University City

Homemade or local compost is best for improving soil

There’s nothing wrong with this area’s red clay soils. In fact, they can become wonderfully productive.

The key to unlocking red clay’s potential is compost.

Soil scientists call compost’s magic ingredient “organic matter” (“OM” for short). OM transforms the clay, opening it up so beneficial soil critters can thrive, and so that water, air and roots can penetrate it.

Mother Nature, Queen of OM, patiently grows fertile soil by adding OM to the top layer. We’re seeing the process now, as autumn leaves fall to blanket the earth.

It is a slow, steady effort; building an inch of rich topsoil in the Carolina Piedmont takes Nature 500 to 1,000 years.

Making compost is essentially the same process, except that it is managed so that decomposition occurs quickly and completely. Made from “waste materials” such as fallen leaves, compost is a rich and easy-to-use source of OM.

The best way to get compost is to make it yourself. It’s fun and easy, and saves money, too, but the process takes several months. (Don’t be suckered by the hype for “two-week compost.”)

Let’s say you need some compost right now, so you head down to the garden center. There you confront a bewildering selection of bags filled with brown-looking stuff, decorated with messages proclaiming “rich in organic matter” and similar slogans. How do you know what to buy?

Focus on key words. Though the contents may all look about the same, each product is different, and some won’t help your soil at all.

• “Compost” is what you are looking for. Compost has gone through a process where beneficial microbes create stable organic matter you can safely add to soil.

A one-cubic-foot bag of compost provides enough to treat about 12 square feet of garden, mixed into the top six inches of soil.

For a new project in unimproved soil, you can use a heavier application, but be careful: New types of persistent weed killers can make it through commercial composting processes. Until these chemicals are removed from the market, gardeners are wise to limit commercial compost application to no more than an inch per year.

For larger applications, it makes sense to call Mecklenburg County’s Compost Central operation, or the similar program in Cabarrus County.

Buying in bulk is much cheaper than buying in bags. Neither county has reported herbicide contamination to date.

Meanwhile, what’s in all those other bags? Here’s a quick overview:

• “Mulch” (anything from bark chips to shredded pine) is undecomposed organic materials. You put this on top of soil to beautify and protect it, and to help control weeds. It may actually make soil LESS fertile if you mix it in.

• “Potting soil” (sometimes called “soil-less mix”) usually contains bark fines, a drainage and aeration medium such as Perlite, and sometimes a bit of compost. This won’t improve your soil as well as compost does.

Quality potting soil is expensive, too, so I use it only for pots. (Garden vocabulary note: In other English-speaking countries, “compost” can also mean “potting soil.”)

• “Manure” is, well, just that. Cow manure (the numbers 0.5-0.5-0.5 on the bag indicate that it is also a fertilizer) has been composted and is OK for use in vegetable gardens. But manure sometimes contains salts that can cause problems if you use too much.

Bagged chicken manure should be used only as fertilizer, not as a soil improver. Compost works better.

• “Topsoil” is a completely unregulated mix of soil from unknown sources, sometimes containing other materials. There’s no way to know what you are getting, and it won’t improve your existing soil.

If you want to fill large planters (“raised beds”), consider a good commercial topsoil blend in bulk, but buying bags for this purpose is ruinously expensive.

• “Peat moss” is organic matter mined from bogs. Even though Canada, our main supplier, does a good job of managing peat sustainably, I still don’t use it in the garden. It is very acidic, just what we usually want to avoid in our already acid soils.

Though it holds water well when wet, when peat dries out it actually sheds water. Advanced growers often keep some peat moss around for special applications, but to improve clay, use compost instead.

• “Sludge.” No, it is never sold by that name, since sludge is to humans what manure is to cows (but with heavy metals thrown in.) Lately, the PC term is biosolids.

Best known is Milwaukee’s sludge, marketed as “Milorganite,” which is cut with brewery wastes from beer. I avoid it, particularly around edibles and where kids might put soil in their mouths.

Everything has its place in the garden, though: The latest word is that Milorganite’s smell helps repel deer. I might try that along the back fence.

The best idea, of course, is to start making your own compost in a corner of your yard. Leaves are falling now, so try Felder Rushing’s easy recipe:

Don’t bag leaves. Just pile them up someplace. In a couple of years, you’ll have leaf mold compost. If you want compost quicker, take a county composting class.