Chapter 8
Commercial Compost: A User’s Guide
Compost
Why is it that some can only measure
By their rule things others treasure?
By their standards some may think
That a compost bin has to stink;
But if you will give it proper care,
No one will even know it’s there.
Because it’s always in demand—as a valuable addition for starting a new garden or maintaining an established one, for planting containers, or for sowing seeds indoors—there never seems to be enough compost. Picking up a bag of compost at the local garden center, or a truckload from a municipal composting center, may seem simple enough. And even though one bag or one pile may look just like another, not all compost is created equal.
Commercially made composts are highly variable in organic matter and nutrient content. Some can even harm soil and plants. Learn in this chapter how to identify safe commercial composts and what to look for when assessing municipal compost offerings. As an added bonus, see “Growing Right in the Bag” for instructions on growing vegetables right in bags of compost or topsoil.
1. Buying Bagged Compost
While the safest bet is to make compost at home from materials that don’t contain pesticides, herbicides, or other harmful chemicals, sometimes a gardener’s need for compost exceeds the backyard pile’s supply. Commercially made compost is available by the bag from local garden centers and home-improvement stores. Unfortunately, buying compost isn’t as simple as looking for a particular brand or finding a bag with a reassuring label. The truth is that most commercial composts are produced and sold locally—different products are available in South Dakota and Tennessee, for example, or even in Nashville and Memphis.
The quality of commercial compost varies, because the compost is usually made from whatever local waste materials are available at the time. For instance, one batch might be made with low-salt manure and the next with high-salt manure. That means that unless the producer monitors each batch carefully, a brand that tests at the top of its class one month could flunk the next time around. A few years ago, Organic Gardening magazine and Woods End Laboratories in Mount Vernon, Maine, tested 30 brands of bagged compost and found a variety of problems. Most importantly, they discovered that more than half the brands tested were either too old or were diluted with soil, which resulted in a low organic-matter content. High-quality finished compost contains 30 to 60 percent organic matter, and if the level of organic matter is too low, the compost won’t improve the soil. In addition, while a near-neutral pH is optimum for most plants, nearly half of the samples tested outside the preferred 6.5 to 7.5 pH range. Nine of the samples tested too high (as alkaline as pH 8.3), while four tested too low (as acidic as pH 4.5). A quarter of the samples contained too much salt. If compost production isn’t managed properly, or if a large amount of chicken manure is used, salts can accumulate to a level high enough to injure plants—especially seedlings. A quarter of the samples were sticky and wet, making them difficult, if not impossible, to spread. So is it possible to find worthwhile bagged compost? Absolutely yes: Just follow the advice below.
Buying Quality Compost
Fortunately, there are several ways to find a good-quality product. If possible, inspect the compost at the garden center or ask to take home a few samples, or purchase a single bag to take home and examine before stocking up. Put the samples through a few simple tests using the criteria below. If the compost doesn’t pass these tests, consider returning it (any reputable merchant would want to know if a product was sub-standard) and asking for a refund, or dump it onto a home compost pile to enrich or dilute the pile.
Inspect the color. Finished compost is a deep, dark brown, almost black. If it’s lighter, it may be low in organic matter and contain soil as filler. It’s easiest to tell the true color if the compost sample dries out.
Assess the texture. The texture should be loose and granular, with little or no recognizable bark or wood. If the compost isn’t loose enough to spread and work into garden beds easily, don’t buy it. Good compost shouldn’t have any big clumps, chunks, or identifiable unfinished materials. Pieces should be no larger than a half-inch in diameter.
Check for moisture. Grab a handful of compost and squeeze it. Ready-to-use compost is slightly damp, kind of like a wrung-out sponge. If the compost is too dry, it’s likely past the point of optimum biological activity. Wet compost tends to be anaerobic (without oxygen) and fermenting. In this case, beneficial aerobic bacteria are replaced with harmful anaerobic ones, undermining the compost’s vital microbial balance. In the soil, compost can hold up to 200 times its weight in water, but in bagged products, excess water makes the compost difficult to spread. Lifting a bag of compost will give a good idea of its moisture content. If it feels like a big glob, the compost probably is too wet. If it feels loose, it’s probably drier.
Take a sniff. Mature compost has a pleasant, earthy smell. A sweet, fresh, woodsy odor when the bag is opened indicates mature, good-quality compost. If the contents smell a little musty or like a barnyard, that’s okay—plastic bags restrict the oxygen supply to the organisms that release the earthy aroma. After a day or two, the compost should acquire that earthy odor. Avoid compost that has any strong or unpleasant odors, like the scent of rotten eggs or ammonia, that signal the compost wasn’t finished when it was bagged.
Measure organic matter content. Mature compost contains 30 to 60 percent organic matter. To test the organic matter content of any compost (commercial or homemade), spread some out on a thin layer of newspaper and let it air dry for about a week. Then measure exactly 1 cup of the dried compost and weigh it. If it weighs between 4 and 6 ounces, it contains the desired amount of organic matter. If it weighs less than 4 ounces, it’s probably immature. If it weighs more than 6 ounces, it’s probably old or diluted with soil and won’t be as effective as compost at its peak.
