We collected 30 bagged composts from stores across the country. Right away we saw that there was a problem with the labeling—or rather, with the lack of information on the labels—of every single bag. The recommended application rates on the labels varied widely, too—from 4 inches to only 1/10 of an inch. When we opened the bags, we were in for more unpleasant surprises. Several manure-based composts were so wet and gooey that they looked like brown Play-Doh. A couple of the bags reeked of ammonia, which is a sure sign that they weren't fully composted. And several more contained shredded wood and bark that were mislabeled as compost.
Then we sent the samples to compost expert Will Brinton, Ph.D, president of Woods End Research Laboratory in Mount Vernon, Maine. His lab tests revealed even more problems, including excessively acidic or alkaline composts, high salt levels, and improper application rates on the labels. As we analyzed all the data, we discovered an easy way you can test compost quality right at home.
4 Ways to Tell Good Compost from Bad
Buying compost isn't as simple as looking for the best brand. We can't even give you a brand-by-brand comparison because most commercial composts are produced and sold locally; you won't find the same products in South Dakota and Tennessee—or even in Nashville and Memphis.
Fortunately, a simple look (and sniff) can be all you need to do to find a good-quality product. Here's how to check out compost before you buy.
Touch
The texture should be loose and granular, with little or no recognizable wood or bark. If the compost isn't loose enough for you to spread and work into your garden beds easily, don't buy it.
Look
The color should always be dark brown or almost black in color. Avoid products that are light in color. They probably contain too little organic matter and too much soil. It's easiest to tell the true color if you let the compost sample dry out.
Lift
Compost should be moist, not dry or soggy. One of compost's biggest benefits, once it's in the soil, is that it can hold up to 2 1/2 times its weight in water. But in bagged products, excess moisture makes the compost difficult to spread. It also means that if the compost is sold in 40-pound bags (as most of them are) and you buy a wet product, you'll be paying for water, not compost. (Hefting a bag will give you a good idea of its moisture content. If it feels like a big glob, the compost is probably too wet; if it feels loose, it is probably drier.)
Sniff
Ideally, mature compost will have a nice earthy smell, but this isn't a reliable test for bagged compost—at least not right away. That's because the plastic bags restrict the oxygen supply to the organisms that release the earthy odor. If you do find an earthy, woodsy odor, you've probably struck "black gold"—a mature, good-quality compost. Most bagged composts will probably have a slight musty or barnyard odor when you first open them, and that's fine. Avoid any products that have a strong unpleasant smell (ammonia or sewer gas, for example) because the odor indicates an immature compost that might damage plants. If you don't want to smell it, don't put it on your garden. (Some stores may not let you inspect a bag before you buy it. So if you buy a bag and find it's not up to these standards, either take it back or dump it onto your home pile to dilute and fully compost.)
Using Compost
You can apply compost to any garden bed once or twice a year. To improve the structure and fertility of poor soil quickly, give it a thorough compost treatment in the fall. Spade it 6 to 12 inches deep.
Vegetable gardens
Put compost in the furrows when you sow seeds and in the holes when transplanting seedlings. When the plants begin to grow rapidly, mix compost with equal amounts of soil and spread it on top of the soil.
Flower beds
Apply a half-inch-thick layer of finely screened compost as a mulch around all flowering plants when they come up in spring.
Rose bushes
When hilling up the soil around rose bushes for winter protection, mix compost with the soil—the roses will get a better start the following spring.
Lawns
To renovate an old patchy lawn, dig up the bare spots about 2 inches deep, work in plenty of compost, tamp and rake well, and sow your seed after soaking the patches well.
Containers
Twice a year, scratch an inch or so of compost into the soil in pots where houseplants and others are growing.
