Quite a number of investigations have stated that optimization of phosphate conditions is a priority for South Russian soil fertility preservation. The ability of soil to provide plants with phosphorus is closely connected with the stability of interaction among various forms of phosphates against the background of various agrogenic and antropogenic burdens. However the problem still remains understudied. Stated has been space heterogeneity in the soil group phosphate composition factor depending on the soil genesis and within a definite timing loop. All phosphate fractions, Ñà–ÐIII included, are subject to strong variations. High variability of black soil mineral phosphate fraction composition is not only characteristic for its absolute values, but also for the balance which was expressed as ISOD indexes. High phosphate fractions variability in the South of Russia should be associated with the degradation changes in the soil absorptive complex saturation with calcium, as well as with deulmification and strong correlation among various phosphates, due to which a change in one phosphate form fraction inevitably leads to a change in other phosphate forms. We have revealed statistically authentic correlative links between a phosphate organic form and a variation of their flexible form determined by the Machigin method (R=-0.69), as well as a stable balanced interaction of this phosphate form with the ratio between film calcium phosphates and their flexible forms (R=0.67). The variation of monosubstituted phosphate forms authentically (the determination factor being 74%) correlated with humus, organic phosphates and pH content in soil. Thus, preservation of the phosphate stock in the most widespread South Russian black soil types depends on the solution of the problem of stability preservation and optimization of the most important agrochemical soil factors.