The research team, led by Adrián González-Guzmán of the University of Seville with colleagues from the University of Granada and the University of Córdoba, formulated four different residue-based amendments (RBAs) from 14 distinct waste materials collected from nine companies across southern Spain and Portugal. Before being used as soil amendments, these material combinations had already served a first life as multi-layer filters capturing nitrogen gases such as ammonia and nitrogen dioxide from pig slurry, enriching them with carbon, nitrogen, and sulfur in the process. The study then tested whether this dual-purpose strategy — mitigating gaseous emissions from livestock while recycling the captured nutrients — could deliver real agronomic value for wheat cultivation.

The four amendments were not standardized products. Each was deliberately formulated to correct the dominant physicochemical constraints of its target soil, following the requirements of EU Regulation 2019/1009 on fertilizing products. Soil 1 was acidic with low phosphorus and potassium availability, low cation exchange capacity, and dangerously high manganese levels that risked plant toxicity. Soil 2 was alkaline with low organic carbon and poor potassium supply. Soil 3 suffered from low zinc availability and weak cation exchange capacity, while Soil 4 contained high active lime that locked away micronutrients. The amendments were designed with weighted means from their constituent residues to address these problems: RBA1, for example, combined ash and cork to raise soil pH, increase phosphorus and potassium availability, and simultaneously reduce manganese solubility in the acidic Soil 1.

The experimental design was rigorous. Four treatments were compared in each soil under a completely randomized pot experiment conducted between December 2024 and May 2025 at the University of Córdoba’s Rabanales campus: an unfertilized control, the residue-based amendment alone, conventional inorganic fertilizer supplying identical amounts of nitrogen, phosphorus, and potassium, and a 50:50 mix of amendment and fertilizer as an integrated nutrient management (INM) strategy. Bread wheat (Triticum aestivum L. var. Átomo) was grown as the model crop, and performance was evaluated through germination, leaf chlorophyll (SPAD values), total biomass, grain yield, plant nutrition, nitrogen use efficiency, recovery efficiency, and physiological efficiency. Soils and fertilizers were incubated for two weeks before sowing to allow initial interactions between soil particles, amendments, and microbial activity.

The central result was a significant soil × treatment interaction for nearly every variable analyzed — a statistical finding with profound practical implications. It means that no single fertilization strategy wins everywhere. RBA1 delivered the highest yield increase over the unfertilized control in Soil 1, a remarkable 97 percent improvement, while RBA4 achieved a 39 percent gain in Soil 4. In contrast, inorganic fertilizer dominated in Soil 2 with a 223 percent yield increase, and the combined RBA + IF treatment produced the best result in Soil 3, boosting yield by 183 percent. These divergent outcomes demonstrate that the agronomic value of waste-derived amendments depends less on their origin than on their capacity to alleviate the specific physicochemical constraints of the target soil.

RBA1 stands out as the clearest success of the soil-tailored formulation approach. Applied to the acidic Soil 1, it produced the highest grain yield, plant phosphorus uptake comparable to inorganic fertilizer, and significantly reduced manganese uptake despite the soil’s naturally high manganese availability. The researchers attribute this to choosing materials that supply the right functions — nutrient supply, pH modification, and contribution to finer soil texture — even when combined with widely different residues such as ashes with cork. Early phosphorus availability is particularly critical for tillering and root development in cereals, while potassium supports photosynthesis, assimilate translocation, and water regulation, all of which likely contributed to RBA1’s superior performance.

RBA2’s results reveal a subtler challenge: nutrient-release timing. Although the amendment included a labile organic fraction (40 percent orange peel) intended to acidify the alkaline Soil 2 and accelerate nutrient availability during early phenological stages, plant performance under RBA2 remained below that of inorganic fertilizer. Potassium recovery was only 18 percent under RBA2 versus 29 percent under inorganic fertilizer, suggesting that a longer incubation period may be needed to improve nutrient mineralization when soil potassium sits below critical levels. High carbon-to-nitrogen ratio materials such as cork and uncooked biomass from cellulose extraction may have slowed decomposition and caused temporary microbial nitrogen immobilization, competing with plants for nutrients during crop growth.

