Understanding the difference -- topsoil, mulch, compost

(L-R) Topsoil – mulch – compost

Topsoil-mulch-compost: what’s the difference?

A: Topsoil-mulch-compost … when landscaping, understanding the difference between this dynamic soil trio is essential. Even more important is knowing what to use where and when.

A well-dressed landscape will include layers of different materials. This establishes an ideal environment for healthy growth. The foundation is topsoil, enriched with the compost and, finally, a mulch to blanket it all.

Take a peek inside a McGill high-rate composting facility

WHAT IS TOPSOIL?

Topsoil — it’s the layer between the layer of decaying matter on 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. Land clearing, farming, and erosion has reduced this layer over time. And 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 also possible to build to this level over time. Make lighter, but more frequent, compost applications raked into the top layer of soil.

But know these two products — compost and topsoil — are not interchangeable. Compost is not topsoil.

The purpose of compost is to build or improve topsoil. But is the wrong product for many applications that call for topsoil. Don’t use 100 percent compost as fill dirt, for example. Mix compost with excavated soil to make an enriched backfill.

The opposite is also true. Topsoil is not compost and will not perform like compost. Adding topsoil alone does not ensure soil performance. Some “topsoil” may be almost inert with little to no organic matter or active soil microbes.

WHAT IS MULCH?

Mulch goes on top of soil

Mulch is a material applied to the soil surface to discourage weeds. A mulch supplies shade and reduces moisture loss through evaporation. Bark, wood chips, shredded yard waste, and sawdust are all used as mulch.

But fresh wood mulches can compete with plants for nutrients. Plus, weed seeds, untreated pet waste, and lawn chemicals can contaminate uncomposted mulch.

A well-managed composting process breaks down many pollutants. It kills weed seeds and pathogens. Thus, compost makes an excellent mulch for holding moisture. At the same time, it shades roots from the summer sun.

Mulching with compost also allows earthworms to till the compost into the soil. The worms rebuild topsoil with no extra work on the part of the landscaper or gardener.

WHAT IS COMPOST?

Compost is a soil amendment. Always mix compost with soil except when used as a mulch.

Using compost completes the natural soil cycle. It returns 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 may be mulch masquerading as compost. Or, it is compost manufactured to a low standard.

Immature, woody composts compete with plants for nutrients. They may contain pockets of material that are not fully composted. Pass on these inferior offerings in favor of a higher quality product.

Manufacturers of STA-certified composts make laboratory analyses available to customers. These reports include many quality indicators, including stability and maturity.

SlideShare: Compost, manure, topsoil, mulch, and peat