Topsoil, as the name implies, is the top layer of soil (top 2 to 8 inches), which is usually darker than the subsoil because of organic matter accumulation. Sand, silt and clay mixed with organic matter, water and air make up topsoil. Topsoil's compacted nature makes it tough and hard and ideal to handle loads of weight. Topsoil is important for food production, soil management and degradation control.
The National Center for Appropriate Technology, which operates the national sustainable agriculture information service, says, "An acre of living topsoil contains approximately 900 pounds of earthworms, 2,400 pounds of fungi, 1,500 pounds of bacteria, 133 pounds of protozoa, 890 pounds of arthropods and algae, and even small mammals in some cases." This makes topsoil a living community rather than just plain dirt or inert body. Topsoil becomes home for the creatures that in turn are beneficial in making the soil fertile, providing nutrients to plants and trees to produce healthy crops that feed humans and animals.
Wholesalers and retailers of construction and landscaping materials sell topsoil to builders and home gardeners to use in their building and landscaping projects. Topsoil makes the foundation of homes and buildings stronger when mixed with gravel and concrete.
Topsoil is an important ingredient in many landscaping projects, such as installation of hard surfaces (patios made of concrete, flagstones and gravel), laying down sods and creating planting beds. Topsoil gives additional strength to other landscaping materials to prevent erosion when mixed together. Compacted topsoil makes a good foundation for solid concrete, rocks, flagstones and other hard surface materials. Sods laid down on solid topsoil are firmer and will not sink. Firm and compacted topsoil bordering planting beds will prevent runoff during heavy rains, while finely textured, loosened topsoil is a good planting medium for vegetables, flowers and trees.
In agriculture, topsoil rich in organic matter and humus (a brown or black substance resulting from the partial decay of plant and animal matter) have many benefits. It aids in the rapid decomposition of crop residues and granulation of soil into water-stable aggregates. Topsoil also decreases crusting, clodding and internal drainage by improving water infiltration, which increases the holding capacity for water and nutrients.
Improving the soil's physical structure makes it easier to till, which is essential in growing crops. It also increases water storage capacity, reduces erosion and improves formation and harvesting of root crops. Plant root systems also become deeper and more prolific, thereby increasing crop production.