Manure-Tuned Fertilizer Prescriptions Boost Greengram Yields and Soil Life in Indian Alfisols

Farmers have long been told to fertilize for yield, but a new field study from Tamil Nadu, India, argues that the smartest fertilizer prescription is one that feeds the soil’s living machinery at the same time. Researchers at Tamil Nadu Agricultural University and partner institutions tested a soil-test-based, target-yield fertilizer system, adjusted with farmyard manure, on greengram grown in a red Alfisol, and found that it not only pushed grain yields past their targets but also measurably improved the biological quality of the rhizosphere soil. The work, published in Discover Soil, offers one of the most integrated validations yet of a fertilizer prescription framework that is usually judged on yield numbers alone.

The approach at the heart of the study is known as the soil test crop response integrated plant nutrition system, or STCR-IPNS. Rather than applying a blanket fertilizer recommendation to every field, the STCR method translates the measured fertility status of a specific soil into a precise fertilizer dose calculated to achieve a chosen yield target. In this experiment, the researchers paired that prescription with the integrated plant nutrition system concept: nutrients supplied by farmyard manure were credited against the calculated inorganic fertilizer requirement, so that manure and mineral fertilizer worked together rather than in parallel. The team compared this manure-adjusted prescription against fertilizer-only STCR prescriptions, organic-only inputs, the recommended dose of fertilizer, the recommended dose plus farmyard manure, and local farmer practice, across yield targets of 0.8, 1.0, and 1.2 tonnes per hectare.

The field trial was conducted at Poolampatty in the Vagarai block of Dindigul district, on a red, non-calcareous sandy loam of the Palaviduthi series, classified as a Typic Rhodustalf. The soil was mildly alkaline with a pH of 8.02, low in organic carbon at 0.35 percent, and contained available nitrogen, phosphorus, and potassium at 230, 25, and 370 kilograms per hectare respectively. Greengram cultivar CO 8 was grown at 30 by 10 centimeter spacing in a randomized block design with three replications, under a warm semi-arid monsoonal climate with only 26.6 millimeters of rain during the cropping period. Because the study covered a single season, the authors are careful to frame it as short-term validation evidence rather than a final recommendation.

The results were striking. The manure-adjusted STCR-IPNS treatment at the highest yield target of 1.2 tonnes per hectare achieved 105.8 percent of its target, meaning the prescription slightly over-delivered. That treatment produced plants 56.63 centimeters tall, with a leaf area index of 3.98 and SPAD chlorophyll readings of 50.76, the strongest canopy response in the experiment. Yield components followed suit, with more pods per plant, more seeds per pod, and heavier test weights than in fertilizer-only prescriptions at matched targets. Importantly, the yield advantage came from greater total biomass production rather than a shift in harvest index, indicating that better nutrient supply fueled overall growth rather than simply reallocating assimilates to grain.

Nutrient acquisition told a similar stoichiometric story. At harvest, the top treatment recorded peak uptake of 56.01 kilograms of nitrogen, 16.96 kilograms of phosphorus, and 41.43 kilograms of potassium per hectare across grain and haulm. The researchers mapped the relative balance of the three nutrients and found that manure-adjusted prescriptions moved the crop closest to balanced uptake, while control and organic-only plots sat far from that ideal. At equivalent yield targets, the integrated system recovered more of the applied nutrients than the fertilizer-only prescription, suggesting that the organic component improved the synchrony between nutrient release and crop demand, a long-standing goal of precision nutrient management.

Grain quality also responded. Crude protein, calculated from grain nitrogen using a conversion factor of 6.25, and true protein, measured by the Folin phenol method, both peaked under the manure-adjusted integrated treatment, along with crude protein yield per hectare. Fibre and sugar fractions shifted as well, indicating that the treatment altered carbohydrate partitioning in the grain alongside its protein enrichment. For a pulse crop whose value rests on protein density, this matters: the study suggests that yield gains need not come at the expense of nutritional quality, and may in fact reinforce it when nitrogen supply is well synchronized with crop demand.

But the most novel part of the study lies underground. The researchers measured a battery of rhizosphere indicators across the vegetative, flowering, and harvest stages: the activities of urease, alkaline phosphatase, beta-glucosidase, nitrate reductase, and dehydrogenase, which together represent nitrogen, phosphorus, carbon, and redox-linked biochemical processes; microbial biomass carbon and nitrogen; potentially mineralizable nitrogen; populations of bacteria, fungi, and actinomycetes; basal respiration; and the metabolic quotient, which expresses respiration per unit of microbial biomass. The integrated treatment lifted microbial biomass, most enzyme activities, and microbial populations above both fertilizer-only prescriptions and non-prescription benchmarks, pointing to a more functionally active rhizosphere community.

The authors are notably careful about what they claim. Soil organic carbon, labile carbon, and water-soluble carbon showed treatment-related trends but did not separate statistically across all stages, so they interpret the carbon response as short-term substrate support from manure rather than a confirmed improvement in carbon pools. Beta-glucosidase, likewise, was not consistently distinguishable from the next-best treatment and is treated as part of an overall enzyme pattern rather than standalone proof of enhanced carbon turnover. The metabolic quotient did not decline uniformly across stages, so it is discussed as a stage-dependent indicator rather than conclusive evidence of improved microbial carbon-use efficiency. This restraint strengthens the credibility of the findings that do hold up.

To integrate all these signals, the team built a soil quality index using principal component analysis. Indicators were standardized, components with eigenvalues above one were retained, and variables with absolute loadings of at least 0.70 formed a minimum data set, which was scored and weighted to produce a single index value. The index clearly separated the manure-adjusted integrated treatment, especially at the highest yield target, from fertilizer-only prescriptions, organic-only inputs, blanket recommendations, farmer practice, and the unfertilized control. Enzyme activity, microbial biomass, respiration balance, and nutrient availability emerged as the dominant drivers of the index, confirming that the treatment’s superiority rested on coordinated changes across multiple soil functions rather than any single variable.

The broader significance is a validation framework. Target-yield fertilizer prescriptions have historically been judged almost exclusively on whether the crop hits its yield goal, with little attention to whether the prescription sustains the enzyme-mediated nutrient cycling and microbial regulation on which long-term soil fertility depends. By coupling yield achievement, nutrient uptake, grain quality, enzyme activity, microbial functional response, and a multivariate soil quality index in one evaluation, this study shows that a prescription can be designed to do both jobs at once. The authors caution that the reliability of the approach depends on site-specific soil test calibration, accurate assessment of manure nutrient composition, and realistic consideration of input availability and economics, and they call for multi-season, multi-location trials across contrasting Alfisols and other pulse systems. If those trials confirm the pattern, manure-adjusted, soil-test-based prescriptions could become a practical template for pulse production that treats soil health not as a constraint on yield, but as part of the yield equation itself.

Subject of Research: Target-yield based integrated nutrient management for greengram productivity and soil biological quality in an Alfisol

Article Title: Target yield based integrated nutrient management improves greengram productivity nutrient uptake and soil biological quality in an Alfisol

Article References: Abhirami, P., Venkateswarlu, M., Maragatham, S., Rajeswari, R., & Balachandar, D. (2026). Target yield based integrated nutrient management improves greengram productivity nutrient uptake and soil biological quality in an Alfisol. Discover Soil, 3(1), Article 169. https://doi.org/10.1007/s44378-026-00328-4

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00328-4

Keywords: greengram, STCR-IPNS, target yield prescription, farmyard manure, integrated nutrient management, Alfisol, soil quality index, soil enzymes, microbial biomass, nutrient uptake, rhizosphere, Tamil Nadu