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How Organic Matter Turns from a Nitrogen Source to a Carbon Source

Written by Angus Stewart

When it comes to composting we often hear about carbon and nitrogen and the importance of getting the balance between these two elements right. Let’s look at how these two elements interact with each other during the composting process and where they end up.

We are all familiar with the beautiful, soft succulent new growth that plants produce as they establish themselves, sometimes referred to as the ‘greens’ that we need for successful composting. Over time most plants start to produce firmer, often woody growth that enables them to grow taller to reach for the light that enables them to become shrubs or trees. This strengthening tissue in plants is made up largely of the carbon-rich substance called cellulose, sometimes known as ‘browns’ . In other parts of the plant the woody tissue provides support for softer tissues that are much higher in nitrogen-rich proteins.

Looking at some of the organic materials that might go into the average home compost heap, we can predict to a large extent whether it is richer in carbon or nitrogen. Fibrous or woody materials such as woodchips, straw, twigs, paper, and cardboard are going to be almost all carbon.

On the other hand, organic materials such as flowers, fruit and vegetable scraps, grass clippings and soft leaves contain a much higher proportion of nitrogen than carbon and as such they provide a key ingredient that compost microbes need to thrive and break down whatever materials you are using to make your compost. It should also be said that these materials do still contain carbon, but the relatively large amount of nitrogen is what makes them so useful to us in constructing a successful compost heap.

If we use too much of the carbon-rich materials, we end up with a compost heap that tends to just sit there and sulk because the compost microbes do not have enough nitrogen to break down the materials in the heap.  Alternating layers of ‘greens’ and ‘browns’ enable us to create the right balance of carbon and nitrogen to allow our trusty compost microbes and worms to do their magic.

As the creatures of the compost such as earthworms, fungi and bacteria get established, they chop up the raw materials that have gone into the compost heap into smaller and smaller pieces until they are unrecognisable in their original form. This process results in the carbon and nitrogen being released from their original substances and being reformed into new substances. In the case of carbon, it can end up in a variety of end products. We need to aerate our compost heap throughout the process as this results in a high proportion of the carbon being used to create a carbon-rich substance called humus, which is the ‘black gold’ of that we can add back into our soils to not only boost its water and nutrient holding capacity, but also incorporate carbon back into the soil rather than into the atmosphere. It should be said that a proportion of the carbon in a healthy compost heap will end up as carbon dioxide but that is much better than what happens when we bury organic materials in rubbish dump landfills. When such materials are buried a different group of composting microbes break it down to release far more potent greenhouse gases such as methane (carbon with hydrogen)

Nitrogen in a healthy compost heap breaks down in the presence of oxygen that is added through regularly aerating the heap, and forms substances called nitrates (nitrogen atoms combined with oxygen atoms). Nitrates represent the way in which nitrogen is recycled in the soi ecosystem to become available for the next crop in your vegetable garden, or to help stimulate new growth in your ornamental garden plants. On the other hand, if we do not aerate our compost heap enough and there is not enough oxygen, the nitrogen atoms instead combine with hydrogen to form ammonia, a gas that we can sometimes smell leaking from a poorly aerated compost heap. The problem there is that we are losing the nitrogen from the finished compost because it is being emitted as a gas rather that becoming water soluble nitrates that plants can take up from the soil.

Successful composting is all about balance, and as a general rule a ratio of carbon-rich ‘browns’ to nitrogen-rich ‘greens’ of 2 to 1 will give a good result.

 Happy composting!

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