SOIL-PH AND COPPER EFFECTS ON YOUNG HAMLIN ORANGE TREES

Soil pH is an important factor affecting the forms of Cu in soils and, in turn, its effects on plants. A field experiment was conducted on a Candler fine sand (sandy, hyperthermic, uncoated Typic Quartzipsamment) to investigate the effects of high soil Cu (as a result of repeated spray applications of Cu on the soil surface in 14 doses, for a total of 120 kg Cu ha-1) on growth and mineral content in the foliage of young 'Hamlin' orange [Citrus sinensis (L.) Osbeck] trees on three different rootstocks at four soil pH regimes (pH 5.0-7.0 at 0.5-unit increments). The recovery of Cu (15 mo after the last application), by Mehlich 3 (M3) extraction, in the top 15 cm of soil accounted for 22 to 80% of applied Cu depending on the soil pH. Stem diameter and canopy volume of the trees (3 and 5 yr old) were significantly influenced by soil pH, Cu, and rootstock. Effects of Cu on tree growth were more pronounced at pH 5.5 to 6.0 than at the lower or higher pH regimes. The concentration of Cu in mature spring flush foliage of the trees did not exceed the optimal concentration range. There was no significant relationship between M3 soil Cu and Cu concentrations in the foliage of the trees on all three rootstocks. A significant relationship was found between the concentrations of feeder root Cu and M3 soil, suggesting that the former can be used as an index of soil Cu status.