The Influence of Organo-Mineral Fertilization on P and Ca Management in Festulolium braunii (K.Richt.) A. Camus and Their Content in the Soil

The management of spent mushroom substrate (SMS) from the cultivation of the button mushroom Agaricus bisporus poses a significant environmental challenge in Poland. This study analyzed the effect of applying SMS on the phosphorus and calcium content in Festulolium braunii plants, their uptake and use efficiency, as well as the increase in the concentration of these elements in the soil. Treatments involving no fertilization and mineral fertilization with NPK alone were compared with treatments where SMS was applied at rates of 3, 4.5, and 6 Mg ha−1 D.M. in combination with NPK mineral fertilization. The following experimental treatments were established: 0 (control), NPK, NPK + SMS1, NPK + SMS2, and NPK + SMS3. All SMS application rates resulted in a 24–29% increase in plant P content and a 35–48% increase in P uptake compared to NPK fertilization alone. SMS application did not affect plant Ca content compared to NPK; however, increased Ca uptake was observed at rates of 4.5 and 6 Mg ha−1, and the 6.0 Mg ha−1 rate improved the Ca:P ratio. The combined application of SMS and NPK in all analyzed treatments significantly increased the phosphorus use efficiency index compared to NPK fertilization alone. The highest values for P and Ca utilization were recorded with the application of SMS at a rate of 4.5 Mg ha−1. Furthermore, increasing the share of SMS in the fertilizer combination significantly increased the P (by 50–200%) and Ca (by 167–400%) content in the soil after two years of the study.