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Much like humans, microbes require optimal quality food for them to function at their best. Without good nutrition, your body is prone to disease, infection, fatigue, and poor performance. The same is true for soil microbes.

In a series of blogs (link below), I established the role played by each organism in the soil. For these roles to be optimally fulfilled the soil must offer nutritious and balanced food. This is where the carbon-to-nitrogen (CN) ratio comes in. It gives a measure of the quality and quantity of the food provided to soil microbes.

The soil food web

What is carbon-to-nitrogen ratio?

Well, it’s pretty much self-explanatory. The CN ratio refers to the relative amounts of carbon to nitrogen in the soil. For instance, a ratio of 10:1 means that there are 10 units of carbon for each unit of nitrogen in the soil. This ratio is important because, without enough carbon in the soil to balance the nitrogen, soil cannot support beneficial microbial activity. A CN ratio of between 20:1 and 30:1 is recommended; with 24:1 being optimal for peak soil microbial processes.

CN ratio of microbes

Microorganism Bodily CN ratio
Bacteria 5:1
Fungi Yeast 10:1 and Mould 20:1
Protozoa 30:1
Nematodes 100:1

The above mentioned microbial CN ratios give an indication of the quality and quantity of food required by each organism in order to meet their bodily requirements. Due to the varied microbial diets, a CN ratio of 24 is able to satisfy these requirements for the diverse food web.

What happens when the CN ratio is less than 20?

A low CN ratio results in rapid depletion of carbon (the food). As a result, soils with lower CN ratios usually have bacteria dominating as it has a lower C requirement. All other organisms, such as fungi and protists, become limited. The disadvantage of losing this carbon quickly is not only to microbes but also to the soil and plants. Remember, carbon not only feeds the microbes, but it also helps hold nutrients and water which are later used by the plant. The other downside to low CN ratios means that there will be excessive N being made available in the soil. The plant won’t be able to use all of this resulting in most of the mineralised N being lost through leaching and volatilisation.

What happens when the CN ratio is more than 30?

This is also a scenario you don’t want in your soil. A high CN ratio means that there is too much carbon and not enough nitrogen for the microbes to consume along with the carbon. In order to overcome this, microbes start to feed on the available (inorganic) nitrogen sources to satisfy their bodily requirement. This is called nitrogen immobilisation. The disadvantage in the process is that, instead of releasing nitrogen; the microbes will consume plant-available nitrogen.

Keep a balance

Earlier, I mentioned that microbes’ requirements for food aren’t different from our own. Our balanced diet includes dairy, fruits, vegetables, grains, and meats.  The soil’s equivalent of a balanced diet is multispecies pastures. For example, a fully matured oats crop can have a ratio of up 70:1. If it’s blended with a crop such as lucerne or vetch that has a ratio of 11:1, it can pull the below ground ratio closer to 24.

The ultimate goal is a healthy soil, with a fully functioning soil food web. One of the important steps in achieving this is ensuring the correct diet for the microbes which make up the full food web.

Portia Phohlo