Transforming Biomass Power Wastes to Innovative Fertiliser: Effect of Poultry Manure Ash-Based Fertilisers on the Growth and Mineral Nutrition of Lettuce and Maize Plants

The natural sources used to make phosphorus fertilisers are limited, making it essential to develop alternative sources for sustainable plant production. This study explores the feasibility of using boiler ash (BA), a waste byproduct from biomass power plants, as a raw material for fertiliser. It also aims to identify the optimal combination of sulfuric and phosphoric acids to convert the phosphorus (P) in BA into a water-soluble form. Boiler ash was acidified with 1M sulfuric and phosphoric acid mixtures in ratios of 100/0, 75/25, 50/50, 25/75 and 0/100. The molecular structure and functional properties of the resulting materials were analysed using SEM and XRD, and their total and water-soluble nutrient concentrations were measured. These materials were then compared with BA and the reference fertiliser, triple superphosphate (TSP), in effectiveness trials using lettuce and maize. Increasing the proportion of phosphoric acid boosted total P content, with the highest water-soluble P observed in the 75/25 acid ratio treatment. This ratio also significantly impacted the water-soluble concentrations of iron (Fe), zinc (Zn), copper (Cu) and manganese (Mn). The dry weights of lettuce and maize increased with all P fertiliser applications compared to the control, with acidified BA-based fertilisers performing particularly well, providing at least as much P as TSP. However, P fertilisers generally led to a reduction in plant microelement content, with BA-based fertiliser applications showing the least reduction in microelements. Boiler ash has potential as an alternative, balanced fertiliser source, with acidified formulations at sulfuric/phosphoric acid ratios of 100/0 and 75/25 demonstrating the greatest effectiveness for plant nutrition. Furthermore, as a recovered waste product, it offers both economic and environmental benefits.