Chelated Zinc Boosts Groundnut Yields in Tricky Calcareous Soils

Zinc is one of those nutrients that plants need only in trace amounts, yet when it runs short, entire harvests can falter. For groundnut, the world’s fourth most important source of edible oil and third major source of plant protein, this hidden hunger is a chronic problem in calcareous soils, where high pH and abundant calcium carbonate lock zinc into insoluble forms that roots cannot absorb. A new field study from Tamil Nadu, India, published in the journal Discover Soil, has now put two competing zinc fertilizers head to head and identified a sweet spot for dosage that could save farmers money while lifting yields by more than 20 percent.

The research team, led by S. Praveen Kumar of Annamalai University together with colleagues from Tamil Nadu Agricultural University, conducted a factorial randomized block experiment on a farmer’s field in Thiruvannamalai District between January and April 2025. The experimental soil was unmistakably hostile to zinc: a pH of 8.11, a calcium carbonate content of 6.54 percent, and a DTPA-extractable zinc level of just 0.57 milligrams per kilogram, well into deficient territory. The team grew the groundnut variety Kadhiri 9 and compared two zinc sources, inorganic zinc sulphate and the chelated compound zinc EDTA, each applied at four rates: zero, 5, 10, and 15 milligrams of zinc per kilogram of soil.

The chemistry behind the problem is worth appreciating. In calcareous soils, zinc solubility collapses because the alkaline environment drives the precipitation of zinc as insoluble zinc carbonate and zinc hydroxide. Free zinc ions are also strongly adsorbed onto calcium carbonate particles, and elevated calcium competes with zinc at the root surface, while bicarbonates interfere with zinc transport inside the plant. The result is that even soils with reasonable total zinc can leave crops starving. Chelated forms such as zinc EDTA sidestep this trap by binding the zinc ion within an organic cage, protecting it from precipitation and keeping it mobile in the soil solution until the root can take it up.

That theoretical advantage translated into striking field results. Zinc EDTA outperformed zinc sulphate across every growth, yield, and nutrient-uptake parameter the researchers measured. Plants receiving the chelated form grew taller, branched more profusely, and accumulated more dry matter. The best-performing combination, zinc EDTA at 15 milligrams per kilogram, produced plants reaching 53.82 centimeters at harvest, compared with just 43.88 centimeters in the unfertilized control. Branch numbers rose from 3.95 per plant at flowering in the control to 5.11 under the top chelated treatment, and dry matter production climbed from modest control levels to 5344 kilograms per hectare at harvest.

The yield data tell the most compelling story. Pod yield peaked at 2269 kilograms per hectare with zinc EDTA at 15 milligrams per kilogram, against a control yield of only 1837 kilograms per hectare, a gain of roughly 23 percent. Kernel yield followed the same trajectory, reaching 1472 kilograms per hectare versus 1148 in the control, while haulm yield, the vegetative fodder that many smallholders value as livestock feed, rose from 2507 to 3011 kilograms per hectare. Shelling percentage, a measure of how efficiently pods fill with kernels, also improved, topping out at 64.87 percent under the highest chelated dose compared with 62.49 percent in the control. Notably, 100-kernel weight did not change significantly, suggesting that zinc primarily boosted reproductive efficiency and kernel number rather than individual seed size.

Here is the twist that makes the study economically important: the 15 milligram rate was statistically indistinguishable from the 10 milligram rate across virtually every measured trait. Pod yields of 2227 versus 2269 kilograms per hectare, kernel yields of 1444 versus 1472, and haulm yields of 2987 versus 3011 all fell within the critical difference, meaning the extra zinc simply did not pay. The researchers attribute this plateau to a physiological saturation point. Once the plant’s internal zinc requirement is met, zinc ceases to be the limiting factor, and consistent with Liebig’s law of the minimum, additional supply cannot drive further growth. The practical recommendation is therefore zinc EDTA at 10 milligrams of zinc per kilogram of soil, capturing the full agronomic response at lower cost.

The nutrient-uptake analysis revealed how deeply zinc fertilization reshapes the plant’s entire mineral economy. Nitrogen uptake climbed from 43.53 kilograms per hectare in pods under the control to 52.93 under the top chelated treatments, reflecting zinc’s role in activating enzymes such as nitrate reductase and glutamine synthetase. Phosphorus and potassium uptake followed parallel patterns, as did zinc uptake itself, which reached 62.78 grams per hectare in pods under the highest chelated dose. Because zinc supports auxin synthesis, chlorophyll formation, and carbohydrate metabolism, adequate supply ripples through photosynthate translocation, pollen viability, and pod set, linking micronutrient status directly to the machinery of reproduction.

The soil itself bore the fingerprint of better-fed crops. Post-harvest available nitrogen, phosphorus, and potassium all declined as zinc rates increased, not because zinc depleted them chemically, but because the larger plants extracted more of each nutrient. Residual soil zinc, by contrast, was highest and most persistent under zinc EDTA, with the 10 and 15 milligram chelated treatments leaving 0.75 and 0.76 milligrams per kilogram respectively, compared with 0.45 in the control. The chelated form’s resistance to fixation means it keeps feeding the soil’s available zinc pool after harvest, a bonus for the following season’s crop.

Perhaps the most elegant piece of the analysis was a principal component assessment of all measured variables. The first principal component alone explained 98.80 percent of the total variance, with plant height, dry matter production, pod yield, haulm yield, phosphorus uptake, and zinc uptake all clustering tightly along its positive axis. In plain terms, nearly all of the variation among treatments could be summarized by a single underlying factor: zinc nutrition. Available phosphorus pointed in the opposite direction, consistent with its depletion by hungrier plants, while available zinc sat near the origin. The multivariate picture confirmed what the individual comparisons suggested, that zinc fertilization was the dominant driver of crop performance in this system.

The authors are candid about the study’s limits. The experiment ran for a single season at one location, and long-term effects on soil fertility were not assessed, so multi-location and multi-season trials will be needed before the recommendation travels widely. Still, the implications are significant for the roughly 228.8 million hectares of calcareous soils in India, nearly 69 percent of the country’s land area, and for the millions of tonnes of groundnut grown across Asia and Africa on similar terrain. Zinc deficiency alone can shave 15 to 20 percent off groundnut yields in these soils, and this study suggests that a modest, precisely targeted dose of chelated zinc can recover much of that loss without waste. For farmers weighing fertilizer budgets against harvest prospects, the message is refreshingly concrete: choose the chelated form, apply 10 milligrams of zinc per kilogram of soil, and let the chemistry do the rest.

Subject of Research: Effect of zinc fertilizer source and application rate on groundnut productivity in calcareous soil

Article Title: Effect of zinc source and application rate on groundnut (Arachis hypogaea L.) productivity in calcareous soil

Article References: Kumar, S. P., Sathiyamurthi, S., Poonkodi, P., Muthukumararaja, T., & Prabudoss, V. (2026). Effect of zinc source and application rate on groundnut (Arachis hypogaea L.) productivity in calcareous soil. Discover Soil, 3(1), Article 180. https://doi.org/10.1007/s44378-026-00337-3

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00337-3

Keywords: groundnut, zinc, zinc EDTA, zinc sulphate, calcareous soil, soil chemistry, micronutrients, nutrient uptake, crop yield, fertilizer management, oilseed crops, Tamil Nadu