Texture and structure. Clay soils are finely textured and are often referred to by farmers as heavy soils, meaning it is difficult to pull a plow through them. A clay soil must contain at least 35 to 40 percent clay-sized particles. Sandy and silty clays are included in this textural class. Sands are coarse-textured soils that may also include loamy sands containing some clay and silt.
Loams represent roughly even mixtures of sand, silt, and clay.
Loams can be further classified as sandy, silty, or claylike, depending on the element that most influences the properties of the soil. Most of the world's prime agricultural soils are loam. Soil separates and organic matter are often found together in natural soil structures called soil peds, or aggregates. Cycles of wetting and drying and freezing and thawing promote ped formation. For this reason farmers often leave their fields exposed in winter to mellow the soil.