The aim of this study was to determine the effect of use of spent mushroom
substrate in fertilisation of arable soils on the content of available phosphorus and on the activity of acid and alkaline phosphatase. The vegetation experiment was carried out in 2008 and 2009 on the production field of an individual farm in the village Zając (Siedlce Upland). On the basis of the analyses it was found that the use of spent mushroom substrate in soil fertilisation increased the share of available forms of phosphorus in relation to the treatment without fertilisation. Moreover, it was found that the acid phosphatase activity in the humus horizon of soil of the individual experimental treatments was higher than alkaline phosphatase activity, and depended significantly on the years of experiment and on the applied fertilization.