Soil Texture Characteristics

Basic Textures

• There are three kinds of soil particles: sand, clay and silt. Sand particles are rough and obvious to the eye. A microscope is necessary to see particles of silt, which are smooth and slippery when wet. Smaller yet are particles of clay, which are sticky when wet and require an electron microscope to be viewed. Combined, their relative proportions determine the classification of a soil's texture.

Texture Triangle

• The U.S. Department of Agriculture (USDA) uses a diagram called a "textural triangle" to divide soils into 12 different classes. The left side represents amounts of clay from 0 to 100 percent. The right side notes percentages of silt, and the base marks off amounts of sand. Inside the triangle 12 quadrants mark off the classes. For example, a soil that is 10 percent silt, 30 percent clay and 60 percent sand is a "sandy clay loam," the kind of soil in which you might successfully grow melons. Closer to the center of the triangle, a soil that is 20 percent clay and equal parts sand and silt would comprise a loose, pure loam -- the kind of soil that most garden plants prefer.

Sands

• Sandy soils are gritty and warm up quickly in the spring. After a rain, they drain easily. But sandy soils don't do a good job of retaining moisture or nutrients. They need to be watered and fertilized frequently. Without vegetation, they erode easily.

Clays

• Soils rich in minerals and abundant in the finest particles are referred to as clays. If they contain too much water, they are gummy; if too dry, they can seem as hard as concrete. Their poor drainage and easy compaction can deprive plant roots of oxygen. Clays are described as expansive.Their extreme water retention, when coupled with freezes and thaws, can lead to expansion and contraction problems. In the spring, they take longer to warm up; in summer heat, their crusting inhibits germination and their cracking tears roots. But clays are nutritious and can be loosened with organic matter.

Silts

• Although not as nutrient-dense as clay soils, silts are rich, agricultural soils. But they are light and erode easily. Silty soils wash downstream to delta areas and blow away in dust storms. This slightly gritty material is such an important component in agricultural soils that one farming community in Colorado is named Silt.

Loams

• A soil containing roughly even amounts of sand, clay and silt is called a loam. Half of the soil classes on the USDA texture triangle are kinds of loam, including silty clay and sandy clay loams. A pure loam is what is sometimes called "just right." It contains plenty of spaces between soil particles for roots to grow and receive oxygen.