Soil is defined by type: sand, silt or clay. Each type has a certain degree of permeability, a characteristic that determines how water flows through the soil and specifically the rate at which a measured quantity of water drains away from the soil. A well-draining soil is one with an optimal rate of permeability, as the water drains slowly enough for plant root uptake but quickly enough to avoid oversaturation.
Soil requires water to support plant growth. The soil needs to retain a certain amount of moisture for a length of time to allow plant roots to access and take up that moisture. If a soil drains too slowly, the plant roots suffocate. If the soil drains too quickly, the plants dehydrate. Determining the permeability of soil allows gardeners and agriculturists to amend soil as needed and determine irrigation rates.
Soil, as well as rock and sand, are considered solids. Though soil is a solid, it still contains spaces between the grains that make up its content; it is a porous solid. These spaces are referred to as pores. The rate at which water flows through these pores is the measure referred to as permeability.
Sandy soil has a higher permeability than silt. The permeability of silty soil is less than that of sandy soil but greater than that of clay. The particles in sandy soil are larger and coarser than those of silt, which in turn are smooth in texture but still larger than those of clay. The size of soil particles influences the level of porosity, in turn influencing permeability.
A bucket of water dumped onto sand at the beach permeates the surface and travels downward within minutes. A bucket of water dumped onto clay soil pools on the surface and slowly drains downward. The difference in time, in rate of flow, determines the permeability of the soil.
Permeability is not to be confused with infiltration. The infiltration rate of water in soil is measured by the ability of the soil to retain moisture. As the water flows downward, a certain amount becomes trapped within the soil particles. This is moisture retention. Sand is highly permeable but has a low rate of infiltration. Clay soil has low permeability but a high rate of infiltration; it retains moisture for an extensive period of time as the water flows downward.