Soil is a mixture of weathered rock fragments and organic matter at the earth’s surface. It is biologically active—a home to countless microorganisms, invertebrates, and plant roots. It varies in depth from a few inches to 5 feet or more. Native soil is roughly 50 percent pore space. This space forms a complex network of pores of varying sizes, much like those in a sponge. Soil provides nutrients, water, and physical support for plants as well as oxygen for plant roots. Soil organisms are nature’s primary recyclers, turning dead cells and tissue into nutrients, energy, carbon dioxide, and water to fuel new life.
Soil and Water
Soil pores, permeability, and water supply
A productive soil is permeable to water and is able to supply water to plants. A soil’s permeability and water-holding capacity depend on its network of pores:
- Large pores (macropores) control a soil’s permeability and aeration. Macropores include earthworm and root channels. Because these pores are large, water moves through them rapidly by means of gravity. Thus, rainfall and irrigation infiltrate the soil, and excess water drains through it.
- Small or fine pores (micropores) are typically a fraction of a millimeter in diameter. They are responsible for a soil’s water-holding capacity. Like the fine pores in a sponge, micropores hold water (acting against the force of gravity). Much of the water held in micropores is available to plants, however, some is held so tightly that plant roots cannot use it.
Soil that has a good balance of macropores and micropores provides adequate permeability and water-holding capacity for good plant growth. Soils that contain mostly macropores drain readily, but are droughty and need more frequent irrigation. Soils that contain mostly micropores have good water-holding capacity, but take longer to dry out and warm up in the spring. Rainfall and irrigation water runoff are also more likely with these soils.
Factors that affect soil porosity
Soil properties that affect porosity include texture, structure, compaction, and organic matter. Gardeners can evaluate these properties to understand how well their soil moves and holds
Texture describes how coarse or fine a soil is. The coarsest soil particles are sand. They are visible to the naked eye and give soil a gritty feel. Silt particles are smaller than sand—about the size of individual particles of white flour. They give soil a smooth, flour-like feel. On close inspection, sand and silt particles look like miniature rocks. Clay particles are the smallest—about the size of bacteria and viruses—and can be seen only with a microscope. They typically have a flat shape, similar to a sheet of mica. Soils rich in clay feel very hard when dry, but they are easily shaped and molded when moist. Although all of these particles seem small, the relative difference in their sizes is quite large. If a typical clay particle were the size of a penny, a sand particle would be as large as a house.
Soil texture directly affects porosity. Pores between sand particles tend to be large, while those between silt and clay particles tend to be small. Thus, sandy soils contain mostly macropores and usually have rapid permeability but limited water-holding capacity. Micropores predominate in soils containing mostly silt and clay, creating high water-holding capacity, but reducing permeability. Particle size also affects the surface area in a volume of soil. Surface area is important because soil surfaces are the most active part of the soil. They hold plant nutrients, bind contaminants, and provide a home for microorganisms. Clay particles have a large surface area relative to their volume, and a small amount of clay makes a large contribution to a soil’s surface area.
Nearly all soils have a mixture of particle sizes (Figure 1). A soil with roughly equal influences from sand, silt, and clay particles is called a loam. Loams usually make good agricultural and garden soils because they have a good balance of macropores and micropores. Thus, they usually have good water-holding capacity and moderate permeability.
A sandy loam is similar to a loam, except that it contains more sand. It feels gritty, yet has enough silt and clay to hold together in your hand. Sandy loams usually have low to moderate water-holding capacity and good permeability. Silt loams are richer in silt and feel smooth rather than gritty. They are pliable when moist, but not very sticky. Silt loams usually have high water-holding capacity and low to moderate permeability.
Clays and clay loams are very hard when dry, sticky when wet, and can be molded into wire and ribbon shapes when moist (Figure 2). They generally have high water-holding capacity