One possible way to improve the solubility of phosphate rock is by co-composting it with organic substances. Four variants of composts were made in a biomass composting bioreactor. Ground phosphate rock (GPR) and shredded barley straw, pine sawdust as well as beet pulp pellets were used as compost components. The four composts were different from one another in the type and amount of organic components. The composts were granulated in a pelleting press. Changes in the solubility of phosphorus were assessed via chemical analyses and P-recovery efficiency calculated from the data achieved in a pot experiment. Solubility of ground phosphate rock was increased resulting from co-composting with organic substances, which meant that bioavailability of phosphorus increased. All the tested composts were characterized by a higher ratio of ammonium citrate soluble phosphorus to total phosphorus than non-composted GPR. Co-composting GPR with all the tested organic components yielded better effects than composting it with straw alone. The four composts were characterized by a slow release of P, which justifies our expectation that they will produce residual effects in the years following their application.