Digestate Delivers Strongest Overall Wheat Performance, Field Trial Shows

A two-season randomized field experiment at the Indian Agricultural Research Institute in New Delhi found that adding a modest amount of solid digestate to the recommended mineral fertilizer dose offered the strongest overall balance of wheat yield, soil fertility, profitability and energy performance. The treatment produced 6,476 kg ha-1 of grain, 6.9% more than mineral fertilizer alone, and generated the highest net return among the 16 strategies evaluated. Yet it also recorded the largest apparent phosphorus and potassium deficits, illustrating the difference between short-term agronomic performance and long-term nutrient sustainability. The experiment compared mineral fertilizer alone with treatments involving biochar, solid and liquid digestate, spent wash incinerator ash and elemental sulphur under irrigated winter wheat production on a sandy loam soil in the Indo-Gangetic Plains.

The investigators used the wheat cultivar HD 2967 during the 2022–23 and 2023–24 rabi seasons. The soil was slightly alkaline, non-saline and relatively low in organic carbon and available nitrogen, but medium in available phosphorus and potassium. Each 4- by 6-m plot received the same treatment in both years, allowing the researchers to assess treatment effects across consecutive wheat crops. Sixteen strategies included the full recommended fertilizer dose of 150–60–60 kg ha-1 of N–P2O5–K2O; fertilizer top-ups; partial replacement of mineral fertilizer with biomass-derived materials; and alternatives for either nitrogen or potassium. After error variances were found to be homogeneous, the researchers pooled the two seasons because no variable showed a significant year-by-treatment interaction, allowing treatment rankings to be compared across both crops.

The most productive option combined the full recommended mineral fertilizer dose with 0.4 t ha-1 of solid digestate incorporated before sowing. Grain yield increased from 6,056 to 6,476 kg ha-1 relative to mineral fertilizer alone, while effective tillers, spike length, grains per spike, grain weight per spike and test weight rose by 21.9%, 17.3%, 8.3%, 20.0% and 4.9%, respectively. Basal biochar, liquid digestate used as a potassium source and spent wash incinerator ash used as a potassium source produced statistically comparable grain yields. These integrated treatments generally improved reproductive growth, whereas strategies relying predominantly on organic nitrogen produced more straw and biological yield but less grain. Supplying the entire crop nitrogen requirement through 15 t ha-1 of solid digestate, for example, yielded 5,608 kg ha-1 of grain despite generating 18% more straw than the highest-yielding treatment.

That contrast exposed a central agronomic trade-off. Organic nutrients released gradually did not appear to synchronize fully with wheat demand for nitrogen during reproductive development, although amendment mineralization rates were not measured directly. The all-digestate treatment also received no mineral phosphorus or potassium, and its digestate supplied little potassium. By contrast, the winning 0.4-t digestate top-up added only about 4 kg N ha-1, so its yield improvement cannot be explained by additional nutrient supply alone. The investigators suggest that combining readily available mineral nutrients with slowly released digestate nutrients may have supported grain formation, but root growth, rhizosphere properties, enzyme activity, chlorophyll status and assimilate translocation were not assessed. The proposed mechanism therefore remains a hypothesis rather than a demonstrated causal pathway.

Soil measurements showed that integrated amendments also improved post-harvest fertility. The full fertilizer treatment supplemented with 0.4 t ha-1 of solid digestate had 35.7% more available nitrogen, 23.8% more available phosphorus and 18.3% more available potassium at harvest than mineral fertilizer alone. Its soil organic carbon was 21.3% higher, and microbial biomass carbon at anthesis was 27.6% greater. Digestate treatments generally supported more microbial biomass than corresponding biochar treatments, while replacing potassium with spent wash incinerator ash produced the largest relative increase in measured soil organic carbon, 53.2% above the mineral-only treatment. Soil electrical conductivity did not differ significantly among treatments, providing no evidence of salt accumulation at the rates tested, while pH remained within a relatively narrow range of 7.41 to 8.04 at flowering and converged to statistically similar values after harvest.

The carbon results nevertheless require caution. The solid digestate in the highest-yielding treatment added only about 35 kg C ha-1, too little to explain the full difference in measured soil organic carbon by itself. More vigorous roots, rhizodeposition, shifts in labile carbon and analytical or spatial variability may also have contributed. Two seasons cannot establish long-term carbon sequestration, particularly without comprehensive measurement of bulk density and carbon fractions. The apparent nutrient balances were similarly limited to plant-available nutrients in the upper 15 cm of soil and did not account for non-exchangeable reserves, subsoil supply, atmospheric deposition, irrigation inputs, leaching or gaseous losses. They are therefore indices of mean single-season nutrient removal and replacement, not complete mass balances or cumulative two-year budgets.

