REACTIONS OF COPPER AND SULFATE IN SANDY SOILS UNDER CITRUS GROVE AND UNCULTIVATED CONDITIONS

Repeated applications of copper (Cu)-containing fertilizers and fungicides have contributed to Cu accumulation in several soils used for Florida citrus production. New plantings on such soils may result in Cu toxicity to young citrus trees. The objective of this study was to examine the reactions of Cu and SO4 when applied as CuSO4 to three Spodosols (Immokalee fine sand, Oldsmar sand, and Myakka sand) and one Alfisol (Boca fine sand) sampled (0-15 cm) either from citrus groves or from uncultivated locations under natural vegetation. The soil pH varied from 6.6 to 7.2 and from 4.8 to 7.6 among the grove and uncultivated soils, respectively. Sorption of Cu by the Boca-grove and uncultivated soils was much greater (99% of applied Cu) than that by the other soils. The soil pH, organic carbon content, and cation exchange capacity (CEC) were greater in the Boca fine sands (in grove and uncultivated conditions) compared with those of the rest of the soils examined in this study. The concentrations of Cu in the equilibrium soil solutions were under-saturated with respect to CuCO3, Cu3(OH)2(CO3)2 and Cu2(OH)2CO3 solid phases in all of the grove and uncultivated soils evaluated in this study, but were supersaturated with respect to CuFe2O4 solid phase in all three grove soils and in the Boca-uncultivated soil. The quantities of Cu sorbed by the soils were 2- to 3-fold greater than that of SO4 sorbed. The sorption of SO4 was greater in high pH soils than in the lower pH soils. There was no evidence of release of hydroxyl ions in response to retention of SO4 by the soils employed in this study. Simultaneous sorption of SO4 along with that of Cu, although at much reduced quantity, may indicate that every one of three Cu ions sorbed may be associated with SO4. An increase in sorption of Cu increased the concentrations of Fe, Mn, and Zn in equilibrium solution.