Topsoil Classification

Scientists classify topsoil according to how much sand, silt and clay it contains.

Soil scientists define topsoil as the first eight inches of material that covers the earth's land surface. Rich with minerals and organic nutrients, topsoil supports plant life that is the foundation of all life. Because of its importance, science has developed several different standards and complex systems to classify topsoil. However, all topsoil is a mix of sand, silt and clay in different proportions, and one of the primary ways to classify topsoil is to rate it according to these three components or textures.

The 12 classes of Topsoil

The U.S. Department of Agriculture uses 12 classifications of topsoil texture based on the percentages of sand, silt, clay and a fourth definition, loam, which is topsoil that is considered to have an ideal balance of all three materials. The amount of sand, silt, clay or loam determines whether topsoil falls into the class of silty clay, loamy sand, silt or one of the other nine categories. The USDA has created a triangular chart that shows what percent of each material is present in each of the 12 categories.

Sand

As rocks and minerals slowly erode, they form a gritty type of topsoil that is generally classed as sandy. The relatively large particles in sandy soil drain moisture well. Although sandy topsoil is considered good for cultivation, it can dry out rapidly during hot weather and deprive plants of nutrients. Sandy topsoil can also lose some of its nutrients during heavy rains.

Silt

Silt topsoil is composed of tiny pieces of minerals and decomposing organic matter that are rich in nutrients. Most scientists define silt topsoil as highly fertile and one of the best types of soil for cultivation. Silt topsoil has a finer texture than sand, often described as similar to flour, that allows it to retain moisture. Although it has some important advantages, silt's fine texture also makes it vulnerable to erosion.

Clay

The tiny organic and inorganic particles in clay topsoil are smaller than those in sand or silt. Clay's ultra-fine texture allows it to retain a high level of moisture and nutrients, in fact sometimes too much. Clay soil drains poorly, and plants often do not receive the amount of oxygen they need to grow and thrive. Gardeners and scientists describe clay soil as thick, gummy and difficult to work. Gardeners point out that topsoils with high percentages of clay are slow to thaw in the spring, which can delay planting and cut short the growing season.

Importance of Texture

Although texture is only one parameter used to classify soil, it affects other important properties. Texture determines soil structure or the way particles group together. Soil structure, in turn, affects how much moisture soil will retain and how far root systems can stretch and grow. Texture also determines how much organic material, such as decomposing leaves and plants, fungi, bacteria and animal material, is present and able to enrich the soil with nutrients. Texture is also a major indication of the types of improvements, such as tilling, irrigation and fertilization, needed for to prepare soil for a particular use.

References

• Rutgers: Soil Texture
• Electronic Data Information Source: Soil Texture
• North Dakota State University: Soil, Water and Plant Characteristics Important to Irrigation
• European Agricultural Investment Services: Types of Soil
• University of Wisconsin-Stevens Point: Soil Properties: