Biomass used for energy purposes generates two important byproducts: the ash produced after its combustion and the biochar obtained through pyrolysis. Both of these materials are used as soil additives, attracting much interest in the last decade. Biomass (e.g., wood) ash is increasingly treated as a substitute for lime and mineral fertilizer—when added to soil, it reduces its acidification and enriches it with nutrients (Błońska et al. in Geoderma Reg. 34:e00676, 2023 [1]). Biochar, on the other hand, is defined as a highly carbonized, porous material formed from biomass under anaerobic conditions that has a high content of stable carbon. Research indicates that biochar is a multifunctional, sustainable soil improver that increases soil fertility, captures contaminants and contributes to improved plant growth (Sharma in Saudi J. Biol. Sci. 28:7539, 2021 [2]). Also important is the long-term fixation of carbon in the soil after biochar application, which promotes carbon sequestration and climate change mitigation (Singh et al. in Biochar 4:1–17, 2022 [3]). Both ash and biochar introduced into the soil have a complex effect on a number of physical, chemical, and biological properties of that soil.