   Building Blocks of Compost - November 7, 2001 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County I made my first compost pile when I was in high school. I raked up leaves from under our big silver maple tree, piled them up, wetted them down, then watched and waited. It didn't get hot and took a long time to decompose. What was missing from my pile? Read on and you will see. Too much carbon in your raw materials, and you will see very little activity: the microbes are not able to reproduce. Too much nitrogen, and ammonia is formed. Ammonia is a gas, which is quickly lost to the atmosphere. Capturing nitrogen is one of the primary goals of composting. After all, it is usually the most limiting nutrient for plant growth in native soils worldwide. Sending perfectly good nitrogen to the atmosphere is not just a waste: it smells bad too. Reviewing the material covered in last week's column, bacteria, actinomycetes, and fungi are the primary microbial workers in compost. Keeping them happy, productive, and reproductive, is critical. Of the carbon present in the raw, uncomposted material, about 2/3 of it will be burned away as it supplies energy to the microbes. This fraction is lost to the atmosphere as carbon dioxide. The remaining 1/3 is incorporated into their cellular structure: amino acids, nucleic acids, lipids, and polysaccharides. For every pound of usable carbon, approximately 5 1/3 ounces of it goes directly into microbial cells. For successful compost, the ratio of carbon to nitrogen (C:N) present in the raw, uncomposted materials is the most critical factor. Bear with me while I do some more math. If analyzed, the average compost microbe cell would have a C:N ratio of between 5:1 and 8:1. Assuming that 2/3 of the carbon is used for energy, this would indicate that our raw materials should have a C:N ratio between 15:1 and 24:1. In fact, to conserve nitrogen, it is best to supply a little more carbon. Most compost publications tell us to strive for C:N ratio in the raw materials of 30:1. This is often easier said than done, but luckily, it's not difficult to compensate along the way. Time to get into the nuts and bolts of compost materials (hopefully, I did not lose everyone with the numbers and math). Think of your raw materials as being in one of two categories: green or brown. Green materials are relatively high in nitrogen and include grass clippings, weeds, pruned vegetation, kitchen waste, coffee grounds, moldy alfalfa or grass hay, and manures. Brown materials include dried plant leaves or small stems, straw, pine needles, newspaper, eggshells, wood chips, sawdust, etc. Brown materials need to be chopped up to increase the surface area for microbes to utilize. Shredders are convenient for this. I always empty the vacuum cleaner and the hair we brush out of our dog into the compost too. Mixing the green and brown materials together is critical. Homogeneous mixtures will activate more readily. I also sprinkle a few shovels of soil into the compost as I build the pile. This inoculates the pile with an assortment of native soil microbes. Moisture content is very critical to successful compost. It should feel about like a well wrung-out sponge (about 40-60% moisture). If you can squeeze moisture out of it, then it is too moist. Heat generated can tend to dry compost, so monitor moisture and adjust as it progresses. Compost bins that are very well aired may dry out very rapidly. I am currently composting in two-foot high, solid sided bins made out of old galvanized steel roofing. Solid wood is also good. Some compost enthusiasts use a thermometer to monitor microbial activity. I just poke a pitchfork it and feel it with my hand. If it's hot, then you have microbial activity. Some composters go for maximum heat to kill weed seeds and plant diseases. If it's not warm, after a week or so, then you probably have an inadequate nitrogen or water supply. To remedy this, add more green material or water and remix. Manure is an ideal material for increasing nitrogen content. Conversely, if it makes your eyes water and smells like ammonia, it has an overabundance of nitrogen. Add some brown materials to bring it back into balance. If it is muck, it needs some coarse materials to help with aeration. Finished compost should have a C:N ratio of about 15:1. Most of the nitrogen and much of the carbon is contained in the "bodies" of the microbial population. The carbon that is not in the microbial cells is in a form that cannot be easily metabolized. As the microbe population dies, the nitrogen they contained becomes slow release fertilizer in the soil. Over time, slow microbial decomposition of the remaining carbon compounds drive the C:N ratio toward 10:1, which is the C:N ratio of humus. Humus is quickly lost in our alkaline soils, so seasonal additions of compost will improve soil dramatically. By the way, there are some materials that should not be composted by home gardeners. Do not compost meat, fat, plastics, cat litter, dog waste, or human waste. Some of these materials are composted on a commercial scale. Furthermore, well-managed compost piles do not stink. They should smell earthy and fecund. At least, that is my opinion. The University of Arizona Cooperative Extension has publications and information on gardening and composting. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 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://ag.arizona.edu/yavapai/anr/hort/byg/. |