Clay is one of the three primary soil types, alongside silt and sand. According to the U.S. Geological Survey (USGS), "most clay minerals form where rocks are in contact with water, air or steam." Although clay soil is viewed as a bane by many gardeners, clay is a necessary component of good soil. Loam, which is considered ideal for most applications, consists of roughly equal parts of clay, silt and sand, according to Purdue Consumer Horticulture.
Small Particle Size
Clay soils have small particles. According to the USGS, "the term 'clay' is applied... to materials having a particle size of less than 2 micrometers." Clay soils consist primarily of clay, but can also include other materials, with larger particle sizes.
Affinity for Water
According to the USGS, "clay minerals all have a great affinity for water. Some swell easily and may double in thickness when wet." This causes clay soils to retain water better than other materials, draining very slowly, which can contribute to root rot in plants that require good drainage. The heavy water content in clay soils can also deprive plant roots of necessary oxygen, further damaging them. As clay soil swells and shrinks, it can damage buildings built on its surface and harm the root structures of plants growing in it. Clay soil also sometimes causes "heaving," according to Gardenline. As it swells, it can push plants up and out.
Water isn't the only substance clay holds. According to Gardenline, clay soils are extremely good at holding on to nutrients. While nitrogen, phosphorus, potassium and other nutrients tend to wash out of coarse-grained, sandy soil, clay soils hold onto them. Because of this, clay requires less frequent fertilization than other soil types.
Clay soils are some of the most difficult to work with. When they are too wet, the particles stick together, creating a gummy consistency. When they are too dry, the clay particles form a solid, stone-like mass. This can make it extremely difficult to garden or farm clay soils.
Clay soils are denser than other soil types, which causes them to warm up quite slowly in the spring. In areas with short growing seasons, this can be a big inconvenience; according to Gardenline, clay soil can remove a week or more from an already short season.
Like all less-than-ideal soils, clay can be improved. Bachman's websiterecommends using equal parts coarse or "builders'" sand and coarse organic matter such as compost to improve clay. Cover the clay soil with 3 to 4 inches of sand and use a tiller to mix it into the top 6 inches of soil. Then, add an equal quantity of coarse organic matter such as coarse compost or aged manure and work it in with the sand and clay.