More On Composting - June 9, 1999
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

This week's column continues with more on compost. Composting will happen with or without our help, so why not use the process to our advantage. No lame excuses. Compost need not stink or breed vermin, can be done on a small scale, and just makes good sense. People with yards are at a distinct advantage, but even apartment dwellers can compost under the kitchen sink with the help of earthworms. The only excuse that I will listen to is "my dog ate it".

In theory, compost can be finished in as short as two weeks (this is the claim with compost tumblers/rotating drums). This all depends on the proper mix of air, water, and raw materials. Last week, I gave a lengthy discussion about the carbon to nitrogen ratio (C:N). Remember, a C:N ratio between 25:1 and 30:1 is the optimal range for fast composting. As the amount of carbon increases, the composting organisms still work, just more slowly. The more slowly it composts, the cooler the temperature within the pile.

This brings us to the two schools of thought on compost temperatures: hot vs. cool. Of course each has advantages and disadvantages. A realistic time frame for hot compost is eight weeks. Hot composting can be likened to combustion: more air (oxygen) results in hotter compost and faster decomposition. Hot compost piles require a minimum size: at least four to five feet square and four feet high. Piles smaller than this lose heat too quickly. Hot compost must be turned often to maintain the necessary level of aeration. Hot compost reaches temperatures between 113 and 158 degrees F. The hot method utilizes thermophilic microorganisms that thrive at these high temperatures.

Cool compost is more forgiving with respect to C:N ratios. If nitrogen is in shorter supply (i.e. a high C:N ratio), the microbial populations do not reproduce as fast. Cool compost still heats up, but only to a maximum of about 120 degrees F. This is a "laid back" method of composting. It can take between six months and two years to finish composting.

A comparison of the methods shows distinct advantages and disadvantages. Advantages of hot composting are speed of decomposition, efficient use of space, kills most weed seeds and pathogens present in the raw materials. Disadvantages of hot composting are amounts of labor needed, a solid knowledge of C:N ratios in materials, some nitrogen is volatilized (turned into a gas and lost) to the atmosphere, and beneficial soil microbes can be killed by high temperatures.

Cool compost has some obvious advantages to the casual gardener - less work. It also preserves beneficial soil microbes, conserves nitrogen (no volatilization), and new materials can be added any time. Disadvantages to cool composting are potential for nutrient losses from exposure and weathering, it takes more time to finish, does not heat kill as many weed seeds and pathogens, requires some attention to C:N ratio and moisture as materials are added, and it can have some material that is not composted mixed in with the finished compost.

Choose a method that fits your needs and personality. My compost is usually the cool version although I always start out thinking it will be the hot version. This is mainly because I continue to add materials to one pile. Having multiple bins can allow you to have the best of both worlds: a batch of hot, a pile of recent raw materials awaiting their turn to be added to the next hot pile, and a bin full of ready to use, finished product.

If these composting articles have only piqued your interest, then I have the book for you: The Rodale Book of Composting. The 1999 Yavapai County Master Gardener Class just bought me the new, revised paperback edition. This book contains all the information you need to start composting. It includes information about raw materials, bin designs, large scale systems, and much more.

The University of Arizona Cooperative Extension has publications and information on composting and cover crops. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at and be sure to include your address and phone number.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: March 15, 2001
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