Phosphate-induced zinc retention in a tropical semi-arid soil

Zinc (Zn) deficiency symptoms and sporadic responses to applied Zn are being observed frequently in the Nigerian savanna, and one cause is thought to be the growing use of phosphorus (P) fertilizers. This study was designed to test the hypothesis of P-induced Zn retention in the soils. Soil mixed with P was incubated at field capacity for 3 weeks at 30 +/- 2 degrees C. P levels added to the soil were 0, 500, 1000 and 2000mg per kg soil. After 3 weeks of incubation, water-soluble Zn in soil decreased by 92% and exchangeable Zn by 78% with 2000 mg kg-l of applied P. Zn levels ranging from 0 to 200 mg kg(-1) were added to the P-incubated soil to determine the Zn sorption isotherm and retention capacity. The P-treated soil retained 93 +/- 2% of added Zn compared with 52 +/- 2% of the control soil. P treatment changed the Zn sorption isotherm from an L-curve isotherm to an ii-curve isotherm, indicating strong affinity of P-treated soil for Zn, probably as a result of the formation of Zn-phosphate complexes on the soil surface and precipitation at sufficiently large concentrations of P and Zn. At 2000 mg P kg(-1), up to 90% of Zn retained by the soil was bound in solid form as ZnHPO4. Varying the soil pH from 3.5 to 9.0, Zn retention by the soil was related to Zn hydrolysis with maximum adsorption occurring at pH7.3 +/- 0.2. The dependence of sorbed Zn on Zn(OH)(2)degrees at pH3.5-7.4 of P-treated soil indicated that significant van der Waals forces might be involved in Zn retention. The implication of the results of this study for the region is that fertilizer-P placement around a growing crop plant, commonly practised to maximize fertilizer-P efficiency, can potentially limit Zn solubility and availability.