EFFECTS OF HIGH COPPER CONTENT OF SANDY SOIL ON GROWTH OF CITRUS SEEDLINGS

Abstract

Recently some mature Florida citrus orchards on sandy, acid, well-drained soils have become affected with a chlorotic disorder of the foliage suggesting Fe deficiency. Field plot studies indicated that soil applns. of basic materials such as CaCO3, Na2CO3 and wood ashes caused substantial improvement of affected trees, but not complete recovery. Massive soil applns. of FeSO4 were ineffective. Soil analysis indicated that topsoil in affected orchards usually contained between 100 and 200 ppm. of total Cu, and were very acid (pH 5 or below). Similar virgin soils contained between 1 and 5 ppm. of total Cu, indicating a strong accumulation of Cu in soils as a result of common orchard practices. Pot studies with one type of orchard soil indicated that at pH 4.6 chlorosis of foliage and abnormal root development of citrus seedlings occurred when the total Cu content of the soil was 150 ppm. or more. At pH 6.1, apparently normal growth occurred even when the soil contained as much as 300 ppm. of Cu. It is postulated that the Fe chlorosis of citrus orchards as descr. above is caused in most instances by a toxic effect of Cu on the roots when the soil becomes quite acid.