INFLUENCE OF UPWARD SOIL-WATER FLUX ON SOIL-MOISTURE IN THE POTENTIALCROP ROOTING ZONE OF A FLUVISOL, KENYA

Soil matric suctions were measured using tensiometers installed at dif ferent depths at three different locations. Ground water table depth f luctuations were monitored at each of these locations by sinking obser vation wells next to the tensiometer positions. Over the experimental period of seven months, the ground water table depth varied from about 40 to 200 cm. The contribution of upward soil water flux (capillary r ise) to crop water requirements of a citrus crop (oranges) with an eff ective rooting depth of 120 cm on sandy loam soils was evaluated and a soil water balance established for the soil type. Results obtained sh owed that when the ground water level was at 100-150 cm below the soil surface, the unsaturated soil above had an average soil matric suctio n below 1.0 bar. With a soil matric suction of 0.3 bars taken as the o ptimum, and 0.5 bars as the upper limit that should be maintained for optimum crop performance, upward soil water flux adequately supplied s oil moisture to meet the crop water requirements of the citrus crop.