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Soil moisture content, drainage, and aeration are critical for terrestrial plant growth and health. Soil acts as a reservoir to hold water and supply moisture to plant roots. Water is a primary factor in a plant's ability to produce organic matter and energy from photosynthesis. It is also used as the solvent for nutrients and as an agent to transport food through a plant's vascular system. Plants use water to cool themselves through transpiration and as a tool to vary turgidity for root and shoot growth. Terrestrial plants also depend on sufficient soil aeration. Without oxygen in the soil, most plant roots will quickly rot. Internal, or subsurface, soil drainage is the process responsible for maintaining moisture levels and aerating the soil.
Drainage can be divided into two parts. The most visible part of drainage, surface drainage (or runoff), occurs above ground. Surface drainage is responsible for most water-related erosion and for the bulk of water removal from many sites. The less-seen component of drainage is subsurface drainage (or internal soil drainage). Subsurface drainage supplies moisture and nutrients to plant roots and is responsible for gas exchange in the soil.
Water and gasses occupy the pore space formed between soil particles. The volume of pore space relative to the total soil volume is known as porosity and varies with soil texture (particle size) and structure (particle arrangement). Soil porosity usually ranges between 25% to 70%, with typical soils having a porosity of somewhere around 50%. When all the pore space in a volume of soil is filled with water, the soil is referred to as saturated.
Size of the pore spaces is even more important than porosity for internal soil drainage. Pore size varies considerable because particle size, mix of particle size, and particle arrangement varies. The movement of water and gasses into and through the soil, and ease with which roots grow through the soil, is dependent upon pore size rather than particle size.
Pores with a diameter of greater than about .05mm ?? are known as noncapillary pores or macro pores. Pore shape and size is actually much more complicated than a single dimension can show, but the dimension can approximate effective size. Water can easily drain from these pores and be quickly replaced by gasses. Gravity is one of two major forces responsible for internal soil drainage. The water that quickly drains from noncapillary pores is called gravitational water because it flows through the pores because of gravity. Not all water is removed from these pores, by this action, because of adsorption (or more generically, adhesion). Adsorption is the attraction of a dissimilar molecule or other particle to a solid, in this case, water attracted to the walls of the pores. Adhesion is the attraction of dissimilar molecules or other particles, regardless of state.
Pores with a diameter of between .03mm ?? and .05mm ?? are known as capillary pores or micro pores. Gravity is insufficient to pull water from these smaller pores because of capillary forces, which are greater than the gravitational forces. Capillary forces result from the combination of adsorption and cohesion. Cohesion is the attraction of similar molecules, in this case water molecules, and is responsible for water clumping together into droplets and for surface tension. Water is held in capillary pores, against the pull of gravity, by the combination of adsorption and cohesion. The water left in a volume of soil after gravitation water is removed is called capillary water and its volume is referred to as field capacity (abbreviated FC). About half of the capillary water can be effectively accessed and used by plant roots. When the moisture levels drop to the point that plants can no longer effectively pull sufficient water from the pore space to overcome wilting in 100% humidity, the soil has reached its permanent wilting point (abbreviated PWP). The remaining water is called hygroscopic water. The difference between field capacity and permanent wilting point is called available water, and is the soil moisture available for use by vegetation.
A second, and highly important, part of internal soil drainage is due to capillary force (capillarity). Once a soil has reached field capacity, capillarity is responsible for movement of water between pores. Capillarity can move water horizontally as well as vertically, in either direction............
Clay - soil comprised mostly of particles less than 0.002 millimeters in diameter. Various standards define the upper limit of clay particle size between 0.001 and 0.004 millimeters. Clay soils usually retain moisture well but have poor drainage and aeration and tend to easily compact.
Silt - soil comprised mostly of particles between 0.002 and 0.05 millimeters in diameter. Various standards define the lower limit of silt particle size between 0.001 and 0.004 millimeters and the upper limit between 0.05 and 0.075 millimeters. Silt holds water better than sand and has better drainage than clay, but may have poor drainage and surface crusting.
Sand - soil comprised mostly of particles between 0.05 and 2.0 millimeters in diameter. Various standards define the lower limit of sand particle size between 0.05 and 0.075 millimeters. Gravel is the standard classification of particle size above sand. Sandy soils have good drainage and aeration but poor moisture retention.
Loam - soil composed of relatively equal amount of clay, sand, and silt. Loamy soils typically have good drainage.