LIMITATIONS OF AUTO-IRRIGATORS FOR CONTROLLING SOIL MOISTURE UNDER GROWING PLANTS

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Abstract
Auto-irrigators have been reported to permit close control of soil moisture under growing plants "from below the wilting point to soil saturation." Considerations of capillary flow, however, set a lower limit for soil moisture control at about twice the wilting point %, with a mois-ture tension of some 50 cm. Hg. In practice auto-irrigators of a most efficient type have failed consistently to maintain soil moisture percentages under moderately transpiring plants with supply tensions as low as 2 cm. Hg. Failure is due to the rapid drop in capillary conductivity of drying soil. Rapid removal of soil moisture by the roots of transpiring plants creates local dry areas. These areas are only very slowly remoistened by capillary action if the supply water is under appreciable tension, and with continued root absorption equlibrium is never regained. In contrast to these failures, auto-irrigators can be used with excellent results to control soil moisture under conditions requiring a minimum of capillary flow.