Compost - Is it Done Yet?
One of the most common questions asked about composting is “How long does it take?” This is usually followed with “How do you know when it is ready to use?” As is the case for many important questions, there are no simple answers.
How long does it take? Mature compost can be made in a minimum of twelve weeks of warm weather if the needs of the microbes carrying out the compost process are met. The microbes need a balanced diet that provides about 30 times more carbon than nitrogen at the start of the process. Dry materials will not break down because the microbes need moisture to function. The compost process works best when the mixture has a moisture content of 50% – a state that is often compared to a moist sponge. Oxygen is the other important ingredient. Oxygen-using (aerobic) microbes break down materials quickly and release extra energy as heat. Compost piles over a cubic meter in size can reach temperatures (55°C or higher) that kill disease-causing organisms and seeds. Using a mix of different material types and different particle shapes helps build air spaces or pores. Turning remixes piles and rebuilds pore spaces.
Why is maturity important? If the compost has not broken down completely, or if the breakdown process was largely anaerobic (no oxygen available), the compost will contain acids and alcohols that are toxic to plants. Compost maturity is an important issue if compost is used in potting soil mixes or applied to soil at planting time.
How can you tell if compost is mature? There is no single criterion that is used to judge compost maturity. The simpler ones are used by home composters; commercial operations also use lab tests. Here are some common ways that compost maturity is judged:
Appearance : It should be dark and crumbly and most of the original ingredients should be unidentifiable.
Odour : The compost should smell ‘earthy’; there should be no sour smells or ammonia or sulphur-like odours coming from finished compost.
Temperature : The heating phase should be complete; the compost pile should be the same temperature it is outdoors. It is best if it can ‘cure’ at the outdoor temperature (above freezing) for several weeks to allow the lower temperature organisms to complete the job.
Germination test : A simple test can be done comparing germination rates of a given seed type in both compost and potting soil. Garden cress (Lepidium sativum) is considered the most sensitive plant to detect problems in compost maturity. Germination rates in the compost should be at least 90% of those in the soil. Relative plant size and health is also observed.
Solvita Test Kits : Woods End Lab ( Mt. Vernon, Maine) has developed a relatively inexpensive test kit for compost maturity. The kit measures carbon dioxide and ammonia levels being released into the air from compost samples. See www.woodsend.org for more details.
Standard Lab tests : Laboratory tests are an important investment for commercial composters. Tests for compost maturity (CO2 levels, carbon/nitrogen ratios) are part of a larger array of tests for compost quality. These can include tests for nutrient analysis, salt content, pH, disease-causing bacteria and heavy metals. The Compost Council of Canada is launching a Compost Quality Alliance program to promote standardized laboratory testing methods for compost and to boost consumer confidence in the compost they purchase.
Microbial testing : There is increased interest in the microbial population in mature compost. Compost and brewed aerobic compost teas can be used to bring soil microbe populations back into balance. A lab which specializes in this type of testing has opened in Vulcan Alberta. It is a branch lab of the Soil Foodweb Lab based in Corvalis Oregon. Visit www.soilfoodweb.ca or call (403) 485-6981 .
(Source: June 2005 WasteWatch)
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