Soil-Test Equations Promise Sharper Fertilizer Doses for India’s Field Bean Farmers

For decades, farmers growing field bean in the semi-arid Eastern Dry Zone of Karnataka have received the same blanket fertilizer advice regardless of what their soils actually contain. A new study published in BMC Plant Biology argues that this one-size-fits-all approach is quietly costing yields, and it offers a data-driven alternative: a set of validated, soil-test-based prescription equations that tell a farmer exactly how much nitrogen, phosphorus and potassium to apply to hit a specific yield target on their own land.

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