Written by Simon Hodgson
What is the Carbon to Nitrogen Ratio?
The Carbon to Nitrogen ratio is a ratio of the mass of Carbon to the mass of Nitrogen in a substance; for example, a C:N ratio of 10:1 means that for every ten parts of Carbon there is one part of Nitrogen.
Why is it important?
The carbon to nitrogen ratio is important in the soil and in the organic matter which goes into the soil. The ratio is a determining factor in how easily organic matter is broken down by microorganisms and how effectively those microorganisms work in the soil.
A soil with a C:N ratio of 24:1 allows the microbes to work at high efficiency and under these optimum conditions, soil microbes can affect release of nutrients like N, phosphorous and zinc. This ratio also influences the amount of soil-protecting cover that remains on the soil. The 24:1 ratio strikes a balance between the two.
As a rule of thumb hard brown organic material has high proportions of carbon (C) and soft green material has relatively high proportions of nitrogen (N) The C:N ratios can differ markedly in organic materials. On one side, wheat straw has a C:N ratio of 80:1. On the other extreme is hairy vetch with a C:N ratio of 11:1.
Consider that the faster the microbes consume the residue the less time the soil is covered. This means that if one of your goals in cover cropping is to provide soil cover and protection against erosion the cover crop should not be managed when it is green and growing vegetatively, you should rather wait until the cover crop reaches the reproductive stage when harder carbon compounds start to form as the plant physiology changes to keep the flower and seed load off the ground.
This will give a higher C : N ratio in favour of the carbon and the organic residue will remain on the soil surface longer.
With legume cover crops the optimum time to incorporate or manage them is when the whole field has reached 10% to 30% flower. This will ensure maximum N availability to the following crop.
These same cover crop residues need to eventually decompose to release plant nutrients and build soil organic matter so timing of management of cover crops is all important.
We must provide our soil microbes with a balanced diet. Think about a microbe, probably a fungus, eating surface-covering wheat straw with its high C:N ratio of 80:1 – it will take a while to consume it and the predominant nutrient is Carbon. The wheat straw is lacking in the required amount of Nitrogen, so the microbes must get it from somewhere else. Here they turn to the soil to balance out the excess Carbon. This can lead to a “negative nitrogen period” where the microbes consume the luxury levels of N available to them from the soil. Essentially the microbes immobilise the soil N and this is only remedied when the microbes die and are recycled – i.e. are mineralised. This could take some time. The things that speed this up are temperature and moisture.
Microbes (mostly bacteria) rapidly devour a low C:N ratio crop like hairy vetch or woolly pod vetch (11:1 C:N ratio). This has little nutritional benefit to the microbes and it is digested relatively quickly. These digested (mineralised) products become dissolved in the soil water solution and are available to the plants for uptake. These nutrients are recycled rapidly. As the material is eaten quickly by the soil microbes there remains very little cover on the soil surface.
C:N ratios of crop residues and other organic materials:
Wood chips: 100 – 500:1
Stooling Rye Straw: 82:1
Wheat Straw: 80:1
Oat Straw: 70:1
Maize Stover: 57:1
Stooling Rye Cover Crop (Flowering): 37:1
Forage Pea Straw: 29:1
Stooling Rye Cover Crop (Vegetative): 26:1
Mature Lucerne Hay: 25:1
Ideal Microbial Diet: 24:1
Rotted Manure: 20:1
Legume Hay: 17:1
Beef Manure: 17:1
Young Lucerne Hay: 13:1
Hairy Vetch Cover Crop: 11:1