Critical Tension

The critical tension of a soil indicates how much water (measured by the height / depth of a column of water) is held in a soil (onto by a soil) after natural drainage has occurred.

Smaller particles have higher critical tensions, resulting in lower drainage rates and producing a more saturated soil profile. The smaller the soil particles and the gaps between them, the stronger their suction capability and the stronger the attraction of water in slowing down the speed at which water can drain away.

A medium sand might, for example, have a critical tension of 25cm with a drainage rate (hydraulic conductivity) of say 200 cm/hr. This will produce a suspended water table of 25cm. It will only drain further if more water is added from above (for example, by rainfall or irrigation), as this effectively pushes out excess water as it cannot be held onto by the soil, or have water used up by plant evapo-transpiration: This is available water, which is that held between field capacity and permanent wilting point and is held within micro-pores and being attached to organic matter within a soil.

Critical tension can be measured by saturating a column of soil in a cylindrical container, and then lifting the column of soil out of the water to allow excess water to drain away. Once the soil has drained, due to gravity, it can be said to have drained naturally, but it will still retain a certain amount of water within the soil column due to its critical tension. The height of water retention will depend on the fineness-coarseness of the soil: A coarse sand, with large diameter pore spaces, would only have a small column height of retained water so will drain well but will also produce a sward which is more prone to drought stress.