A particularly striking finding concerned grain zinc concentration. Across all four soils, inorganic fertilization consistently reduced grain zinc compared with the unfertilized control. This matters enormously for wheat, a staple cereal and primary dietary zinc source in many developing countries where affordable biofortification strategies are often unavailable. The result suggests that conventional phosphate fertilizers may inadvertently dilute grain micronutrient density, whereas well-designed waste-derived amendments could help preserve or enhance nutritional quality.

Food safety was also scrutinized. Grain concentrations of lead, cadmium, arsenic, and nickel were compared against EU Regulation 2023/915 and Commission Regulation (EU) 2024/1987 limits, while straw concentrations were checked against Directive 2002/32/EC for animal feed. No regulatory exceedances occurred except in the unfertilized control of Soil 1, where lead reached 0.48 mg/kg and nickel 1.26 mg/kg against maximum allowed levels of 0.2 and 0.8 mg/kg respectively — likely reflecting the lower biomass dilution and naturally higher element concentrations in that acidic soil. Interestingly, barium in grain was consistently higher under inorganic fertilizer than under the waste-derived amendments, probably because barium occurs naturally in the phosphate rock used to manufacture superphosphate. Chromium and aluminum in straw also showed higher concentrations under certain inorganic treatments, highlighting that trace-element impurities deserve attention in both conventional and recycled fertilizers alike.

The study also quantified the nitrogen bonus from the amendments’ previous life as slurry gas filters. Gaseous nitrogen recovered during filtration represented an additional nitrogen input of approximately 34, 27, 13, and 40 percent for RBA1 through RBA4 respectively — nitrogen that was captured from livestock emissions and reintroduced into the soil-plant system rather than lost to the atmosphere. The residue-based amendments produced crop performance comparable or superior to inorganic fertilizer in three of the four soils, indicating that total nutrient concentration alone does not determine crop response; nutrient availability and release kinetics during early crop establishment are what count. The combined RBA + IF treatment consistently raised harvest index above inorganic fertilizer alone across all soils, likely reflecting complementary nutrient supply: readily available nutrients from mineral fertilizer during early growth, and more sustained release from the organic amendment during reproductive and grain-filling stages.

The authors conclude that the agronomic value of residue-based amendments depends less on their origin than on their capacity to alleviate the main physicochemical constraints of the target soil. Contrary to their initial hypothesis, the integrated nutrient management approach did not consistently produce the best results, indicating that nutrient-release timing must be more accurately synchronized with crop demand. The researchers acknowledge that their findings come from a pot experiment with small experimental units, without leachate analysis, and under controlled incubation conditions, so field validation is needed — particularly to improve understanding of nutrient dynamics, salinity, and material mineralization under real farming conditions. Still, the work provides a conceptual framework for advancing circular fertilization strategies beyond conventional waste valorization, suggesting that the same material combinations can simultaneously mitigate ammonia emissions from livestock, recover nitrogen that would otherwise be lost, and improve soil health and crop productivity. As technological advances enable more efficient integration of data on soils, crops, residues, legislation, and environmental conditions, the optimization of waste-derived amendment formulations could become a cornerstone of Europe’s transition toward a genuinely circular agricultural economy.

Subject of Research: Soil-tailored waste-derived amendments for wheat within integrated nutrient management

Article Title: Designed residue-based amendments tailored to soil-specific requirements for wheat within an integrated nutrient management framework

Article References: González-Guzmán, A., Vela Herrador, J., Navajas, A., Barrón, V., & Sánchez-Rodríguez, A. R. (2026). Designed residue-based amendments tailored to soil-specific requirements for wheat within an integrated nutrient management framework. Frontiers in Soil Science, 6, Article 1955073. https://doi.org/10.3389/fsoil.2026.1955073

Image Credits: AI Generated

DOI: 10.3389/fsoil.2026.1955073

Keywords: Designed, residue-based, amendments, tailored, soil-specific, requirements, wheat, within, integrated, nutrient, management, framework