Fertilisation has the greatest impact on soil properties, and they in turn decide on the conversion and availability of fertilising compounds. The aim of the tests was to evaluate the influence of low-temperature (300oC) conversion of pig manure and poultry litter on (i) K, Mg, and P contents in biochars and (ii) reduction of acidification, cation exchange capacity, and availability of K, Mg, and P in sandy soil after their application. The tests were conducted in laboratory conditions using 0.5%, 1%, and 2% addition of pig manure, poultry litter, or biochars. Thermal conversion of pig manure and poultry litter increased the content of total forms of K, Mg, and P in biochars and did not significantly increase the content of these elements extracted by water. The introduction of organic materials into the soil, in particular biochar, caused significant reduction in soil acidification and an increase in cation exchange capacity. The contents of available potassium and magnesium increased together with the increase in dose of pig manure-derived and poultry litter-derived biochars. No available forms of phosphorus were found in the soil into which lower (0.5% and 1%) doses of biochar were introduced. Only the highest (2%) dose of biochars and poultry litter caused the release of available P.