Soil Water Storage

Soil water storage is a function of the surface area of soil particles (i.e., particle-size) and the amount of porosity occurring between these particles (i.e., soil structure). Soil pores occur across a wide range of diameters and are often categorized as macropores (>60 µm) and micropores (<60 µm). water is present in macropores following precipitation events and is drained by the force of gravity. after water has freely drained due to the force of gravity, the soil is at field capacity and has a soil water potential generally between 0.01 to 0.03 mpa. water in macropores is not available for plant use because it freely drains from the soil profile and is lost from the rooting zone. water held in very small micropores (<0.2 µm) is held so tightly that plants are not able to extract if for use. the permanent wilting point is the soil water potential to which plants can effectively utilize water and corresponds to a soil water potential of approximately 1.5 mpa. thus, the pores in the diameter range 0.2 to 60 µm are the primary storage pores for plant available water (i.e., water held between approximately 0.01 and 1.5 mpa). the distribution of pore sizes is primarily a function of the soil texture and structure. the amount of water storage as a function of soil texture is illustrated in this figure.