THE DYNAMICS OF SOIL PHOSPHATES AND THEIR CHANGES IN CONDITIONS UNDERIRRIGATION

In field trials performed in the years 1984 to 1987, in maize growing region, established on chernozem, soil phosphate regime under irrigati on and without it was studied with three variants of fertilizing, untr eated variant, variant treated with fertilizers and manure and the var iant fertilized solely with industrial fertilizers, in crop rotations with lucerne arid liming, and without liming and lucerne. The studies were carried out to the depth of 0 to 60 cm, with 15 cm layers. Fracti ons of inorganic phosphorus were determined by the method after Chang- Jackson (first fractions of L - P, Al - P, Fe - P, Ca - P and residual phosphorus as fraction V) in soil samples. Exchangeable pH/KCl was de termined as well. Irrigation reduced significantly pH compared with un irrigated variant, in particular in interaction with fertilizing. This unfavourable effect was eliminated by organic manuring. Long-term irr igation resulted usually in significant reduction of the content of al l phosphorus fractions, except for Fe - P. The most significant change s were recorded with the fraction Ca - P and residual phosphorus (frac tion V). These differences were caused by more intensive formation of organic phosphorus and increased crop yield.