The research, led by Krishna Murthy Rangaiah of the All India Coordinated Research Project on Soil Test Crop Response at the University of Agricultural Sciences, Bengaluru, was conducted between 2023 and 2025 under the Soil Test Crop Response, or STCR, framework. This approach, long championed by the Indian Council of Agricultural Research, rests on a simple but powerful idea: the amount of fertilizer a crop needs is not a fixed number but a function of the yield the farmer wants and the nutrients the soil can already supply. By quantifying how much of each nutrient comes from the soil, from fertilizer, and from organic amendments such as farmyard manure, the method converts a routine soil test into a tailored fertilizer prescription.

Field bean, Lablab purpureus, is a multipurpose legume of enormous importance in the Eastern Dry Zone, valued for grain, fodder and green manure, and prized for its tolerance of the region’s erratic rainfall. Yet its productivity has remained stubbornly sub-optimal, in part because existing recommendations ignore the wide variation in soil fertility from farm to farm. The Alfisols that dominate the zone are inherently variable, weathered soils whose nutrient-supplying capacity can differ dramatically even between adjacent fields, making them an ideal test case for site-specific nutrient management.

The study unfolded in three carefully sequenced phases. First, the team established a fertility gradient experiment using fodder maize as an exhaustive crop. By applying differential doses of nutrients and organic matter across a single field, they created three distinct fertility strips: low, medium and high, each with measurably different levels of available nitrogen, phosphorus and potassium. This artificial gradient is the statistical engine of the STCR method, because it generates the wide range of soil-test values and crop responses needed to fit reliable equations on a manageable plot of land.

With the gradient in place, the researchers moved to the second phase: a main calibration experiment with field bean. Across the three fertility strips, they applied a factorial combination of fertilizer levels and farmyard manure, then measured both seed yield and the total uptake of nitrogen, phosphorus and potassium in the harvested crop. From these paired observations they derived the core parameters of the targeted-yield equations: the nutrient requirement, expressed in kilograms of nutrient per quintal of grain; the contribution of soil nutrients to yield; the contribution of applied fertilizer; and, in the integrated treatment, the additional contribution of farmyard manure. Separate equations were fitted for the NPK-only system and for the NPK plus farmyard manure system, allowing the organic amendment to be priced into the prescription.

The mathematics behind these equations is straightforward once the parameters are known. For a chosen yield target, the required fertilizer dose equals the nutrient requirement multiplied by the target, minus the product of the soil test value and the soil contribution coefficient, minus the manure contribution where applicable. In practice, this means a farmer with a phosphorus-rich soil test value receives a smaller phosphorus recommendation than a neighbor with depleted soils, even when both aim for the same harvest. It is a level of precision that blanket recommendations, by design, cannot achieve.

The third phase put the equations to the test in validation trials at two locations, where STCR-based prescriptions were compared head-to-head with the general recommended dose of fertilizer and with the soil fertility-rating approach, a coarser method that adjusts recommendations only by broad fertility classes. The results were striking. Treatments targeting a yield of 12 quintals per hectare under the integrated NPK plus farmyard manure system delivered the highest seed yields of the entire study, reaching 13.42 and 13.21 quintals per hectare at the two validation sites. That performance exceeded the general recommended dose by 20.2 to 23.7 percent and the soil fertility-rating approach by 35.1 to 38.8 percent.

There is an honest caveat in the data that the authors themselves flag. Observed yields under the four STCR treatments overshot their specified targets by 7.8 to 17.6 percent, indicating positive prediction errors rather than the uniform agreement within plus or minus ten percent that a perfectly calibrated model would show. In other words, the equations lean generous, prescribing slightly more nutrients than strictly necessary to hit a target. For farmers, this bias is arguably the safer direction, since under-fertilizing a legume crop risks forfeiting yield, but the overshoot suggests the equations may warrant refinement as more validation data accumulate.

One of the study’s most economically significant findings concerns farmyard manure, a resource many smallholders in the region already possess. Integrating FYM into the nutrient budget reduced the calculated fertilizer requirements for nitrogen, phosphorus pentoxide and potassium oxide by roughly 8 to 15 percent at comparable soil-test levels. The organic amendment did more than substitute for purchased fertilizer; it also improved the efficiency with which the crop used applied nutrients. Apparent recovery of potassium, a measure of how much of the applied nutrient ends up in the harvested plant, climbed to 2.74 to 2.81 kilograms per kilogram under the integrated system, compared with 1.82 to 1.89 kilograms per kilogram under fertilizer alone. Agronomic efficiency followed the same pattern, reinforcing the case for combining organic and inorganic sources rather than treating them as substitutes.

