There are four types of clay, one of which we commonly bring to mind when thinking of the term. As a group, they are called mineral clay and form through gradual chemical exchange. Typically, weak acid eats away at a silicate-bearing rock until only a purified, moisture-absorbent, porous silicate structure remains. The presence of mineral clay is a determinate factor in soil taxonomy, in which there are three orders of soil containing clay.
Vertisol is a soil order extremely rich in clay, comprising no less than 50 percent by mass. Because of this high-clay content, the soil experiences extreme changes in moisture content. During rains, it can swell up to twice its original mass. When it dries, it shrinks, leaving deep cracks. This creates a shifting landmass with no constant features, which is incapable of sustaining significant plant life. Texas hosts an estimated 76 percent of vertisol worldwide. Vertisol is divided into the udert, torrert, ustert, aquert and cryert suborders.
Ultisol is an order of soil that's highly acidic and has lost the majority of its nutrients to plant growth through a period of millennia, particularly in regards to calcium, magnesium and potassium. It has a thick subsurface strata composed of clay. While pure clay is chalky white, clay in ultisol is a yellow or ochre red color because of high concentrations of oxidized iron. It occurs primarily in equatorial and tropical regions, such as the American Southeast and Southern Asia, where the humid climate is conducive to plant life and native species have adapted to acidic soil. Ultisols are divided into the humult, ustult, aquult, udult and xerult suborders.
Alfisol soil retains its nutrient content through time and possesses a neutral pH. Clay that accumulates under the surface of alfisol is almost pure, meaning it appears as a pale gray or white. Because of its inherent fertility, alfisol often hosts large deciduous forests and is predisposed for agricultural use. Appearing primarily in the American Midwest, West Africa and scattered throughout Europe, this clay soil makes up 10.1 percent of all the world's ice-free land. Alfisol is divided into the udalf, xeralf, ustalf, cryalf and aqualf suborders.