A controlled water table irrigation system for hydroponic lettuce production

The Controlled Water Table (CWT) irrigation system consists of a capillary mat placed on a level bench with one end of the mat suspended in a small trough of nutrient solution. The vertical distance between the surface of the water (water table) in the trough and the surface of the bench determines the water potential in the capillary mat. The water table is maintained with a liquid level controller with nutrient solution supplied via gravity from a larger reservoir. The water potential may be changed by raising or lowering the water table relative to the bench surface. A finely woven material on the mat surface provided nutrient movement to the roots but prevented root penetration into the mat. Lettuce seedlings; germinated in 2.5 cm(3) cubes, were placed in holes cut in a rigid foam sheet. Lettuce roots grew on the surface of the root barrier. Lettuce grew most rapidly when the water table was near 0 compared to I or 2 cm below the bench surface. At early stages of growth, plants grew uniformly; at later stages, plants nearest the trough were larger because of access to higher nutrient solution concentration. Increasing nutrient solution concentration increased uniformity along the length of the bench. The CWT concept of hydroponic production does not require pumps nor large storage reservoirs. No runoff occurs; the only nutrient solution used is that required by plants and a minimum amount of evaporation from the ground cover surface. Disease potential should be lower than with other systems.