   Composting Organisms - October 31, 2001 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County With the arrival of fall, we start ripping out our summer gardens, raking leaves and cleaning up other garden debris. Once it's gathered up, I hope each of you has a compost pile to feed this waste into. Compost is the key to gardening in the southwest. Periodic additions of partially decomposed organic matter helps soil retain moisture, have ample aeration and permeability, and, most importantly, provide a steady supply of essential plant nutrients. We think we live in a brutal world, but our world is a walk in the park compared to living in a compost pile. Even if you happen to be at the top of the compost food chain, an insect for instance, a bird, skunk, or some other bug eating life form can gobble you up. So, lets dive in and learn who or what rules the compost heap. The smallest organisms in a compost pile are bacteria. Most bacteria are between 0.2 to 2 micrometers in diameter. To put this in perspective, it would take 25,000 bacteria lined up end to end to span one inch. They are simple, free-living cells that come in several shapes: rods, spheres, and spirals. Most are beneficial or harmless to humans. In fertile soil, they bacteria range from 7.5 to 750 billion cells per cubic foot. Here they form the foundation of the food web. Composting bacteria use organic compounds such as protein, carbohydrate, and lipids as their energy sources. They do this by exuding enzymes that break down the organic compounds into smaller pieces that can be absorbed through their membranes. In addition, many bacteria can survive without oxygen by conducting anaerobic respiration. However, this type of metabolism is much less efficient that aerobic respiration (with oxygen). Therefore, it is best to keep a compost pile well aerated. Actinomycetes are another small but important life form in soils and compost. They are a more highly developed form of bacteria sometimes called "thread bacteria". Actinomycetes are quite common in soil, especially in higher pH and drier soils such as those found in our area. In healthy soil, there can be from 1/4 to 2.6 ounces of actinomycetes per cubic foot. If this seems small, keep in mind this represents 3.5 million individuals per ounce of soil and comprise 5% of the soil's bacterial population. Actinomycetes are responsible for giving soil and compost that earthy smell. They are very important in the breakdown of organic matter to form humus and release plant available nitrogen. As an aside, they are also the source of antibiotics such as those found in Neosporin and other topical antibiotic creams. Protozoa are single-celled animals that may inhabit compost piles at some times, but are not key players in the big picture of composting microbes. This is because they feed much like bacteria but are much less competitive. They are much better suited to aquatic systems. Some of these may be familiar to you from the microscope viewing sessions of biology classes or Gary Larson's Far Side cartoons (Paramecium and Amoeba). We typically think of fungi as mushrooms, but mushrooms are only the visible reproductive structure of a relatively small set of fungi. Fungi are actually primitive plants that lack the ability to make their own food through photosynthesis. Like bacteria and actinomycetes, decomposer fungi exude enzymes that break down organic matter. The white button mushrooms (Agaricus bisporus) we buy at the store are grown on compost piles of horse manure and straw. Bacteria do most of the initial breakdown preparing to organic matter to be digested by the actinomycetes, protozoa, and fungi. All of these organisms are primarily responsible for chemical decomposition: breaking the organic matter down into their sub-microscopic constituent parts and ultimately making them into plant available nutrients. Keep in mind the decomposition organisms are living and dying in the system as well. The dead bodies of the microbes are food sources for other microbes. To maximize biological activity in a compost pile, proper temperature, aeration, and moisture conditions must be maintained. Hot compost will have different microbial populations than cool compost. Composting bacteria can tolerate up to 170 degrees F while fungi prefer 70-75 degrees F. Most compost piles start hot with bacterial activity then cool down as the fungi take over. Invertebrates such as insects, mites, centipedes, sowbugs, snails, slugs, nematodes, and worms can all be players in the compost game. Many of these feed directly on some part of the microbial population while other utilize organic matter broken into smaller pieces by microbial activity. All of invertebrates churn and turn the compost as they seek resources. They also further digest the organic matter and leave behind waste that plants will use as fertilizer. All of the organisms discussed above work together: living and dying for our compost. It is a noble effort and a thankless job. As you make your compost pile this fall, take a moment and reflect on the complexity of the mini-ecosystem. If you are thinking about starting a compost pile, some composting basics can be found in the June 2, 1999 (Gardening Composting), and June 9, 1999 (More On Composting) Backyard Gardener columns (available on the web site shown below). Worm composting (Vermiculture) is also covered in the August 25, 1999 Backyard Gardener column (also available on the web site). The University of Arizona Cooperative Extension has publications and information on gardening. Note: the Cottonwood Master Gardeners have a new E-mail address: mgardener@verdeonline.com. 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/. |