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Buying Compost

by Cheryl Long

The Good News and the Bad

We tested 30 brands and found that it’s a mixed bag.

Homemade compost is the best thing you can use to feed your plants, improve your soil, and recycle yard wastes. But it seems as if there’s never enough-no matter whether you’re starting a new bed, trying to cover your entire lawn, or living in an apartment and tending a community-garden plot. What’s a gardener to do?

The obvious answer is to head for the garden center or home-improvement store and load up the car with bagged compost. However, when we took a close look at what was in those bags, we discovered the bad news: Some brands tested well, but many flunked out. The good news is that there are simple steps you can use to select a top-quality compost product.

Our Testing Methods

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-Do. 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 this data, we discovered an easy way you can test compost quality right at home.

Be a Compost Connoisseur

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. The quality of commercial composts varies because they are usually made from whatever local “waste” materials are available at the time; the contents will differ from batch to batch. (For example, one batch might be made with low-salt manure; the next, with high-salt.) That means that unless the producer monitors each batch carefully, a brand that tested at the top of the class this month might flunk out the next time.

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 the texture, color, moisture, and “bouquet.”

  1. 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. (See the photo on page 51 for examples of what to avoid.)
  2. 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.
  3. 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.)
  4. 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.)

What Does It All Mean?

We started this project because we suspected there might be some poor-quality compost on the market, and we wanted to help gardeners buy the best possible products.

Until the compost industry cleans up its act, we recommend that you continue to make as much compost at home as you can and use it as soon as it decomposes to the point that you can no longer recognize the original ingredients. If you just can’t make as much as you need, head for your local yard-waste recycling center. At the one here at Organic Gardening magazine’s home base in Emmaus, Pennsylvania, city residents (including OG staff members) can pick up as much compost as their sagging vehicles can carry. It’s easy, it’s pure composted yard wastes, and it’s free!

If your city doesn’t have a yard-waste composting site, you can sometimes save money by buying compost in bulk from garden centers. Bulk composts are often cheaper than bagged products: 1 cubic yard of bulk compost (the equivalent of about 25 40-pound bags) usually costs under $30, whereas the good-quality bagged composts sell for $2 to $4 or more per 40-pound bag.
If you decide to buy bagged compost, inspect the contents carefully before you buy multiple bags. And if you need a large amount of compost, it’s probably worth your time to insist on seeing lab-test results so that you can check them against the quality standards outlined in “The Woods End Lab Report”. (Any reputable compost producer should have lab-test information available.)

Gardeners have a right to expect good-quality compost, fully tested and correctly labeled. Having informed consumers ask tough questions may be the only thing that will force the industry to improve. Years ago, farmers had to fight a long, hard battle to pass laws requiring that all fertilizers be fully labeled. Right now, compost producers can avoid those laws if they don’t call their compost “fertilizer.”

We hope you’ll use this special report to educate local suppliers about compost quality. If you find compost producers that are doing a better job of meeting quality and labeling criteria than those that created the products we surveyed for this report, we would love to hear about them. And if you obtain lab-test results to help you evaluate composts being sold in your area, please write to us and share what you have learned.

Reprinted with permission from Organic Gardening Magazine