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Carbon drives the compost process.
A proper carbon-to-nitrogen (C:N) ratio, such as straw-to-manure, is key to a proper composting process, explains Virginia Nelson, a composting specialist with the AgTech Centre in Lethbridge.
The ideal is a 25:1 C:N ratio and not more than 30:1. Depending on the manure, it may mean adding more carbon to the mix to achieve the proper ratio. Also, the composting materials initially should be at about 55 percent moisture.
"The combination of materials, moisture and oxygen should produce an ideal composting environment with an internal composting windrow temperature above 50 C," says Nelson. "These are optimum conditions for microbial activity."
While straight manure will compost, the carbon component improves the process, says Nelson.
The C:N ratio is important because if the C:N is lower than 20:1 all available carbon is fully utilized without stabilizing all the nitrogen. The excess nitrogen might then be lost to the atmosphere as ammonia or nitrous oxide, and odour can then become an issue. Composting mixes with higher than 40:1 C:N ratios require longer composting times for the micro-organisms to use the excess carbon.
In the composting process, micro-organisms decompose organic materials progressively, breaking them down from complex to intermediate to simple compounds to obtain nitrogen for new cell material. Some of the nitrogen is converted to ammonia. If the nitrogen becomes available faster than it is used, ammonia accumulates. Eventually this lighter-than-air gas can escape the windrow.
A similar nitrogen loss can also occur in stockpiled manure through denitrification - a process where certain types of bacteria reduce nitrates into nitrite, nitrous oxide, ammonia or elemental nitrogen. Denitrification is responsible for the loss of much of the soil's natural and synthetic fertilizers. The process favours warm, anaerobic conditions found in a manure pile.
"In simple terms, when microbes first become active in compost they digest the easy parts of the nitrogen and let the ammonia pass through them," explains Nelson. "The ammonia will typically bind to the carbon and be held in place. After the microbes complete the first pass, they go back and start working on the more difficult compounds such as the combined carbon and ammonia compounds.
Even with properly composted material there will be some nitrogen loss as some ammonia escapes from the composting pile. But over all, the composting process retains most of the nutrients supplied by the raw materials and stores them within stable organic compounds.
"If you don’t have the carbon source to act as a binding agent, much more ammonia is simply lost to the atmosphere," says Nelson. "The composting process is over sooner because the microbes have no food left."
The best way to retain ammonia is to match the rate of nitrogen availability to its rate of uptake by the micro-organisms. The micro-organisms use the nitrogen in proportion to the amount of carbon available. Therefore, high C:N ratios tend to limit ammonia loss.
In properly composted manure, the remaining nitrogen is left in a more organic form, making it much less susceptible to leaching. It is released at a more gradual rate and is more readily available to plants when applied and mixed with the soil. |