Physical Properties of Soils
Soil hydraulic conductivity is a function of the water potential of the soil. Conductivity measures the ease with which water moves through the soil. As water content (and hence the water potential) decreases, the hydraulic conductivity decreases drastically (Web Figure 4.2.A, note the logarithmic scales).
The decrease in conductivity as the soil dries is due primarily to the movement of air into the soil to replace the water. As air moves in, the pathways for water flow between soil particles become smaller and more tortuous, and flow becomes more difficult.
The overall shape of this curve is representative of many soils, but the shape for a particular soil may be influenced by the size distribution of its particles and by its organic matter content.
The field capacity (labeled on the curve) is the maximum amount of water the soil is able to retain against gravitational forces. The permanent wilting point (labeled on the curve) is the soil water potential value at which plants cannot regain turgor pressure even at night, in the absence of transpiration.