BORON IN CITRUS TREES

Abstract

During the past twenty-five years attention has been given to the effects of boron on trees. Practically all soils in southern California were then considered to be alkaline in reaction and under such conditions boron availability might be interfered with. Recently, however, it was found that soils in healthy citrus orchards are generally acid (11) at some time during the year ; some soils are becoming so acid as to require calcium carbonate. Also recently (10) it was shown that most citrus soils in southern California contain adequate amounts of available boron. Boron is an essential element for citrus (8). The response of olive trees to applications of boron to the soil (25) has afforded an opportunity for studying adjoining citrus trees in low-boron areas. Excessive-boron areas on the other hand are numerous. Use was made of trees growing in both of these and in healthy areas in supplementing studies of trees growing under controlled conditions in artificial cultures. In Rhodesia (18) a condition known as hard fruit'' in navel orange trees was corrected by applications of boron to the soil. Translucent spots, such as were found in artificial cultures (13), were noted and in addition there were deposits of gum around the fruit axis and in the albedo of the rind. No corky split veins were observed in the leaves. In Florida (22) grapefruit trees were grown in (coarse builders' sand) cultures and when the trees were boron-deficient, the fruit showed brown discolorations in the albedo but no gum was noticed around the axis of the fruit such as was described in Rhodesia. In Florida but not in Rhodesia there was observed a thickening of the midrib and veins, the midrib of the old leaves becoming corky (1) on the dorsal surface. Other observations (3, 4) in Florida groves revealed symptoms similar to those reported from Rhodesia but the symptoms were not consistent over a period of time in the same grove, disappearing rather quickly when a resumption of rains ended a period of unusual drought. This view is consistent with the results obtained on the avail-