Organic matter stability — why is it so important?

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Organic matter stability is one of the basic markers of a quality compost product, stability being one of the most essential delivery mechanisms of compost’s many benefits.

Biodegradation is nature’s way of recycling all plants and animals, breaking down organic matter (OM) into complex molecular structures, then simple compounds and, finally, chemical elements – the basic building blocks of life on this planet.  Composting is a manufacturing process based on the application and control of this natural recycling system.

Complete biodegradation takes a long, long time.  But between the death of the organism and its eventual disorganization into atomic parts exists a phase where the degradation process becomes resistant or stabilizes. While it can take centuries to reach this stage naturally,  composting can achieve OM stabilization in a matter of days or weeks, depending on the  process employed.

Using a stable compost adds stable organic matter to the soil.  Stable organic matter can absorb six times its weight in water (some authors suggest much higher OM absorption numbers) and  hold more nutrients.  While organic matter in the form of living microbial biomass and detritus contribute to soil fertility, it is the stable fraction – humus – which is responsible for the soil’s structure, tilth and cation exchange capacity (nutrient-holding capabilities).

Stable organic matter gives compost many of its unique properties.  Compared to compost, the organic matter content in fresh manure is of low stability.  As for peat moss, apart from the negative environmental impacts associated with its use, the product is generally regarded as too expensive for large-scale commercial and agricultural projects, and one study even suggests it may reduce overall soil stability.

“Stability” isn’t necessarily something one can see, but there are tests which can determine if organic matter (like compost) has reached a stable state.  Stability is a standard testing parameter for manufacturers participating in the U.S. Composting Council’s Seal of Testing Assurance (STA) program, and as participants in the program, McGill’s U.S. facilities test every month. The STA is a compost testing, labeling and information disclosure program designed to give you the information you need to get the maximum benefit from the use of compost. The program was created in 2000 and the tests were compiled by the leading compost research scientists in the United States. When you buy from a producer with the STA seal you know the product has passed rigorous testing and that you can rest assured that you’re buying from amongst the best products on the market.

Set a minimum target of five percent organic matter for improved soil performance and ask for the STA.  When it comes to economy, ease of use and efficacy, a stable compost is the better amendment choice. More information on the STA and what the program entails can be found on the U.S. Composting Council‘s webpage.

SOURCES:  Cornell Cooperative Extension’s Agronomy Fact Sheet #41, University of Minnesota Extension, Organic-matter effects on soil strength properties