EFFECT OF SOIL FUMIGATION ON GROWTH AND CHEMICAL COMPOSITION OF CITRUS PLANTS

Abstract

Fumigation of old citrus soils prior to replanting of citrus trees or seedlings appreciably increased growth. In some tests growth was nearly doubled. Growth stimulation was accompanied by occasional but not consistent changes in the chem. composition of citrus plants. In some greenhouse tests and in most field tests with lemon treees, Ca in the leaves was moderately increased, Mg slightly increased, and S decreased by soil fumigation. In a field trial with oranges, the K concn. in the leaves was slightly increased. Soil fumigation with D-D, CS2, chloropicrin, or propylene oxide increased Mn in leaves of citrus seedling and of young lemon trees but not in leaves of trees more than 1 yr. old. Soil fumigation with ethylene dibromide did not affect the Mn content of the leaves. In both greenhouse and field tests the quantities of N, Na, P, and several trace elements in roots or leaves were not influenced by soil fumigation. Fumigants containing Cl sometimes increased the Cl content of the plants, and those containing Br greatly increased the Br content. Citrus seedlings utilizing ammonium N largely or partly as a source of N were comparable in growth to, and were not significantly different chemically from, those utilizing nitrate N only. Citrus seedlings grown in old citrus and non-citrus soils were not significantly different in chemical composition. All plants contained adequate amts. of most nutrient elements normally found in healthy citrus plants. With the exception of increased leaf Mn in some tests, the few changes resulting from fumigation did not change the level of any nutrient from a deficiency to a sufficiency status. This fact and the further facts that fumigation of non-citrus soil produced similar chemical changes in the citrus plants but did not increase growth, and that growth of citrus seedlings in non-fumigated non-citrus soils was better than that in fumigated old citrus soils, indicate that, so far as the soils in this study are concerned, the changes effected in the status of the nutrient elements in old citrus soils by fumigation do not account for the increase in growth of the citrus plants. It is concluded that destruction of parasitic and possibly other detrimental soil organisms best explains the growth-stimulating effect of soil fumigation on citrus replants.