The main factor involved in soil drainage is soil texture, which refers to the size of the particles in the soil. Generally, the larger the soil particles, the more porous the soil is. One of the surest signs of poor drainage is standing water that only disappears through evaporation. Places with poor drainage can also be identified by the types of plants growing there: algae, mosses, and ferns frequent damp areas while clay soils shrink and crack in drier areas. Since evaporation is greater in dry areas, white salt crusts can typically be found on rocks and vegetation.
Smaller soil particles pack together more tightly, thereby slowing the process of drainage. Sand — the soil with the largest particles — can range in size from .083 inches (about 2.11 mm) down to .004 inches (about .102 mm). Soil with particles around .002 inches (about .051 mm) is classified as silt, which is soil with medium-sized particles. The soil with the smallest particles is defined as clay, which has particles less than .00008 inches (about .00203 mm) in size.
The worst drainage problems typically occur in clay soils that bind together, thereby creating a dense, almost solid surface. These soils are also referred to as adobe, gumbo, caliche, hardpan, or heavy soil. Due to their tendency to be watertight, such clay soils are often used in the making of ceramics.
As soil is washed from the mountains down into valleys and basins, the smallest particles travel the farthest. This leaves the biggest rocks in the mountains and the smallest particles in the lowest, flattest areas. For this reason, the poorest drainage is typically found in the lowest areas. These areas are where clay soils with poor drainage are most often found.
A test for poor drainage is to dig a hole about 2 feet (about 60.96 cm) deep, fill it to the top with water, and let it drain. Once the hole has drained, the hole should be filled again with water. If it takes longer than an hour to drain completely, the drainage can be considered poor and the soil will usually need to be amended before planting.
To determine what type of soil is present, a person can pick up and squeeze a handful of soil that has been watered a day prior. If the soil falls apart, it usually is sandy; if it sticks together in a solid lump, it most likely is clay. The best garden soils are usually made by mixing together all three types of soil; such soil is known as loam. By adding humus — decomposing organic material — to loam, a person can further improve the soil and better facilitate drainage.