Basic Composting - November 16, 2005
Jeff Schalau, County Director, Associate Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County


Much of what goes into our trash cans, and ultimately into our sanitary landfills, can be composted. This time of year, pine needles and leaves from deciduous trees are plentiful. Other compostable materials include manure, yard trimmings, weeds, grass clippings, small twigs, newspapers (black and white only), pet hair, and the contents of vacuum cleaner bags. In fact, almost anything that was once living is a candidate for composting. Grease, oil, meat scraps, human and pet excrement, and diseased plant material should NOT be added to garden compost. The end product is a rich soil amendment of each gardenerís own design.

The most critical factor for successful compost is the ratio of carbon to nitrogen (C:N) in the raw materials. A C:N ratio of 30 parts carbon to one part nitrogen is ideal. This ratio is the optimum proportion needed by micro and macro-organisms in the compost pile. The proper amount of moisture is also important. Too much moisture will prevent oxygen exchange and too little will not provide favorable conditions for the organisms to thrive. A table with C:N ratios of many compostable raw materials is included on the web version of this column (see URL below).

The most critical factor for successful compost is the ratio of carbon to nitrogen (C:N) in the raw materials. A C:N ratio of 30 parts carbon to one part nitrogen is ideal. This ratio is the optimum proportion needed by micro and macro-organisms in the compost pile. The proper amount of moisture is also important. Too much moisture will prevent oxygen exchange and too little will not provide favorable conditions for the organisms to thrive. Table 1 shows C:N ratios of many compostable raw materials.

Table 1. Carbon to nitrogen ratios of some raw materials used in compost.

Raw Material
C:N Ratio
Vegetable Wastes
12-20:1
Alfalfa Hay
13:1
Coffee Grounds
20:1
Grass Clippings
12-25:1
Cow Manure
20:1
Horse Manure
25:1
Horse Manure (with litter)
30-60:1
Poultry Manure (fresh)
10:1
Poultry Manure (with litter)
13-18:1
Pig Manure
5-7:1
Leaves
30-80:1
Corn Stalks
60:1
Straw
40-100:1
Bark
100-130:1
Paper
150-200:1
Wood Chips and Sawdust
100-500:1

Carbon is the main constituent of organic matter. Plants take in carbon dioxide during photosynthesis and incorporate it into cell walls, proteins, sugars, nucleic acids (DNA and RNA) and lipids (fats and oils). Just as we consume foods and convert them to carbon dioxide through respiration, organisms in the compost pile respire using the organic residues that you add to your compost.

Although nitrogen is needed in smaller quantities, without it the compost will not readily decompose. Nitrogen is a major constituent of protein and nucleic acids. Most backyard composters use manure, green plant residues, or nitrogen fertilizers to bring nitrogen into balance with the carbon. The microorganisms that get the process started are in the air and soil. By adding small amounts of topsoil to your compost, you can inoculate the pile.

The organisms living in the compost use both carbon and nitrogen to grow, reproduce, and grow more. Once their populations increase, they begin to break down the organic matter more quickly. A food chain (or a food web) with many different organisms feeding on the raw materials and each other lives within the compost pile. Fungi, bacteria, actinomycetes, insects, and worms feed on the raw organic matter. Springtails, mold mites, beetles feed on the fungi. Protozoans, roundworms, rotifers, and other small organisms graze on bacteria. Larger insects such as centipedes, beetles, mites, and flat worms feed on the smaller organisms. All the organisms produce waste and/or die to create the compost.

Compost bins vary in size, shape, and materials (see October 27, 2004 column). To start a compost pile, add alternating layers of leaves, manure (or other nitrogen containing materials), and native soil. The soil inoculates the pile with locally adapted microorganisms. Wet it down so that it is as moist as a wrung out sponge. Check the pile in a few days-it should be hot.

Kitchen scraps can be buried in the surface of the pile. I have a half gallon plastic bucket with a lid for collecting kitchen scraps. Vegetable wastes, leftover cereal, moldy bread, and thing else that we can catch (except meat, bones, grease, and oil). We even add leftover coffee and tea (liquid, grounds, and leaves). This provides liquid and would just be added to the septic system so why not add it to the compost.

A compost pile that is not working is either too dry, too wet, or lacking sufficient nitrogen. A compost pile that smells like ammonia has too much nitrogen. This can be corrected by adding carbon-rich leaves, twigs, or straw. A well-laid compost pile gets warm and smells earthy. Many people have toys that they use to make and monitor their compost. These include shredders, thermometers, manufactured bins, and handy gadgets to help you turn it. Composting is an art and science. Have fun, watch it, smell it, and use it.

The University of Arizona Cooperative Extension has publications and information on gardening and pest management. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: July 16, 2009
Content Questions/Comments: jschalau@ag.arizona.edu
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