Unraveling the Mysteries of Composting

By Richard F. Hennessey

More than half of what Denver residents send to the landfill consists of organic material, like plant debris and kitchen food scraps. Instead of discarding this valuable material, we can compost most of it and use the resulting humus to enrich the soils of our gardens and lawns.

With these benefits in mind, here is an introduction to composting – how it works, how to do it, how to use the results, and why it is a fun, interesting and worthwhile activity.

Let’s begin by defining a few key terms: composting, decomposers and humus. Composting means the managed or controlled decomposition of organic matter utilizing naturally occurring micro and macro decomposers  — like bacteria and worms – to create humus. The resulting stable, long-lasting, organic component of soil – humus – contributes to healthy plant growth.

Composting happens best in a pile or heap that is created and maintained under a few basic conditions:

1. The pile should be the right size (a minimum of one cubic yard) to provide sufficient volume for the necessary thermal activity.

2. It should be fed with nitrogen- and carbon-rich ingredients, cut to small, 2-inch pieces.

3. The pile requires adequate air and water.

4. The contents of the pile should be turned over periodically.

Key ingredients

Carbon and nitrogen are the two main ingredients of a healthy compost pile. Carbon is the main source of energy for the decomposers, and nitrogen is necessary for decomposers to grow and reproduce.

Carbon-rich materials are called “browns” — think fall leaves; twigs and chipped branches; stalks of broccoli, cabbage, cauliflower and kale; corn cobs and stalks; crushed eggshells; straw; peanut husks; teabags; paper towel and toilet paper cores; paper towels, napkins and tissues; shredded newspaper; cut-up cardboard; and coffee filters.

Nitrogen-rich materials are “greens,” and they are softer, younger, sometimes green, and have higher moisture content – think grass clippings with no pesticide residue; green leaves; hay and alfalfa; tops of root vegetables; coffee grounds, used tea and spent hops; garden waste; food waste (no meat, fat, cheeses or bones); fruit and vegetable peels; hair and pet fur; manure (herbivores only); and weeds (without mature seeds). Allow weeds with creeping underground stems and roots to sunbathe in shallow layers until their roots have died before using them in the compost pile. Warning: Don’t use rhubarb leaves; they are toxic!

The target ratio is about two-to-one by volume, carbon-rich materials and nitrogen rich materials.

Size and location

The ideal size for your compost pile is 3’x3’x3’ because it creates sufficient volume to enable the thermal activity that is part of the composting process. Locate the pile in a level, partially sunny area.

You can make an open pile, build a three-sided bin for the pile (e.g., out of wire, wood, snow fencing, hardware cloth, recycled cement blocks, etc.), or use a pre-constructed plastic bin available in gardens shops, garden catalogues and wholesale outlets like Costco.

Make sure to put the pile in a partially sunny area, but protect it from too much sun and wind, which can lead to overheating or loss of necessary moisture due to evaporation. If you are building your pile on the ground, loosen the soil that will be under the pile to about 1½” depth.

Building the pile

Making good compost is like mixing ingredients for chopped salad. Chopping or cutting makes for smaller pieces with increased surface-areas for the micro and macro organisms to work on.

Mixing the browns and greens puts the carbon-rich and nitrogen-rich ingredients in close proximity, and ensures air infiltration to provide the oxygen necessary for the composting process. These are the basic steps in starting your compost pile:

Step 1. Open the soil, by scratching or turning it to 1 ½ inch depth.

Step 2. Add a 4” to 6” layer of mixed and chopped brown (carbon) materials.

Step 3. Add a 2” to 3” layer of mixed and chopped green (nitrogen) materials.

With respect to both steps 2 and 3, materials with different textures are ideal.

Step 4. Add a sprinkling of soil or compost. This further inoculates the pile with microorganisms.

Step 5. Mix the layers with a three-pronged hand garden tool, garden fork or mattock.

Step 6. Sprinkle – don’t pour – water and mix further. Sixty percent (60 percent) moisture content in the pile is ideal, which is achieved when the layer feels like a wrung-out sponge.

Continue building the pile by repeating Steps 2 through 6.

