Reactions of phosphate and zinc fertilizers in Iraqi calcareous soils

The objectives of this research were: (i) to investigate the phosphorus and zinc status of Iraqi calcareous soils, (ii) to describe the characteristics of both phosphate and zinc fertilizer reactions in such soils, and (iii) to test the nature of phosphate-zinc interactions in soils and on plant growth. Six soil samples were studied: three alluvial calcareous soils from the Lower Mesopotamian Plain and three desert calcareous soils from the southern desert region. Chemical and physical examination showed that the alluvial soils were generally more fertile than the desert soils. However, poor structure and high salinity of the alluvial soils limit their agricultural potential. Reaction of fertilizer P with soils involved initial rapid adsorption of P, followed by a gradual change in solution composition toward equilibration with octocalcium phosphate (OCP). Hydroxyapatite was unlikely to be involved because of its very slow rate of formation. Dicalcium phosphate dihydrate (DCPD) could be produced initially in all soils, but it was less persistent in the alluvial soils than in the desert soils, suggesting that persistence of DCPD was greater in a soil low in salts and/or carbonate. It was further shown that an increase in calcium ion concentration enhanced DCPD hydrolysis as evidenced by the formation of OCP. Formation of amorphous zinc silicate (Zn2SiO4) may play a major role in controlling Zn solubility in such soils. ZnCO3, Zn(OH)2 and Zn3(PO4)2.4H2O are too soluble to account for the zinc concentration. The solubility of Zn was somewhat higher in the desert soils than in the alluvial soils. P/Zn interactions in soils and plants were observed, but the reaction in the soil external to the plant is not adequate to explain crop effects. P-induced Zn deficiency in maize caused by imbalance in P/Zn ratio is basically a consequence of the differential translocation rates of P and Zn from the root to the top.