Soil can be categorized in several ways, the most prevalent of which is by texture. You can identify soil type by its texture, noting the size of its individual particles and its ability to clump together when damp. One property of soil texture that affects gardening is its drainage capacity. The soil type that's best for your drainage needs will depend on what you are planting, its water needs and its root patterns.
Sandy soils have the largest particles of any soil type. Because of this, sandy soils have the largest pores, which are the spaces between individual particles. These large pores allow water droplets to easily pass between the grains of sand, making drainage very rapid and extensive. You can observe sandy soil's drainage capacity by pouring a small amount of water in a bucket full of sandy soil. Note how quickly the surface of the sand appears dry again. While sandy soils are among the most rapidly draining soils, this doesn't necessarily make them the best option for planting. Because liquids pass through the soil so rapidly, many plants' root systems are unable to receive water or nutrients before they leach deeper into the earth.
Instead of sandy soils, many farmers and gardeners prefer the drainage profile of loamy soils. These soils have a mixture of particle sizes and a high content of organic matter. As a result, loamy soil can conduct water relatively easily, but not as rapidly as sandy soil. Loam's relative ability to retain moisture makes water and its nutrients available to plants' roots closer to the soil surface. However, the loamy soil does not retain water excessively, which would keep air from making its way through the soil's pores and reaching the roots.
Because soil drainage can vary considerably even within a particular texture type, soil scientists have devised a more specific method of describing soil drainage properties. The United States Department of Agriculture divides soils into seven drainage classes: very poorly drained, poorly drained, somewhat poorly drained, moderately well drained, well drained, somewhat excessively drained, and excessively drained. Many plant species thrive in well-drained soil, in which the water movement is fluid, but not rapid. Other species, acclimated to wetlands or desert environments, may fare better in soils with different drainage types.
Aside from the form and texture of the soil itself, the ability for water to drain through the soil also depends on other features. For example, organic matter can transform a relatively poorly drained soil type into a well-drained medium. On the other hand, a soil type that is usually well drained may not drain well at all if it has been compacted, as is often the case around recent construction sites, where heavy vehicles and machinery have compacted soil.