Soil acts as more than a means of mechanical support for the roots of your garden. It provides the nutrients your plants need to grow and holds moisture so that water is available even during dry spells. While most gardeners understand that healthy soil equals healthy plants, understanding the difference between soil additives can be confusing. Although they share some traits in common, topsoil and compost are different materials that serve different purposes in your garden.
Topsoil refers to the uppermost layer of the earth's surface. Depending on the location, it may be a few inches to several feet thick. The slow weathering of rocks and decay of organic matter produces topsoil over thousands of years, releasing nutrients back into the soil, where plants use them. Composting refers to the process by which organic material decays. Although compost may form a component of topsoil, it is not complete topsoil by itself.
A major observable difference between topsoil and compost occurs in the rate of formation. In most places, 1 inch of topsoil requires 100 years to form. Composting is a much faster process, and hot composting methods can produce finished compost in as little as six weeks.
Topsoil includes several components. According to the Maine Geological Survey, the primary ingredient in topsoil is mineral grains, which constitute almost half of healthy topsoil. Topsoil also contains water, air and organic matter, the latter of which holds all of the components together. Compost, on the other hand, derives from decaying plant and animal matter and, therefore, contains primarily organic matter.
Topsoil cradles the plant life growing on the surface of the Earth. It provides nutrients, including mineral nutrients and micronutrients, and its structure helps hold water and prevent soil loss through erosion. Composting represents just part of the process by which topsoil forms, breaking down dead organisms and waste products and returning the nutrients they contain to the soil. Gardeners often add compost to gardens to improve the topsoil, increasing water and nutrient retention and restoring nutrients.
Both topsoil and compost are available at a garden supply store, and compost can be easily manufactured at home. However, because topsoil is a precious and not easily obtained resource, what is sold as topsoil is rarely actual topsoil. According to the Washington State University Extension, it is a mixture of other ingredients, including soil, sawdust, manure, sand and biosolids. Both topsoil mixtures and compost can be added to gardens by working them into at least the top few inches of soil to improve water drainage. Compost can also be added as a mulch or side dressing around existing plants.