Dig deeper. When bags are stacked outside, lift and look between them. If there are red worms or red wrigglers squirming around, this is a sign of quality compost. These worms, which feed on decaying matter, thrive around the cooler fringes of healthy compost piles. The presence of worms between the bags indicates that earthworm cocoons are hatching inside. However, not all quality bagged compost will contain worms, so look for other signs of quality.
Check the price. Cost per bag also can be an indicator: Cheap compost is typically unfinished or anaerobic.
Read the fine print. It’s important to read all the fine print about what the compost contains. Some manufacturers use what the industry terms “biosolids,” which means human sewage sludge. This can contain virtually anything that homeowners, hospitals, or industrial plants flush down their pipes—from shampoo chemicals and pharmaceuticals to industrial solvents and heavy metals. The good news is that the use of human sewage sludge is strictly banned in organic agriculture (another reason to eat USDA-certified organic food!). However, many nonorganic farmers do use it to grow food crops, and it’s also allowed to be used in municipal or bagged compost. It is sometimes labeled as an “organic” or “natural” material. (Use of the term “organic” is strictly enforced in the food industry but less so in other industries.) The Environmental Protection Agency declares human sludge safe for use, as long as it passes tests for nine substances and two indicator bacteria—but that hardly covers all possible contaminants. In short, avoid buying any compost that has biosolids labeled as an ingredient. Also avoid buying a product that lists “inert ingredients,” because it is unclear what those ingredients may be.
Ask for more information. If the information on the bag is vague or unclear, start by asking the seller what it contains. If the seller isn’t sure, call the manufacturer’s number given on the bag and ask. Compost made from bark mulch and sawdust won’t be worth carrying home, because it doesn’t provide much nutrition for soil organisms. If the company uses biosolids or unidentified inert ingredients in its compost, choose another compost.
Test it. Bagged compost also can contain herbicides that don’t break down, even during the composting process. (One of these is clopyralid, used on lawn and grain crops.) These can destroy virtually every plant in the garden. To determine if compost harbors harmful herbicides—or at the very least, has a bad pH—try starting a few lettuce seeds in the compost. If germination is poor or nonexistent, herbicides could be present or the pH of the soil could be way off. In either case, don’t use this type of compost.
Buy in Bulk
2. Evaluating Municipal Compost
Municipal compost operations range from small leaf-composting facilities to huge systems capable of turning 800 tons per day of waste into compost. Large facilities create windrows of compost that are either turned every few days with mechanized equipment or aerated using forced-air methods. A few facilities are fully enclosed, automated “in-vessel” compost systems, which take in refuse at one end and sort, grind, mix, moisten, and aerate it on its way to the other end of the process, where it arrives as finished compost. Other municipal compost yards are mainly yard-waste collection sites with huge piles of branches, grass clippings, leaves, and weeds that may be shredded or allowed to compost in place.
Compost from a municipal recycling facility has its pros and cons. On the plus side, it’s often available for free or at low cost, and residents can help themselves to an unlimited supply. On the minus side, some of the yard waste may contain diseased plant material or contaminated soil and could contain weed seeds that will readily germinate in good garden soil. Municipal facilities use materials collected from yards and gardens throughout their regions, so the grass clippings and other materials they use may have been treated with herbicides, pesticides, or other harmful chemicals.
It’s a good idea to test municipal compost for herbicide residues before applying it to gardens or using it to make a potting mix. To test a sample, simply sow seeds of sensitive crops such as cucumbers, tomatoes, or peas (make sure the seed is viable) in containers filled with compost. Watch the seedlings for several weeks. If the seedlings germinate poorly or are not properly formed, it’s a sign of a problem. While the cause can’t be pinpointed to either herbicide residue or simply poor-quality compost, it’s best to avoid incorporating that compost in the garden. Also, watch for weed seedlings to sprout; some weeds, such as thistle and ground ivy, are tenacious and can quickly overrun a garden bed. If the compost is weedy in a short time frame, carefully consider its ultimate cost in labor—whether it was free to pick up or not.
To locate nearby composting facilities, contact local governing bodies responsible for solid waste management; most states maintain listings of compost facilities, as well. The quality of the compost can vary considerably, depending on the nature of the materials used, the skill of the facility’s operators, and how long the product has been allowed to mature. Some good news is that most states prohibit distribution to the public of compost containing heavy metals or other toxic contaminants.
What’s in It?
Because the quality of compost from municipal operations can vary considerably, keep the following tips in mind:
- Never use municipal sludge or compost that contains sludge. Sludge, sometimes called “biosolids,” may contain toxic heavy metals, such as cadmium, nickel, and copper, that can build up in soil.
- Use municipal compost only on ornamental beds, not in food gardens. Municipal compost is made from yard wastes, so some of that plant debris probably was treated with herbicides, fungicides, or pesticides.
