Due to the systematic utilisation of substantial amounts of phosphorus fertilisers on agricultural crops in crop rotation, the amount of available phosphate compounds increases in the soil. It ensures the maximum increase in crop yields. The study of the phosphate level in Chernozem soils in various agrocenoses is really up-to-date. The gist of our research was to determine the specifics of providing mobile phosphorus in typical Chernozem in field crop rotation, especially in sunflower, varying according to fertilisation systems in the forest-steppe zone of Ukraine. The assessment of mobile phosphorus content was conducted using the Chirikov method, which involves extracting mobile phosphorus compounds from the soil using a solution containing acetic acid (CH3COOH) at a concentration of 0.5 mol dmE−3, with a soil–to–solution ratio of 1:25. Phosphorus was determined with the spectrophotocolorimetric method, which is based on the colour intensity of the phosphorus-molybdenum complex. Our investigations showed that the typical low-humus Chernozems have considerable reservoirs of potentially exploitable phosphorus for plant nourishment. We established that the mineral and organic-mineral fertilisation system leads to a greater use of mobile phosphorus with agricultural crops of crop rotation resulting in the formation of elevated phosphate content in the Chernozem soil. By the end of the second rotation, the organic-mineral fertilisation system variant displayed the greatest concentration of mobile phosphorus within the 0–25 cm soil depth, marking a surge of 15.6 mg kgE−1 compared to the unfertilised variant. In the period of sunflower germination, the content of mobile phosphates increased in the soil layers at depths of 0–25 cm and 25–50 cm in the variant of the mineral fertilisation system on 17.6 and 22.2 mg kgE−1 of soil compared to the alternative without the fertiliser. In the sunflower’s ripening period, the mobile phosphorus concentration in the soil at 0–25 cm depth increased significantly in the variant of organic-mineral fertilisation system by 12.0 mg kgE−1 and mineral fertilisation by 14.7 mg kgE−1 of soil if compared with the variant lacking the fertiliser. In the variant of the mineral fertilisation system, the amount of mobile phosphorus increased in the 0–30 cm soil depth by 18.7 mg kgE−1 of soil in two crop rotations compared to the beginning of the first crop rotation.