The study, conducted by researchers at Banaras Hindu University in Varanasi and published in the journal Discover Soil, grew out of a growing anxiety about potassium mining in one of the world’s most intensively farmed regions. The rice-wheat cropping system that dominates the Indo-Gangetic plains supplies more than half of the food consumed in India, occupying roughly 27 percent of the country’s agricultural land. Yet fertilizer use in India has become badly skewed: between 1971-72 and 2021-22, nitrogen consumption surged 11.3-fold and phosphorus 14.2-fold, while potassium rose only 5.71-fold. The current national ratio of nitrogen to phosphorus to potassium stands at 6.7:2.4:1, far from the 4:2:1 ratio long recommended by the Fertiliser Association of India. An estimated 72 percent of India’s agricultural land is now considered potassium deficient.

Why does this imbalance matter? Potassium is not a minor player in crop physiology. It governs enzyme activation, water regulation, photosynthesis, and the transport of sugars, all of which feed directly into yield, grain quality, and the plant’s resilience to stress. When farmers skimp on potassium while piling on nitrogen, they quietly draw down the soil’s native reserves, a process scientists call potassium mining. Crop residues complicate the picture further: rice and wheat straw is extraordinarily rich in potassium, containing 80 to 85 percent of the nutrient taken up by the plant, and returning it to the field can substitute for 20 to 50 percent of potassium fertilizer. In conservation agriculture systems, where residues are retained on the soil surface and tillage is minimized, this recycling loop changes the chemistry of potassium availability in ways that conventional soil tests were never designed to capture.

To find out which chemical extractants best reflect reality under these conditions, the team turned to a long-term experiment established in 2012 at the university’s agricultural research farm. The site sits on sandy loam soils of the Typic Ustochrept family in a humid subtropical climate with about 1,110 millimeters of annual rainfall. The experiment followed a split-plot design with four crop establishment methods, ranging from full conventional tillage to full conservation agriculture with residue retention in both seasons, crossed with three nutrient management levels. After eleven complete crop cycles, the researchers sampled soil from all 36 plots at a depth of 0 to 15 centimeters and subjected it to a battery of twelve different potassium extractants.

The extractants spanned four mechanistic families. Distilled water and dilute salt solutions such as calcium chloride, barium chloride, ammonium acetate, sodium acetate, and sodium tetraphenyl boron target the potassium held in soil solution and on exchange sites. Multi-nutrient extractants, including ammonium bicarbonate-DTPA and the Mehlich solutions, pull several nutrients simultaneously. Acidic extractants, including strontium chloride mixtures and boiling nitric acid, attack the mineral lattice itself, releasing the non-exchangeable potassium locked between the layers of clay minerals. The logic is that plants do not simply sip from the soil solution; as roots deplete it, potassium desorbs from exchange sites and, over longer timescales, is released from mineral interlayers. A good soil test should mimic that whole cascade.

The results were striking. Boiling nitric acid extracted the most potassium, between 1,042 and 1,189 milligrams per kilogram of soil, followed by sodium tetraphenyl boron at 407.8 to 495.5 milligrams per kilogram, because both tap the non-exchangeable reservoir. At the other extreme, distilled water pulled out a mere 13.98 to 20.63 milligrams per kilogram. The workhorse ammonium acetate test extracted between 46.33 and 62.86 milligrams per kilogram, a middling figure that belies its global status. Among exchangeable-potassium extractants, barium chloride was the most aggressive, yielding 251.15 to 277.41 milligrams per kilogram, likely because barium is not preferentially adsorbed and does not collapse the expanded clay structures that hold potassium.

