You can't have compost without microorganisms. Like most living things, microbes that help make compost require favorable temperatures, moisture, oxygen and nutrients. Without them, your pile of leaves and plant debris is going to remain just that -- leaves and debris.
Plant-digesting microbes operate in a temperature range of 70 to 140 degrees F, but breakdown occurs very slowly at the lower temperatures. Well-managed compost rapidly breaks down as summer temperatures quickly reach 120 degrees to 130 degrees F. If summer heat plus the heat produced by active microorganisms causes the temperature of the plant mass to exceed 160 degrees F, the microbes will die. Conversely, Colorado's winter temperatures cool compost and greatly extend the time required to produce a finished product.
Thermometer inserted into composting material
Moisture and Oxygen
Moisture and oxygen are essential to microbial activity. In a region of limited rainfall, such as in Colorado, you need to add moisture regularly to maintain the composting process. If parts of the composting material dry out, many microorganisms in the dry areas die. Microbes that remain require time to multiply and resume plant digestion even when moisture is added.
The net result is slower composting. Excess moisture, however, displaces air and also slows breakdown. Too much water creates low oxygen conditions in which certain microbes multiply and produce foul odors. The proper moisture level for good composting can best described as "moist" or "damp" but not soggy. The entire mass of plant wastes should be moistened uniformly to the point where only a few drops of water can be squeezed from a fistful of plant material.
The size of plant particles that go into the compost affects aeration. Large particles allow a lot of air to circulate around the plant chunks, but breakdown is slow because microbes can act only on the outside, not on the inside of the large chunks. Particles chopped into smaller chunks increase the surface area for microbes to operate, but particles chopped too small will compact and restrict air flow. Moderate-sized plant pieces of 1/2 to 1 1/2 inches are the best size to use and can be produced by hand or machine shredding.
Woody materials need to be chopped into a smaller size. Leave soft plant parts in larger pieces for effective composting. Fluff or turn the material with a pitchfork or aerator tool at regular intervals.
Composting yard waste
In general, avoid plants treated with weed killers. Small amounts of herbicide-treated plants may be mixed in the pile as long as you are careful to allow for thorough decomposition. Weed killers and other pesticides break down at various rates. If you use treated grass clippings, the breakdown of these chemicals should be at least as fast as breakdown in the soil. Plants killed with weed killers that are soil inactive (glyphosate products such as Roundup or Kleenup) should present no problem when composted in small quantities.
If the compost is properly mixed and maintained, you should be able to obtain a final product in one to two months under optimum summer conditions. In other seasons, or with less maintenance, composting may require six months to one year. When complete, the mass will be about half its original size and will give off an earthy smell.
Photographs courtesy of Judy Sedbrook.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010