Compost vs Mulch

Compost vs Mulch by Wayne Bowman, Master Gardener.

Not all mulch is compost, but all compost can be used as mulch. To be clear, the terms are not interchangeable. Unlike compost, some forms of mulch will never decompose– such as shredded rubber tires. Various woods are slow to decompose– like cypress or redwood. This doesn’t mean they are bad mulch products– they still keep the soil moist and at a lower temperature– but they’ll not turn into quickly useable compost. Another way to distinguish between the two is to remember that mulch goes ON the soil, while compost goes IN the soil.

Here in North Texas, the soils vary from sandy to heavy clay. Sandy soil drains well– sometimes too well– and often lacks nutrients. The heavy clay is nutrient rich, but it doesn’t drain well and turns into bricks when it dries. In both cases, compost comes to the rescue.

There are two ways decomposition occurs: anaerobic (without air), and aerobic (with air). Anaerobic occurs when organics are placed in an airless environment, like at the bottom of a pit dug in the ground and then covered with dirt. I’m going to concentrate on the aerobic method.

An aerobic compost pile is a combination of “green” and “brown” materials in the proper ratio– four parts brown to one part green. Greens provide the nitrogen that feeds the microbes; browns provide carbon for energy.

Green material includes fresh grass clippings (to be used sparingly), vegetable scraps from the kitchen, and pretty much any recently living plant material. Manure is considered “green” also, but care should be taken to make sure it comes from grass eaters only, not meat eaters. Meat scraps and pet manure are no-no’s in the compost pile. Resist the urge to take your old plants from your garden to compost, too. You could be transferring disease which will infect your garden next year. Old plants deserve cremation or a proper burial in the landfill. Likewise, avoid grass clippings from a lawn that has been treated with herbicide, or weed and feed.

Browns include dried leaves, wood chips, hay, etc. In addition to carbon, the browns add bulk to the compost.

Properly mixed, the pile should begin to heat up, indicating decomposition; if it doesn’t, add greens. If it smells, add more browns. You can add a sprinkle of high nitrogen fertilizer while alternating the greens and browns during the build and turning. Add some water if the pile seems to be drying out. Expect the pile to decrease in size during the process– you’ll have room to add more fresh material. When to turn the pile depends on how much time you wish to allot to the task. The more frequent the turn, the faster the compost finishes. If you’re in a hurry, turn as often as every three days. If not, a week to ten should work.

There are many options in building the physical container for your pile. I built three side-by-side bins, keeping two full of working compost and transferring the pile to the third during the turn. The most important factors to keep in mind in designing your container are building it large enough for your needs and ensuring air circulation to the pile.

A healthy compost pile will also be home to a few critters. Earthworms love coffee grounds, and help the microbes turn your kitchen waste into garden gold. Insects like pill bugs and soldier flies also make quick work of the decomposition. All that’s needed are scraps and moisture to excite them to come to your buffet.

After several weeks of regular turning, you’ll dig into your compost pile to find that it’s no longer generating heat, but instead has a nice granular brown appearance and a great earthy smell. Any resemblance to grass, chips, etc is gone. It’s finished and ready to head for the garden or flower bed to begin feeding your plants and amending your soil.

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