The Slow Reversible Drying of Sandy Surface Soils Beneath Citrus Trees in Central Florida

S citrus trees in a number of the older groves in the central part of Florida have for. several years shown a peculiar type of decline. The condition will start with one or more trees and spre,ad progressively to those unaffected. The trees have failed to respond to those'treatments that have been used successfully with certain other types of decline caused by mineral deficiencies. If dead or declining trees are replaced by resets, the young trees' are normally vigorous and healthy. The progress of the decline seems most rapid in dry years. An effort is being made to , determine whether soil moisture conditions are in any way related to it. The groves affected are generally on fine or very fine sands of Norfolk or Blanton series. Peech has described these soil types elsewhere. During the 1941 summer rainy season, it was observed that the soil within the leaf-drip of declining trees remained dry while beyond the leaf-drip it was moist. The results of a cross-section study of soil moisture conditions under one of the declining trees is shown in Fig. i. This section was made on the morning of June 25, 1942, following 0.85 inch of rain the day before and 5.90 inches in 3 weeks. A large body of dry soil existed beneath the tree extending from the trunk outward to about the margin of leaf-drip and at a depth of 2 to 12 inches. Occasional small dry bodies occurred outside the leaf-drip. Moisture determinations showed that the surface inch or two under the trees was considerably higher in moisture content than the 2 to 12-inch layer or the soil in the middles. Here and there were wet channels of moist soil where evidently some infiltration had occurred. Frequently the channels were only partially developed in the form of wet pockets. The subsoil at a depth of 8 to 12 inches was moist. Here the transition from the dry to moist condition was not nearly so sharp as in the thin surface layer. Except for retarded downward movement through the small wet zones or by runoff to the area beyond the leaf-drip, the moisture beneath the tree was held in the thin surface layer until it evaporated. The condition shown in Fig. i is typical of a large number .of observations made during the 1942 season. Dry soil bodies were found under young and healthy as well as under declining citrus trees. However,