Correlation analysis revealed the real story. Sodium acetate emerged as the standout performer, correlating strongly with sodium tetraphenyl boron (r = 0.979), with relative grain yield (r = 0.927 to 0.984), with plant potassium uptake (r = 0.930 to 0.974), and with key soil indices such as cation exchange capacity, specific surface area, and organic carbon. The conventional ammonium acetate test performed respectably but less impressively, with correlations of 0.80 to 0.94 against plant indices. The acidic extractants fared worst: strontium chloride at 0.002 molar showed correlations as weak as 0.231 and no significant relationship with yield, while even boiling nitric acid, despite extracting the largest quantity of potassium, correlated poorly with what plants actually achieved. The researchers attribute sodium acetate’s advantage to its ability to pull water-soluble and exchangeable potassium along with a fraction of the non-exchangeable pool, capturing a broader spectrum of the potassium a crop can access during a growing season.

The team also established critical concentration thresholds using the Cate-Nelson statistical procedure, which partitions soil test values into ranges below which crops respond to fertilizer and above which they do not. Most extractants predicted critical levels with high coefficients of determination between 0.63 and 0.76. The critical value for ammonium acetate ranged from 51 to 54 milligrams per kilogram, while sodium acetate fell between 108 and 116 milligrams per kilogram. Boiling nitric acid produced the highest critical concentration, 1,224 milligrams per kilogram, but with the weakest predictive power, a coefficient of determination of only 0.35 to 0.46. Notably, critical values differed between rice and wheat for the same extractant, a reminder that soil test interpretation cannot be divorced from crop identity.

The agronomic findings were equally telling. Plots under full conservation agriculture, with residue retention in both seasons and minimal soil disturbance, outyielded conventionally tilled plots by 8.47 percent in rice and 10.72 percent in wheat, and recorded the highest potassium uptake, reaching 135.7 kilograms per hectare in rice and 112.4 kilograms per hectare in wheat. Residues appear to work through two mechanisms: their decomposition releases organic acids that dissolve potassium-bearing minerals, and the straw itself delivers a substantial potassium input back to the soil. Full conservation agriculture also raised cation exchange capacity by 19.4 percent relative to conventional tillage and boosted easily oxidizable organic carbon by 64.32 percent, changes that increase the number of adsorption sites available to hold and release potassium. Interestingly, additional potassium fertilizer did not significantly raise grain yields in this experiment, underscoring how much the residue-recycling system was already supplying.

The implications ripple outward from a single research farm in eastern Uttar Pradesh. Soil test calibration is inherently local, dependent on soil mineralogy, climate, and cropping system, and no single extractant works universally. But the message from Varanasi is clear: the global default test for potassium, ammonium acetate, captures only part of the story in soils where non-exchangeable potassium feeds crops, and it may systematically misjudge the potassium supplying power of conservation agriculture systems. Sodium acetate and the multi-nutrient extractants AB-DTPA, Mehlich I, and Mehlich III, which tap all or part of the non-exchangeable pool, tracked crop performance more faithfully. For a country where potassium fertilizer is expensive and farmers already under-apply it, a soil test that better reflects what the soil can actually deliver could save money, slow the mining of native reserves, and help sustain the rice-wheat system that feeds hundreds of millions. The researchers caution that their calibration must be validated across wider cropping systems, but the direction is unmistakable: the chemistry of measuring potassium needs to catch up with the agriculture it is meant to serve.

Subject of Research: Evaluation of chemical soil extractants for measuring plant-available potassium in long-term conservation agriculture rice-wheat systems

Article Title: Evaluating suitability of chemical extractants for measuring available soil potassium under long term conservation agriculture

Article References: Deepak, Jha, S., Singh, U. P., & Rakshit, A. (2026). Evaluating suitability of chemical extractants for measuring available soil potassium under long term conservation agriculture. Discover Soil, 3(1), Article 170. https://doi.org/10.1007/s44378-026-00330-w

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00330-w

Keywords: soil potassium, chemical extractants, conservation agriculture, rice-wheat system, ammonium acetate, sodium acetate, soil testing, Indo-Gangetic plains, non-exchangeable potassium, critical concentration, Cate-Nelson, crop residue retention