Occurrence of nutrient and contaminant elements in biomass ashes and their significance for potential soil fertilization

Eight biomass ashes (BAs) generated from agricultural, herbaceous, woody and aquatic biomass types were studied for the occurrence of 14 nutrient elements (NEs) and 6 contaminant elements (CEs) to evaluate their significance for potential soil fertilization. For that purpose, a combination of different mineralogical and chemical analyses, and leaching procedures was used. The data show that the contents of NEs in BAs are highly variable; however, the concentrations of B, Ca, Cl, Cu, K, P, S, and Zn, and, to a lesser extent, Mg, Mn, Mo, Na, and Ni in BAs are perspective for potential soil fertilization or soil conditioners. The inorganic matter of BAs comprises amorphous matter and different minerals among carbonates, silicates, chlorides, sulphates, oxyhydroxides, and phosphates. BAs have high yields of water-soluble solutions with alkaline character, which are favourable for BA use. NEs such as Ca, Cl, K, Mg, Na, P, and S occur significantly in water-soluble and bioavailable salts. Various NEs and CEs in BAs fulfilled the available regulation limits for fertilizing products, excluding some problematic concentrations of Cd, Co, Cr, Fe, Mo, Ni, and P for specific BAs. Therefore, each BA needs to be studied on a case-by-case basis for that purpose. The establishment of advanced regulatory framework for BA utilization as fertilizing products is required to generate strict limit values of NEs and CEs in BA leachates. The water-extractable proportions of elements from BA and their release in time are very important to predict the supply of bioavailable NEs or CEs for crops.