A: Topsoil-mulch-compost … understanding the difference between this dynamic soil trio, as well has specific uses for each product, is essential for a successful landscaping project.
A well-dressed lawn or landscape will include layers of different materials that create an ideal environment for healthy growth. First is the topsoil, then the compost and, finally, a mulch to blanket it all.
Topsoil is the layer of humus (partially decomposed organic matter) between the surface and the subsoil. Once upon a time, topsoil was a deep, rich, organic layer. Today, in developed regions of the world, topsoil is very thin or nonexistent, scraped or eroded away over time. What passes as topsoil may actually be inert subsoil.
If topsoil is poor, make your own. Add 2-3 inches of a quality compost product and incorporate to a depth of 6-8 inches. The goal is to reach a level of about 5 percent organic matter in the soil. It is possible to build to this level over time with lighter, but more frequent, compost applications raked into the top layer of soil. But these two products — compost and topsoil — are not interchangeable.
Compost is not topsoil. It can be used to make topsoil or improve topsoil, but is the wrong product for many applications that call for topsoil. Don’t use compost as fill dirt, for example.
Conversely, topsoil is not compost and will not perform like compost. Adding topsoil alone does not ensure soil performance, especially if the “topsoil” is mostly inert and contains little to no organic matter or active soil microbes.
Mulch is a material applied to the soil surface to discourage weeds, provide shade and reduce moisture loss through evaporation. Bark, wood chips, shredded yard waste and sawdust are all used as mulch, but unless manufactured by a state-permitted composting facility, the resulting product is not compost. In fact, fresh wood mulches can compete with plants for nutrients, and uncomposted organic materials can contain weed seeds, untreated pet waste, and lawn chemicals.
A properly managed composting process breaks down many pollutants and kills weed seeds and pathogens. Compost makes an excellent mulch for holding moisture and shading roots from the summer sun. Any unwanted airborne “volunteers” that take up residence in a planting bed where compost is used as a mulch can be easily removed during routine maintenance. Mulching with compost also allows earthworms to till the compost into the soil, rebuilding topsoil with no additional work on the part of the landscaper or gardener.
Compost is the product resulting from the aerobic (with air) biodegradation of plant and animal (organic) matter. It is a soil amendment. Using compost completes the natural soil cycle, returning organic material to the soil to grow a new generation of ornamental, food and fiber crops.
Look for compost that is dark in color, has an “earthy” aroma, and offers an even texture. If the “compost” is lumpy or contains a lot of twigs and sticks, it’s mulch masquerading as compost or is compost manufactured to a low standard. Immature, woody composts can actually compete with plants or contain pockets of material that are not fully composted. Either way, pass it by in favor of a higher quality product. Manufacturers of STA-certified composts offer users laboratory analysis of quality indicators like stability and maturity.