Soil is defined by its characteristics. One aspect of soil is its texture. Soil texture refers to particle size and the proportion of particle types within a soil. Sandy soil has large particles that are visible to the naked eye. Both silty soil and clay soil contain particles that are visible only with a microscope. The particles in clay soil are smaller than those found in silt, but are also higher in mineral content than either sand or silt.
Clay soil may be red, brown or gray and is a dense soil that is difficult to till or dig. When dried out, the surface of clay soil is cracked and crumbly. When wet, the soil color darkens and water sits on the surface, draining slowly into the soil. This slow drainage is due to the structure of the particles in clay soil.
The particles are microscopic, measuring approximately .002 mm in size. The particles are tightly packed together and are further bonded together when wet, resulting in poor drainage. The dense structure of the soil holds onto the water, the particles attracting and retaining the moisture.
Clay soil, with its multitude of particles, then has a low level of permeability. Permeability may be defined as the rate of flow of water and air through a soil and is connected to soil porosity. The amount of particles in clay soil increases the number of pores, or spaces, created by the irregular shapes of soil particles in proximity to one another. The spaces trap the water in the soil, making it flow downward more slowly. Clay soil, then, has high soil porosity and low soil permeability.
Clay soil is susceptible to compaction due to the size of its particles and high rate of moisture retention. When soil becomes compacted, it's difficult to dig into or till for gardening or landscaping purposes. Plants have a difficult time penetrating the dense soil and establishing roots. When a plant does root, the lack of oxygen in the heavy wet soil may result in root rot.
Clay soil, though, does retain nutrients. While sandy soil drains well, it has little nutritional value. Mixing sand in with clay soil improves the drainage of the soil and allows plant roots to access those nutrients locked in the clay soil. Adding sand and organic matter, such as compost, creates a richer, healthier soil that retains nutrients and drains at an appropriate rate, making it a more hospitable environment for plants.