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SUMMARYThe efficiency of two forms of phosphate -polyphosphate and orthophosphate -was compared at different levels of phosphorus on two zincdeficieiit soils. On both soils, polyphosphate showed its superiority in drymarter productioii compared with diammonium phosphate. This difference appears mainly due to a difference in the reaction of these two forms with soil zinc. Presence of polyphosphate in soil leads to the liberation of soil zinc suseeptible to ammonium acetate extraction, probabty through the formation of a complex between polyphosphate and soll ziiic. On the coiitrary, the reaction of diammonium phosphate with soil zinc reiidered the latter less available and resulted in a decrease of dry-matter production. The differeiices in the two forms were marked particularly in the absence of applied zinc.Liberation of soil zinc, following the application of polyphosphate increased linearly with time, which is important from the agronomic point of view, as the efficiency of polyphosphate in zinc-deficient soils can be iiicreased by its applieation well before the crop is planted. The optimum time of application needs to be worked out under field conditions. Application of zinc increased dry-matter productioii considerably. It increased the coneentration of Zn and decreased the concentration of P and Fe in plant tissues. Zn concentration showed a significant positive correlation with dry weight, whereas the concentration of P and Fe showed a negative correlation.

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