Soil Phosphorus and Potassium Fractions in Response to the Long-Term Application of Pig Slurry and NPK Mineral Fertilizers

The content of bioavailable forms of phosphorus (P) and potassium (K) in soil is essential for the proper functioning of agroecosystems. This study aimed to determine the effects of pig slurry (PS) and NPK mineral fertilizers on soil phosphorus (P) and potassium (K) fractions, the relationship between these fractions and basic soil agrochemical properties, and crop yield. The research material was collected from a long-term experiment established in 1955 in Prague-Ruzyně, Czechia. The effect of two constant factors was analyzed: manure application (control, PS) and different doses of NPK fertilizers (N0P0K0, N1P1K1, N3P2K2, and N4P2K2). A significant effect of fertilization on basic soil properties was demonstrated, including total soil carbon and nitrogen. PS and NPK fertilization also significantly affected the content of water-soluble and moderate labile fractions of P and K. These fractions were positively correlated with plant-available P and K (Mehlich 3). The best fertilization option, which resulted in the greatest increase in yield, was the use of PS and mineral fertilizers at the N3P2K2 level. Increasing the nitrogen dose to the level of N4 resulted in a decrease in the content of bioavailable forms of P and K in topsoil despite the application of PS.