(L-R) Topsoil – mulch – compost

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.

Learn how a quality compost is manufactured

4 replies
  1. Jeremy Thompson
    Jeremy Thompson says:

    It’s good to learn that there is more to soil health and planting than just compose and mulch since topsoil is also important to take into account. As you’ve mentioned if I would like to have a good-looking lawn having the complete layer of the three would be significant. That’s perfect to learn since we’ve just moved and it’s not too late for me to replant most of our greens. My wife would love to see our new lawn once I finish having it fixed. Thanks for the informative read about topsoil, mulch, and composts!

  2. Larry Davis
    Larry Davis says:

    Hello I’m Larry. I have some very old large shrubs on both sides of my driveway and I had to trim them back quite a bit and now they’re brown on one side. The other side used to be brown but now that I trim this side the brown side is now green. Anyway I removed a lot of inferior dirt that was filled with sand that was around the bushes. Should I put topsoil and then mulch? Long story short what can I do to make these bushes healthy again.

    • Lynn Lucas
      Lynn Lucas says:

      Hello, Larry. Before beginning your fill project, check with the horticultural agent at your county’s Cooperative Extension Services office (or other local expert) for advice specific to the shrub variety and your soil type. Dig your new hole 2-3 times wider than the root ball. Keep in mind that compost is a concentrate, so when using compost as backfill, it should be mixed with native soil at a ratio of about 70 percent soil to 30 percent compost. Be sure the compost is a quality product — mature, stable, etc. Finish up by adding a thin layer of compost out to the shrub’s drip line. This will help keep the roots cool and feed the plant a little bit each time you water. Then, cover the compost with a decorative mulch that matches the other mulch products currently in use around your home. If compost will be the only mulch, lay it on a little thicker (2 inches at least) and replenish as needed throughout the growing season, ending with a final “blanket” in the fall. Do not pile compost (or soil) up around the stems of woody plants and tree trunks. Though this may be a common sight throughout suburbia, the practice invites insects and will eventually damage the plant or tree. For help in determining how much compost is needed, use our Compost Calculator.

  3. Camille Devaux
    Camille Devaux says:

    It is nice to know the difference between these different types of soils. Having the insight to look into getting topsoil that will be good enough to grow things is important since there is only a thin layer of it left. This would be a good tip for my cousin who has talked about getting topsoil services in the past.

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