Step 7. Cover the pile to keep it from drying out. If you’ve made an open pile or built a three-sided bin, you can use a plastic sheet, tarp or even a layer of straw. Holes should be punched into any plastic-like sheet covering to facilitate the circulation of air. Most pre-assembled bins come with a lid.

Thermal activity

As soon has your compost pile is constructed, decomposition begins and within days the temperature inside the pile begins to rise, ideally as high has 140° F, and then begins to recede. This thermal activity lasts from several weeks to a couple of months, depending on conditions.

As the temperature varies, different kinds of microorganisms are active. Check with a long kitchen thermometer; if the pile gets too hot (over 160°F), all the microorganisms will die. This is one reason why the compost pile should not be in direct sun.Turning the pile

Just like aerobic exercise is healthy for humans, turning the compost pile introduces necessary fresh air into the pile and provides fresh sources of food for the decomposer organisms at work in the pile. Turning the pile about once a week will re-introduce fresh air, circulate food and working organisms and vastly accelerate the composting process. It also is a good time to check the moisture level.

If you use a pre-assembled, barrel-type composting unit that permits rotation of the unit on an axis or swivel, rotating the unit will turn over its contents.

Finished compost and curing

Depending on conditions – mix and size of materials, volume of the pile, moisture, aeration and thermal activity – the composting process can take between two months and two years. It is complete when the heat producing and active decomposition stages are over.

There are several telltale indications: the original materials are no longer recognizable, and the contents of the pile are dark brown or black in color, relatively lighter weight, crumbly to touch or granular in texture, and smell like a damp forest floor. The finished pile will be about half the volume of the original pile.

At this point, the finished compost should be covered and, before use, left for about another two months to age or cure, for the pile’s ph to reach neutral levels.

Using your finished compost 

The finished compost – now humus – contributes to the healthy life of the soil. It can be used in garden beds, in lawns, for trees and shrubs, and for houseplants. It is a soil amendment, not a fertilizer.

In your garden beds, before planting mix one-to-two inches of compost into the top two-to-four inches of soil. During the growing season, you also can lightly work small amounts into the soil around growing plants.

For trees and shrubs, spread around one-to-two inches of humus into cultivated soil at the drip line.

After your lawn is aerated, a thin layer of humus will work its way into the soil, break-up the clay particles and promote a deep, healthier root structure.

You can also enhance the potting soil for your houseplants by adding up to 20 percent of finely screened humus.

Tips and troubleshooting

•While indoors, keep your kitchen scraps to be composted in a covered container. This will eliminate flies and odors.

•Use grass clippings sparingly. While they are a source of nitrogen, too much will result in matting of materials and interfere with aeration of the pile.

•One of the more common errors is failing to cut or shred material for the compost pile to small enough sizes.

•A compost pile should not produce odors. If it does, it may mean that there is not enough air in the pile, in which case turn the pile. Or, it may be that the pile is too wet; if so, add coarse brown or carbon-rich materials. It could also mean that there is too much nitrogen in the pile. Here again, add coarse carbon-rich material.

•If your compost pile does not heat up, it may mean the pile is too small or that it needs more nitrogen rich materials.

•If the pile is too dry, work in added moisture to the degree of a wrung-out sponge.

•If the pile is attracting flies, it likely means that there are food scraps on top of the pile. They should be buried deep within.

Want to learn more?

From May through October, free composting classes are provided at Gove Community Gardens, between 13th and 14th Avenues on Colorado Boulevard. Classes typically are held on Wednesday evenings, 6 – 8 p.m., and Saturday mornings 9 – 11 a.m. For additional information, check out dug.org, or call 303-292-9900.

Richard F. Hennessey is a retired Denver attorney and resident of Park Hill since 1968. He is a participant of the 2014 Master Composter Training and Outreach Program sponsored by Denver Urban Gardens and Denver Recycles. Mr. Hennessey specially acknowledges Judy Elliott, education specialist with Denver Urban Gardens, whose extensive knowledge, interesting classes and capable teaching made this article possible. Next month: Why composting works so well for Colorado’s soil.