Fundamental Investigations on the Irrigation of Citrus Trees

Abstract

This investigation was designed to find the most intelligent and successful way of irrigation in citrus groves, and some related information obtain-ed is reported in this paper.
(1) Twelve gallons of water which is common-ly recommended as the amount of pouring-water for a hole was applied in the flat ground and brink soils. In the soil of even texture, the water was distributed in cylindrical form, 2 to 3 feet in diameter and 1 1/2 feet in depth. The moistened sphere differed with the properties of soils, but precisely agreed with the results computed from KOBAYASHI's formula. When the soil had reached a moisture content approximating the moisture equivalent before the irrigation, the applied water could moisten the soil, the volume of which was 3 to 5 times as large as that of the water added.
(2) Under dry and hot summer conditions, the drying of the soil progressed at so rapid a rate in the rhizosphere soil that distinct isolation of root zone from the surrounding soils took place. During the midsummer months, the rate of moisture move-ment in the soil seemed to be more important for the use of soil moisture by plants than the amount of water contained in the surrounding root free soil.
(3) The minimum hydraulic gradient neces-sary for the effectively rapid movement of mois-ture was about 3 to 7% on the sandy soil deriv-ed from granite, and it was a little varied with the properties of soils or the direction of movement.
(4) The rate of upward movement of moisture in the soil was too low to meet the demand of the fruit tree, and so digging of not too deep, at the most 1 1/2 feet in depth, but many water-ing-holes must be more efficient for the water-use of citrus than that of deep but few holes.
(5) The branches or twigs of the tree depended closely on the same side of the roots even in the case of water delivery within a tree. Watering-holes, should therefore, be dug evenly in all direc-tions of a tree.
(6) The rate of moisture movement in the soil was unexpectedly low, and so the water-dissipating action of the tree overtook its capillary adjustment during the midsummer months.
(7) The critical moisture content for the growth of citrus organs such as the shoots, fruits or roots was about 1 to 8 atmospheres of tension, while, that for the movement of soil moisture was 1/2 atmospheres which corresponds to the moisture equivalent of soils.
(8) In order to keep the moisture content of rhizosphere soils above the critical point for the growth of citrus trees, that of the surrounding root-free soil should be kept, at least, above the moisture equivalent.