The implications extend well beyond a single crop or district. India’s fertilizer subsidy bill is enormous, and nutrients applied without reference to soil status are simultaneously a fiscal drain and an environmental hazard, contributing to nitrate leaching and phosphorus runoff. Equations of the kind validated here, which are crop-specific and soil-specific by construction, offer a pathway to site-specific nutrient management that a soil testing laboratory can operationalize with routine analyses. For the Eastern Dry Zone, where Alfisols are marginal and rainfall unreliable, squeezing an extra fifth of the harvest from the same fertilizer investment could meaningfully change the economics of legume farming. The study, funded by the Indian Council of Agricultural Research under grant CRP-18 and conducted with collaborators from the ICAR-Indian Institute of Soil Science in Bhopal and the ICAR-Agricultural Technology Application Research Institute in Kolkata, establishes a replicable template: build a fertility gradient, calibrate against yield and uptake, validate in farmers’ conditions, and hand the resulting equations to the soil testing laboratories that serve the fields. If similar calibrations are extended to the region’s other crops, the era of the blanket recommendation may finally be drawing to a close.

Subject of Research: Soil test crop response-based targeted yield fertilizer prescription for field bean on Alfisols in Karnataka

Article Title: Development and validation of STCR-based fertilizer prescription equations for field bean under Alfisol conditions of the Eastern Dry Zone of Karnataka

Article References: Rangaiah, K. M., Nagendrachari, A. N., Nanjundappa, S. M., Kasturappa, G., Nagaraju, B., Krishna, P., Srivastava, S., Haokip, I. C., & Dey, P. (2026). Development and validation of STCR-based fertilizer prescription equations for field bean under Alfisol conditions of the Eastern Dry Zone of Karnataka. BMC Plant Biology. https://doi.org/10.1186/s12870-026-09986-6

Image Credits: AI Generated

DOI: 10.1186/s12870-026-09986-6

Keywords: field bean, STCR, targeted yield, fertilizer prescription, Alfisols, Eastern Dry Zone, Karnataka, soil fertility, integrated nutrient management, farmyard manure, nutrient use efficiency, site-specific nutrient management

The findings, published in the Journal of Agriculture and Food Research, carry serious implications for how growers interpret soil tests, not just in Florida but anywhere crops are grown on phosphorus-enriched soils. Florida’s potato industry is substantial, with roughly 6,800 hectares harvested in 2023 producing 190,500 megagrams of tubers valued at 105.4 million dollars. Because potato plants have shallow root systems, limited root hairs, and a notoriously weak ability to scavenge phosphorus from soil, the crop is heavily dependent on fertilizer inputs. Decades of repeated applications have left many fields with elevated soil-test phosphorus values, well above the thresholds where guidelines say no additional fertilizer is needed.

Yet growers in the region have repeatedly reported yield losses when they withheld phosphorus in high-testing soils. To investigate, the research team ran a factorial experiment over three growing seasons at the UF/IFAS Hastings Agricultural Extension Center. Two adjacent 0.3-hectare sites were planted with the chipping cultivar Atlantic each spring from 2023 to 2025. The sites shared a soil type, a sandy Alfisol of the Ellzey series, but differed sharply in their starting phosphorus status. Site 1 averaged 50.6 milligrams per kilogram of Mehlich-1 extractable phosphorus in the surface 15 centimeters, while site 2 averaged 110.9 milligrams per kilogram. Under the historical interpretation system, both values fall into the high or very high categories, which normally trigger no phosphorus recommendation at all.

Each site received ten treatment combinations: five phosphorus pentoxide rates, from zero up to 269 kilograms per hectare, crossed with two nitrogen rates of 168 and 280 kilograms per hectare. The team tracked soil phosphorus and mineral nitrogen through the profile, measured plant biomass and nutrient uptake at 50, 70, and 90 days after planting, and mechanically harvested and graded tubers according to USDA size classes. A total of 240 plot-level observations across six site-years fed into the analysis.

The results were striking. At site 1, the field with the lower starting phosphorus, fertilization raised total yield from 21.6 megagrams per hectare in the unfertilized control to an average of 30.6 megagrams across rates of 135 to 269 kilograms of phosphorus pentoxide per hectare, a gain of roughly 42 percent. Marketable yield climbed even more dramatically, from 16.4 to about 25.1 megagrams per hectare, an increase of 53 percent. Most of the gain came from intermediate-sized tubers, the A23 fraction, which jumped from 9.1 to 17.4 megagrams per hectare. Tuber specific gravity, a key quality measure for chipping potatoes, also improved. At site 2, by contrast, phosphorus rate had no significant effect on total yield, marketable yield, tuber size distribution, or specific gravity, and the yield gap between the two sites vanished once phosphorus was applied at 135 kilograms per hectare or more.

