Agricultural waste valorization in the circular economy using sunflower husk ash as a secondary resource

Abstract The valorization of agricultural waste within the circular economy provides an opportunity to reduce waste disposal and return valuable mineral resources to agricultural production. Sunflower processing generates substantial amounts of husk, and the accumulation of ash from its industrial utilization represents an important waste-management challenge. In Ukraine alone, more than 100 thousand tonnes of such ash are generated annually. Instead of being disposed of, sunflower husk ash (SHA), a by-product of sunflower processing, can be used as a secondary mineral resource for fertilizer production. Such an approach to waste management can contribute to the formation of added value, increased agricultural production, and the implementation of environmental initiatives. This study aimed to assess the feasibility of using sunflower husk ash-based fertilizer in agriculture and to evaluate its effects on crop yield, soil agrochemical properties, and selected quality parameters of agricultural products. Field experiments were conducted during one growing season at the Slobozhanske Experimental Field of the National Scientific Center “O.N. Sokolovsky Institute for Soil Science and Agrochemistry Research”, Ukraine, on a typical heavy loam chernozem. Seven crops were evaluated under field conditions: barley ( Hordeum vulgare ), maize ( Zea mays ), sunflower ( Helianthus annuus ), potato ( Solanum tuberosum ), tomato ( Solanum lycopersicum ), table beet ( Beta vulgaris ), and cucumber ( Cucumis sativus ). The experimental treatments included without fertilizers (control), conventional NPK 16:16:16 fertilizer, and SHA-based fertilizer applied at rates of 200 and 300 kg/ha using basal application and application during pre-sowing cultivation. The size of the plot for each crop was 0.1 ha. Crop yield, soil agrochemical properties, and selected quality parameters of agricultural products were evaluated. The obtained data were processed using descriptive and comparative statistical methods, including calculation of arithmetic means and absolute and relative differences. The application of SHA-based fertilizer was associated with an increase in crop yield of approximately 10–18% compared with without fertilizers (control). The content of mobile phosphorus and exchangeable potassium in the soil increased by approximately 10–22%, while soil pH changed by approximately 0.3–0.5 units. An increase in selected quality parameters of agricultural products, including dry matter, protein, and fat content, was also observed. The SHA-based fertilizer contained 30% K₂O, 5% P₂O₅, 9% CaO, 10% MgO, and 4% S and did not contain chlorine. The results were obtained during one growing season at a single experimental site and therefore reflect the specific soil, climatic, and experimental conditions of the study. The results should therefore be interpreted descriptively rather than as statistically significant effects. Further studies in different natural and climatic zones and on soils with different properties are required. The results provide initial field evidence that SHA-based fertilizer can serve as a secondary mineral resource for agricultural production within a circular economy approach. Its use may contribute to the valorization of sunflower processing residues, reduction of waste disposal, and return of mineral nutrients to agricultural soils. Further field research is required to assess the consistency and broader applicability of the observed effects under different soil and climatic conditions.