, chlorophyll readings, and seed yield. This enzyme acts as a functional bridge between tissue zinc status and photosynthetic carbon metabolism, so its responsiveness confirms that the foliar sprays were not merely accumulating zinc in inert tissue but actively feeding zinc-dependent metabolism. Nodulation also improved, consistent with recent evidence that zinc participates in nodule signaling and the regulation of symbiotic nitrogen fixation, while phosphorus sustains the adenosine triphosphate demands of nodule function.

The soil measurements added an important caveat about what was happening below ground. Available phosphorus and the activities of alkaline phosphatase and dehydrogenase, markers of phosphorus mineralization and overall microbial oxidative activity, rose significantly with phosphorus supply. DTPA-extractable zinc, by contrast, declined numerically across the phosphorus gradient, an indicative trend the authors attribute plausibly to phosphate-associated zinc sorption and precipitation, though they are careful to note that the underlying transformation pathways were not measured directly. Because the zinc-EDTA was applied to foliage rather than soil, the soil zinc pool reflected basal zinc sulfate, phosphorus rate, and crop uptake, not the sprays. The picture that emerges is of a crop growing in a phosphorus-enriched but progressively zinc-limited rhizosphere, rescued above ground by targeted foliar delivery.

Nutrient uptake data reinforced this interpretation. The balanced treatment recorded the highest numerical uptake of nitrogen, phosphorus, potassium, and zinc, with significant interactions for most seed and haulm fractions, indicating that improved acquisition was efficiently partitioned into reproductive sinks. The authors emphasize that uptake integrates both tissue concentration and biomass, so the response reflects coordinated acquisition rather than simple concentration effects. Foliar zinc likely bypassed the constrained root-zone supply at precisely the stages when phosphorus-driven growth was raising the crop’s zinc demand, a bypass strategy supported by prior work showing that leaf-applied nutrients can sidestep soil chemical barriers when applied at physiologically active stages.

Perhaps the most nutritionally consequential finding concerns phytic acid, the antinutritional compound that binds zinc into stable complexes in pulse seeds and restricts its absorption in the human gut. Under the balanced regime, seed zinc concentration reached 52.75 milligrams per kilogram, the highest in the study, while the phytic acid to zinc molar ratio fell to 22.92, indicating improved potential bioavailability. The authors stress that this molar ratio is an indirect indicator rather than a direct measure of zinc absorption, since no in vitro digestion or cellular uptake assays were performed. Even so, the simultaneous achievement of higher zinc enrichment and a lower phytate-linked constraint is exactly what agronomic biofortification requires, because enriched grain is only useful if the mineral it carries can actually be absorbed by consumers.

Multivariate analysis tied the whole system together. Principal component analysis of twelve treatment means across ten variables, including soil nutrient availability, enzyme activities, carbonic anhydrase, yields, zinc uptake, and seed quality, showed that the first two components explained 95.03 percent of total variance, with the balanced treatment positioned squarely within the favorable productivity-bioavailability domain. Pearson correlations linked soil biochemical activity to biomass, yield, and nutrient uptake, while the inverse positioning of extractable zinc against phosphorus-driven variables mirrored the expected antagonism. The authors are careful to frame these exploratory analyses as supporting evidence rather than proof of causality, with the factorial analysis of variance remaining the primary basis for inference.

The study’s limitations are clearly acknowledged: it covered a single season, a single cultivar, and a single zinc-deficient site, and the proposed soil-chemical mechanisms remain plausible interpretations pending direct measurement of zinc speciation and rhizosphere transformations. The phytic acid ratio awaits validation through digestion models or feeding studies, and economic returns were not assessed. Still, the core message carries real weight for farmers and policymakers alike. In zinc-deficient pulse systems, the winning strategy was neither maximal phosphorus nor maximal zinc, but a calibrated pairing in which moderate phosphorus fueled growth and foliar chelated zinc protected the physiological and nutritional functions that phosphorus pressure would otherwise erode. Before the 50-kilogram-plus-0.5-percent prescription can be broadly recommended, the authors call for multi-season, multi-location, and genotype-specific validation, ideally incorporating direct bioaccessibility assays and economic analysis. If those trials confirm the pattern, the finding could reshape fertilizer guidance for pulse production across zinc-deficient regions, replacing the reflex to add more with the discipline of adding it right.

Subject of Research: Phosphorus-zinc interaction and agronomic zinc biofortification in blackgram grown on zinc-deficient soil

Article Title: 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

Article References: Chama, T., Venkateswarlu, M., & Shanmugasundaram, R. (2026). 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. Discover Soil, 3(1), Article 185. https://doi.org/10.1007/s44378-026-00344-4

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00344-4

Keywords: blackgram, phosphorus-zinc interaction, foliar zinc-EDTA, agronomic biofortification, zinc-deficient soil, carbonic anhydrase, phytic acid molar ratio, nutrient uptake, seed zinc concentration, balanced fertilization, soil enzyme activity, pulses