Clay soil is any type of soil that contains a high percentage of clay particles. When discussing dirt, the term “clay” is basically a catch-all for a family of minerals that are heavy, sticky, and dense. Clay soil can look different in different places, but it usually acts the same way: it tends to be slow to drain, quick to harden, and difficult to use for anything but the hardiest plants. In most cases, there is no single definition with respect to the soil's specific composition, though gardeners typically use the term if there’s enough clay to be both noticeable and problematic.
Where It Can Be Found
At first glance, most dirt looks more or less uniform, but beneath the surface, things tend to be much more complex. Soil typically has varying bands or ribbons, known as “striations” in geological circles, that change over time based on weather, erosion, and tectonic plate shifts, among other things. Clay particles can be found in some form or another in many striations, and they appear in most parts of the world. They are most common in mountainous, rocky regions, but as the earth shifts, particles do, too, and they can often end up relatively far from where they were first formed.
Most clay-heavy soils contain a combination of four types of minerals: layer silicates, metal oxides and hydroxides, allophanes, and crystalline chain silicates. These all carry both negative and positive “charges,” a term that relates to how the minerals react on a cellular level with other nearby elements. In most cases, they tend to make the nearby area alkaline, which can affect the health of plants and other organic life.
The type of minerals clay soil contains tends to vary based on location, temperature, and humidity. Soil with red clay particles is common in hot, arid places, for instance, while grey and white versions are more common in damp, cold climates. Color is usually a factor of both mineral content and chemical composition. How the clay acts, however, is more or less consistent everywhere.
Most people know they have clay-rich soil when the dirt in their gardens or around their homes is thick, sticky, and very heavy when wet. It tends to have an almost “gummy” appearance that sticks to shoes and gardening tools almost like cement. In fact, clay is actually one of the key components of basic cement in many places.
The best way for people to figure out whether or not soil has clay particulates is to perform a pH test to determine the relative ratio of acid to alkaline. Some nurseries and gardening centers sell pH testing strips for home use, but the best results are usually obtained through more professional services. Gardeners can collect samples from varying depths and send them away to a horticultural lab for analysis. This process may be more than casual gardeners need, but anyone looking to grow crops commercially or hoping to develop the landscape may need to know the precise make-up of their land before making extensive investments of both time and money.
Problems for Gardeners
Clay-rich soil is often very hostile to plant life, both in terms of its chemical structure and its physical attributes. As an alkaline, it has a tendency to block root absorption, which prevents many small plants, like flowers and shrubs, from getting the nutrients they need to survive. Established trees and bushes with expansive root structures can sometimes withstand even highly alkaline soils by finding water and energy by bursting through to other striations, but smaller plants may not have this ability.
The soil’s tendency to absorb water also makes it problematic. When conditions are wet, clay particulates tend to wick moisture away from plants; when things dry out, though, rather than returning that water to the nearby surroundings, the soil often simply dries and cracks. Ground that looks crumbly and fractured on hot days usually has a lot of clay in it.
People who suspect that they have a lot of clay but want to plant anyway often blend other topsoil and fertilizers into their gardens to get a better soil balance. Many garden centers sell specially-formulated “soil amendment” mixes that are designed to counteract the alkaline and moisture-wicking elements of clay, but similar results can often also be achieved with mulch, compost, or even manure. The idea is basically to dilute the concentration of minerals and make the ground more hospitable. Gardeners who are concerned about drainage and moisture retention also sometimes choose to plant in raised beds or suspend planters off the ground in order to encourage excess water to run off.
Considerations for Builders
Architects and construction teams are another subset of people who are commonly concerned with soil composition. Building homes or other structures on clay-ridden ground can cause problems when it comes to land shifting and settlement, particularly where sunken foundations are concerned. As the clay particles absorb and release water, they tend to expand and contract, which causes gradual shifting. Over time, this can lead to foundations that are cracked, lopsided or, in very bad cases, actually collapsed.
Mixing in other soil elements to improve the balance can sometimes work in this scenario, but it must usually be done on a very large scale. Builders more often choose to fortify their foundations from the beginning, either by building them with more reinforcements or sinking them lower into the ground to get out of the clay level.
Ceramics and Clay Mining
Ceramics manufacturers often use clay to make pottery, but this does not usually mean that the particulates found in nature can be immediately turned into a bowl or plate. The clay that is present in most soil is “composite” material, which means that it is made up of a great many different things. Clay miners usually start with ordinary clay soil, but they refine it and sift it down into a more “pure” product before marketing it to potters and craft centers.