Those balances revealed a major vulnerability of the top-ranked strategy. The full fertilizer plus solid digestate treatment recorded apparent nitrogen, phosphorus and potassium balances of −89.2, −21.2 and −125.2 kg ha-1 season-1, respectively. Its potassium deficit was the largest among the treatments because crop removal increased while the digestate contained only 0.02% potassium. Every treatment had a negative potassium balance, ranging from −45.0 kg ha-1 season-1 under mineral fertilizer alone to −125.2 under the leading treatment. Positive nitrogen surpluses in several organic-heavy strategies do not necessarily indicate improved sustainability: nitrogen neither taken up by the crop nor retained in the available soil pool may be vulnerable to ammonia volatilization, nitrate leaching or denitrification, none of which was measured. High-yielding treatments are also mechanically more likely to show negative balances because crop uptake is counted as an output.

Economically, the best-yielding treatment was also the most profitable in absolute terms. Its estimated gross return was ₹130,495 ha-1 and its net return was ₹88,601 ha-1, compared with ₹122,029 and ₹83,824 under mineral fertilizer alone. The additional digestate made cultivation about 9.9% more expensive, but the yield gain compensated for that investment. Mineral fertilizer alone retained the highest benefit-cost ratio, at 3.19, while the leading digestate treatment achieved 3.11; the difference was not statistically significant. Replacing 25% of mineral fertilizer by weight with solid digestate was also economically competitive, with a benefit-cost ratio of 3.15 and a net return of ₹81,299 ha-1. In contrast, supplying all nitrogen through solid digestate required 15 t ha-1 of material, raised cultivation cost to ₹156,839 ha-1 and produced a net loss of ₹43,828 ha-1.

Energy accounting produced a parallel divide between productive and efficient strategies. Digestate-based treatments generally performed well because the material was assigned a low embodied energy requirement and could replace energy-intensive mineral fertilizer. Replacing 25% of the fertilizer with solid digestate increased energy-use efficiency by 28.3%, improved the energy efficiency ratio by 15.5% and reduced specific energy by 13.2% relative to mineral fertilizer alone. Biochar-intensive strategies told the opposite story. The treatments designed to supply 25% of crop nitrogen through biochar required about 73.4–73.6 GJ ha-1, roughly 2.4 times the mineral-fertilizer input, and increased specific energy by 143–158%. Their additional biological yield did not offset the embodied energy assigned to biochar production, although this conclusion remains sensitive to the chosen coefficient and to whether useful coproducts such as heat are credited.

A combined ranking using the Technique for Order Preference by Similarity to Ideal Solution placed the full fertilizer plus 0.4-t solid digestate treatment first, with a closeness coefficient of 0.852, followed by the 25% digestate substitution treatment and the liquid-digestate potassium strategy. Principal component analysis supported this result: the first two components explained 86.2% of the variation, separating a productivity-profitability-soil fertility gradient from an energy-efficiency gradient. The ranking is a best-compromise judgment rather than an absolute optimum, because all 12 criteria received equal weight, related measures carried substantial influence, and nutrient balances and environmental losses were omitted. The field trial itself was limited to one location, one soil type, one cultivar, two seasons and three replicates, while the protected least significant difference procedure can increase the risk of false-positive comparisons. Longer-term, multi-location validation is therefore needed before the results are generalized across the Indo-Gangetic Plains, where the practical performance of digestate will also depend on local supply, transport costs, amendment quality, storage and the long-term maintenance of phosphorus and potassium reserves.

Subject of Research: Field evaluation of biomass-derived nutrient strategies for irrigated wheat productivity, soil fertility, nutrient balance, economics and energy efficiency

Article Title: Integrated biomass-derived nutrient sources for sustainable wheat production: Agronomic, soil fertility, economic and energy perspectives

Article References: Rathore, S. S., Padhan, S. R., Sairam, A., Shekhawat, K., Upadhyay, P. K., Babu, S., Singh, R. K., Sarkar, S., Saini, S., Tyagi, V., Nagargade, M., & Singh, A. (2026). Integrated biomass-derived nutrient sources for sustainable wheat production: Agronomic, soil fertility, economic and energy perspectives. Journal of Agriculture and Food Research, Article 103385. https://doi.org/10.1016/j.jafr.2026.103385

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103385

Keywords: wheat productivity, solid digestate, integrated nutrient management, biochar, soil fertility, nutrient balance, energy efficiency, agricultural economics, Indo-Gangetic Plains, circular bioeconomy, Integrated, biomass-derived