Automatic Feedback Control to Maintain Constant Soil Moisture Tension in the Study of Drought Tolerance in Rice 1

We have developed a greenhouse technique by which a chosen soil moisture tension can be maintained continuously. The setup for supplying water at relatively constant soil moisture tension in pots includes five major components—water tank, W-tube, water distribution system, soil moisture tension sensors, and pots. The W-tube, which contains a predetermined amount of Hg, controls the soil moisture tension in the pots. Water can flow freely from the tank to the pots through the distribution system whenever the soil moisture tension exceeds that desired. Response to the sensors' demand for water is rapid. We have successfully used this setup to compare relative drought tolerance of rice (Oryza sativa L.) varieties and breeding lines. Another use of this setup might be to measure evapotranspiration (ET) almost instantaneously; the technique might be useful in some soil-plant-atmosphere-water relationship studies.