What Is the Difference in Organic Matter Soil & Topsoil?

Topsoil is the upper layer of soil in a given area. It typically contains sand, silt and clay in varying proportions, which can account for as little as 40 percent or as much as 80 percent of the soil's bulk. Sandy soils are loose and drain well, but they retain water poorly. Silt soils compact and become dense, while clay soils can become hard and drain poorly (see Reference 1). All of these characteristics can be modified if there is enough organic matter in the topsoil. Some of this organic matter is fully decomposed, and some is actively decomposing.

Above the topsoil, at ground level, is a thin layer composed almost entirely of organic matter. It's made up of varying materials, such as thatch in grasslands and leaf mold in forests. This layer is constantly decomposing as bacteria, fungi, worms and insects digest it and transform it into rich humus (see Reference 1, pages 1 and 2). Through the passage of time and the activity of soil-based organisms, this organic material slowly becomes incorporated into the topsoil beneath it (see Reference 3). Humans can accelerate this process by adding organic matter to this layer as mulch or by adding finished humus to the soil in the form of mature compost.

Soils with increased organic matter demonstrate consistently superior drainage. Mature humus acts as a sponge, creating air pockets where water can seep in and be retained. This is beneficial to all types of soil. Humus improves a sandy soil's ability to retain moisture, reducing the need for irrigation. In hard-packed silt or clay, humus softens and lightens the soil. This allows water to seep in and be absorbed, or "infiltrated," rather than pooling on the surface to create runoff and wastewater (see Reference 1, pages 1 and 2, and Reference 2). This minimizes both erosion and water use, two significant environmental benefits.

The physical properties of organic matter soil, and their importance as a portion of the topsoil, are only part of the larger picture. Organic matter is also crucial to soil's ecosystem. A host of bacteria, fungi, nematodes, insects, worms and other organisms feed on fresh organic material, turning it into humus. The more diverse and prolific these organisms are, the healthier the soil. A thriving, biologically diverse soil provides more nutrients to plants, helps remediate soils containing toxins and encourages plant health by competing with harmful plant pathogens. (See Reference 4).