Phosphorus (P) is an essential nutrient for the plant life cycle. The agricultural management of phosphorus is complicated by the inefficient use of phosphorus by plants, consequent environmental losses, and the rapid consumption of slowly renewed phosphate rock (PR). These issues represent a huge environmental burden and jeopardise food production. In this study, we proposed the combination of this fertiliser with food-processing by-products such as olive pomace, barley spent grain, and citrus pomace to increase phosphate rock solubility and the efficient use of P. Phosphate rock, by-products, and mixtures of phosphate rock and by-products were placed into litterbags and buried in sand. Periodically, one replicate per treatment was collected for the destructive measurement of total and water-soluble phosphorus. In parallel, pH, organic matter, and ash content were measured to investigate the mechanisms behind changes in P content. The mixtures’ P-release values ranged between 80% and 88%, whereas phosphate rock lost 23% of its P over 30 days. Phosphate rock showed a constant water-soluble P fraction at the four sampling times, whereas the mixtures exhibited a highly water-soluble P fraction that tended to decrease over time. Specifically, citrus pomace led to the significant and rapid release of phosphorus, barley spent grain maintained the highest water-soluble fraction over 30 days, and olive pomace was not the best-performing product but still performed better than pure phosphate rock. Moreover, the increased solubility of phosphate rock in mixtures was significantly (p < 0.001) ascribed to the reduction in pH. The results of this experiment are promising for in vivo trials and suggest the possibility of simple and easily achievable solutions for more sustainable production systems and effective P-fertilisation strategies. Proposing such easily applicable and inexpensive solutions can reduce the distance between research achievements and field applications.