The physiological story behind those numbers is revealing. At site 1, phosphorus fertilization increased whole-plant dry biomass from 5.48 to 7.96 megagrams per hectare and nitrogen uptake from 89.5 to 132.2 kilograms per hectare, while nitrogen uptake efficiency rose from 26.8 to 42.3 percent. In other words, when phosphorus is limiting, the constraint ripples outward: restricted root and shoot growth lower the plant’s demand for nitrogen and its capacity to acquire it. Adequate phosphorus supply therefore unlocks the crop’s ability to use nitrogen efficiently. At site 2, the higher-phosphorus field, the only response to added fertilizer was a modest increase in nitrogen uptake efficiency, with biomass and productivity per unit of nitrogen unchanged. Notably, whole-plant phosphorus uptake at site 2 pulled ahead of site 1 after 50 days after planting and stayed ahead through harvest, showing that the richer soil sustained phosphorus delivery during the critical tuber-bulking period.

Nitrogen management, meanwhile, turned out to be a non-player in the phosphorus story. The two nitrogen rates produced clearly different soil mineral nitrogen levels, but phosphorus rate by nitrogen rate interactions were limited to efficiency indices, and the crop’s principal responses to phosphorus were identical under both nitrogen regimes. Raising nitrogen from 168 to 280 kilograms per hectare never increased yield and consistently reduced both nitrogen uptake efficiency and partial factor productivity, the tubers produced per kilogram of nitrogen applied. In the difficult 2024 season, when seed-piece decay cut plant stands by 57 percent, the higher nitrogen rate actually lowered total and marketable yields by about 4.4 megagrams per hectare. The team stresses this does not establish 168 kilograms as a regional optimum, only that extra nitrogen neither overcame phosphorus deficiency nor added yield when phosphorus was sufficient.

The soil itself told a story of accumulation and movement. Over the experiment, surface Mehlich-1 phosphorus in fertilized plots rose by roughly 63 to 67 milligrams per kilogram, while unfertilized controls declined by 7 to 27. Concentrations peaked near crop emergence after pre-plant fertilization and then shifted, with measurable increases detectable down to the 30 to 45 centimeter layer, particularly at site 2 in 2024. Because most potato roots in these seepage-irrigated soils sit within the top 30 centimeters, phosphorus moving below that depth is effectively out of reach for the crop and represents a potential pathway for transport off-site, although drainage losses were not directly measured in this study.

To translate the yield responses into a usable threshold, the researchers modeled relative yield against Mehlich-1 phosphorus measured at harvest using four normalization methods and several calibration models. The best-performing combination, the MAXRATE normalization paired with a modified arcsine-log model, explained 41 percent of the variation in relative yield. It estimated critical phosphorus levels of 63.6 milligrams per kilogram for 80 percent relative yield, 81.4 for 90 percent, and 96.2 for 95 percent, with the response curve flattening above roughly 100 milligrams per kilogram. Those numbers are far above the historical sufficiency threshold of about 30 milligrams per kilogram of Mehlich-1 phosphorus, and they even exceed the high category boundaries of both the Mehlich-1 and Mehlich-3 systems currently used in Florida. The moderate fit and the overlap of high- and low-yielding observations across the range reinforce a point echoed in decades of Ohio fertilizer trials: a soil test is a probabilistic index of potentially available phosphorus, not a deterministic boundary between deficiency and sufficiency.

The practical message is nuanced. A high soil-test classification does not guarantee a field will ignore added phosphorus, and skipping fertilizer in a responsive high-testing soil can be costly; conversely, blanket phosphorus applications to nonresponsive soils simply inflate soil phosphorus reserves and environmental risk. The authors caution that their critical level of 96.2 milligrams per kilogram derives from a specific soil, cultivar, sampling protocol, and set of conditions, and that each initial phosphorus status was represented by a single site with different irrigation infrastructure, so broader calibration and independent validation are needed before any new threshold enters recommendations. Still, the study makes a compelling case that Florida’s provisional allowance of up to 134 kilograms of phosphorus pentoxide per hectare, applied regardless of soil test, is a blunt instrument. A validated critical level could enable a tiered strategy: full fertilization below the threshold, crop-removal-based maintenance near it, and deliberate drawdown above it, protecting both potato yields and the waterways downstream of phosphorus-enriched sands.

Subject of Research: Potato yield and nutrient uptake responses to phosphorus fertilization in phosphorus-enriched sandy soils

Article Title: Initial soil phosphorus status governs potato yield and nutrient uptake responses to fertilization in P-enriched sandy soils

Article References: Oliveira, J. D. M., de Castro, G. F., da Silva, B. A., Clark, M. W., Nunez, G. H., Guzmán, S., & Zotarelli, L. (2026). Initial soil phosphorus status governs potato yield and nutrient uptake responses to fertilization in P-enriched sandy soils. Journal of Agriculture and Food Research, 31, Article 103296. https://doi.org/10.1016/j.jafr.2026.103296

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103296

Keywords: phosphorus, potato, soil testing, Mehlich-1, fertilizer recommendations, sandy soils, nitrogen use efficiency, critical phosphorus level, Florida agriculture, nutrient uptake, soil fertility, alfisols