Soils are usually divided into three types, depending on the size of particles they contain. Sand is the coarsest soil, silt is medium course and clay is made of fine mineral particles. The fine particles in clay largely accounts for its properties, many of which are annoying or frustrating to home gardener.
Water bonds to the surfaces of the fine particles of mineral in clay, mostly feldspar. These fine particles are flaky or flat, meaning that they have a large surface area for their small size. Some clay soils may double in size when they are wet.
Clay can hold a lot of water and nutrients. This means less run-off of irrigation water and fertilizer. By contrast, there is more space between the round particles of sand, and water runs off easily.
Not only will clay bond with fertilizer, it can also bond with the organic molecules in pesticides. Clay soil contaminated by pesticides can disperse the pesticide if it is moved to another area.
Clay soil drains poorly. Clay soaks up and retains water from heavy rains or snow melting after a spring thaw. This deprives plant roots of oxygen and makes them susceptible to fungal diseases that thrive in moisture.
In the spring, clay warms up more slowly than soils with sand or silt. This means gardeners and growers have to delay planting vegetables, annual flowers and crops.
Heaves and Buckles
In cold climates, clay soils are subject to "heaving." Clay soil holds a lot of water; if this water alternately freezes and thaws, the soil can buckle or hump, pushing plants out of the soil.
Turns Gummy and Hard
Too much water can cause clay to turn gummy, gooey and slick. With too little water, clay soil can get as hard as concrete. Foot traffic on a clay trail or pathway will make it compact and turn hard.
Crusts and Cracks
If clay soils suffer drought, a crust may form on top or the soil may crack. Crusting prevents roots from going deeper in pursuit of moisture, and it blocks the emergence of seedlings seeking the sun. Cracking can tear the roots.
Useful in Paint and Pottery
A little clay added to water will form a flurry; add color, and the result is paint. Clays are used by the paint-manufacturing industry as a medium for spreading color. When a little water is mixed with clay, potters or ceramicists can shape, dry and fire it.