Mineral composition and soil texture

The size of the individual soil particles is called soil texture. The three general designations are sand, silt, and clay. Generally, sands and gravels are composed of many small particles bound together chemically or by a matrix of cementing material. Silt is composed of finer particles that are usually too small to see without a hand lens or microscope. Clay particles are even smaller yet. They are visible individually with an electron microscope as even a powerful light microscope will not render them visible. Clay particles are individually called micelles. These particles are negatively charged, sheet-like, and held together by chemical bonds. Because of their negative nature, they attract, trade or capture positively charged ions. The water that adheres tightly to the particle surface acts as a binding agent and a lubricant. This lends to the plastic nature of clay. Clay is also a colloid, meaning it is a suspension of particles that are larger than molecules, yet do not settle out of a fluid medium. The fluid medium is generally water. These three particles in various balances comprise the bulk of soil.

Soils are sometimes referred to as heavy and light. Light soils have low clay content and high sand content; heavy soils have a high clay content and low sand content. Clay soils have a high water content as they do not allow water to pass through between the particles, recall that water is a binding agent in clay. Sandy, or coarse-textured, soils do not retain much water. Organic matter and clay store more plant nutrients in the form of ions than sand and silt. The smaller clay and organic particles have a greater total surface area for the attachment of ions compared to an equal volume of sand and silt particles.