- Gauge the quality of the compost before loading up. Some piles will be more finished than others. Choose compost that has a uniform dark brown color and sweet, earthy odor with no hint of ammonia or sourness. (See “Buying Quality Compost” for more ways to assess municipal offerings.) Municipal compost shouldn’t contain visible wood chips, pine needles, or other materials that still need to decompose.
- Don’t confuse mulch—another offering made from yard waste by many municipalities—with compost. Mulch is produced by simply grinding up trees and branches.
- Remember that the compost won’t be bagged at a municipal compost site. Bring a pickup truck or stock up on containers to transport it.
3. Growing Right in the Bag
Believe it or not, it’s easy to turn that bag of soil or compost into a vegetable or flower garden. That’s right: Leave the compost in the bag, cut a few holes, and plant. In a number of countries, gardeners routinely plant many vegetables in “grow bags,” which are plastic bags filled with potting soil, in order to avoid soilborne pests or contaminants.
Super-Easy Bagged Vegetables
Potatoes, tomatoes, and salad greens are among the easiest crops to grow in a bag of soil or compost. To start a bagged garden, purchase a bag of organic compost—or two, or three, or four. To minimize mess, before cutting drainage holes in the bottom of the bags, decide where the bags are going to sit during the remainder of the growing season. If they’re going to be on a surface that could stain, set them on a watertight tray.
To grow potatoes or tomatoes, stand the bag on end, cut a few drainage holes in the bottom of it, and then set it upright where the crop will stay for the growing season. To grow salad greens, like lettuce and arugula, or even radishes, lay the bag on its front or back and poke holes all over the top. Then flip it over, so the drainage holes are on the bottom, and set it horizontally in place.
Planting potatoes in a bag. Cut open the top of the bag. Tuck two small potatoes about 4 inches deep into the potting mix. Garden centers may sell “seed” potatoes; if not, try using a couple of small organic potatoes from the supermarket. If possible, choose potatoes that already have eye sprouts; cut the potatoes into pieces, with at least one eye per piece. Potatoes are very hardy, so it’s safe to keep the bag outside as long as the nighttime temperature doesn’t drop much below freezing.
Planting tomatoes in a bag. To grow tomatoes, open up the top of the upright bag and insert two or three tall, sturdy 4- or 5-foot stakes into the bag along its back side (there should be room for the plant to grow in front of them). Push the stakes down to the bottom of the bag, being careful not to tear it. They should stick out of the top of the bag by 3 or 4 feet. Plant a single tomato seedling near the top center of the bag. Set the plant deep: Pick off all but the top four leaves and plant, so just this growing tip is peeking out of the soil. The buried stem will send out masses of roots. Tomatoes do not like cold weather, so wait until the nighttime temperature consistently stays above 40°F or so before planting. Bags set on watertight trays can probably be slid indoors if cold weather threatens, or set the bag in a wagon or garden cart so that it can be moved easily. Should a late frost threaten, toss an old sheet over the stakes to protect the plant for the night. Use twine or strips of cloth to tie the vines to the stakes, so the plant will grow upward.
Growing salad greens in a bag. Smooth out the flat top side of the bag, and use a sharp knife or scissors to cut away a rectangle about two thirds the size of the top, leaving a border strip to keep the soil in place. Then plant lettuce, spinach, radish, or other seeds in the exposed soil mix, following their packets’ instructions. Or plant transplants. Start growing greens (and radishes, too) in very early spring, as soon as the nights stay near or above freezing. Greens appreciate some shade in the heat of summer, so during the hottest months, move the salad bag to a spot that becomes shady in the afternoon.
Caring for Bagged Gardens
Water the bag to keep the soil moist but not soggy. Feed with a liquid organic fertilizer or liquid compost every few weeks.
When flowers start to pop up on the potato plants, pull out a few “new” potatoes by digging gently around in the soil. Or harvest the whole lot anytime, before the plants start to turn yellow. Harvesting is easy: Just tip over the bag onto a plastic tarp and search for the potatoes (no digging required). If space allows, start a new bag every few weeks from very early spring through early summer, to extend the harvest.
For salad greens, use scissors to harvest just the outside leaves of the plants to extend the yield; as the plants keep growing, keep harvesting leaves. If a whole plant is removed, tuck in a seed or two to grow another round of greens.
At the end of the season, dump the bag of used soil into a raised bed or more permanent container or spread it around the yard in areas that need leveling or improved soil.
The Glories of Garden Gold
All gardeners who incorporate rich compost into a garden bed have a similar reaction: They want more of the stuff. Fortunately, compost is easy and inexpensive to make—the vast majority of materials are free, after all—and gardeners can choose a variety of methods to make it. In fact, using more than one method often makes sense. Manage a three-bin system, and occasionally augment the supply with a batch or two of compost in a can, for example. Plus, with a bit of extra attention, it’s always possible to buy good-quality bagged compost or bring home a load from a garden center or a municipal facility. Using it is the important thing—and in this case, more is almost always better. It’s simply hard to imagine having too much compost!