The carbon to nitrogen ratio (C/N) of a soil or substrate is a measure that indicates the ratio between the amount of organic carbon and the amount of nitrogen present in the soil. It is a relevant parameter for assessing the quality and availability of the main nutrient in the soil.
Carbon and nitrogen are two essential elements for the growth and development of plants and micro-organisms in the soil. Carbon is found in soil organic materials such as decomposed organic matter, crop residues and decaying roots. Nitrogen is a key component of the nutrients needed for the synthesis of proteins and other compounds essential for plant growth.
A low C/N ratio means that more nitrogen is available in relation to the organic carbon in the soil. This may suggest two different scenarios: in the first one the soil can provide sufficient nitrogen to plants and micro-organisms, which favours good plant growth and efficient decomposition of organic matter. A soil with a low C/N ratio is generally more fertile and prone to higher biological activity. Typically these soils will have C/N ratios of 10:1 to 20:1 and a high percentage of organic matter, above 5%. The second scenario would indicate a soil with symptoms of depletion, where the organic matter content is very low and may be derived from high temperature conditions, which cause an acceleration in the decomposition processes of organic matter or from intensively exploited or eroded soils.
On the other hand, a high C/N ratio indicates that there is a higher amount of carbon compared to nitrogen in the soil. This may occur in highly degraded soils, soils exposed to extreme environmental conditions or soils with a high accumulation of organic matter but limited nitrogen availability. In such cases, decomposition of organic matter may be slower, as micro-organisms require nitrogen to efficiently decompose organic matter. A high C/N ratio is considered up to 40:1 - 50:1.
A balanced C/N ratio is desirable for a healthy and fertile soil, as it indicates an adequate amount of nitrogen available to plants and micro-organisms, which promotes efficient nutrient cycling. Most authors consider an optimal ratio for crop development to be around 12:1 - 25:1. In the case of organic substrates of plant origin, the carbon:nitrogen ratio can be very different, depending on the origin of the material. In the case of conventional cocopeat fiber, the C/N ratio is usually around 30:1 - 45:1 or even, depending on the origin and the treatment to which the cocopeat fiber has been subjected, can be higher than 100:1. Hence the need for fertigation systems that constantly supply nitrogen to the plants in substrate production systems.
To improve the C/N ratio in soils or substrates, the main objective is to increase the availability of nitrogen compared to carbon. Here are some ways to achieve this:
- Adding nitrogen sources:Incorporates nitrogen sources, such as manure or high nitrogen compost in the case of soil cultivation, or nitrogen fertilisers, to increase the proportion of nitrogen in the soil or substrate.
- Fertiliser application:Use fertilisers that provide a balanced supply of nitrogen. These fertilisers will provide a constant supply of nutrients and avoid excessive increases in carbon content.
- Using legumes as cover crops:Legumes, such as clover or alfalfa, are able to fix atmospheric nitrogen and enrich the soil with nitrogen available for other crops. Rotating or intercropping legumes as cover crops can improve the C/N ratio of the soil.
- Avoid excessive accumulation of high C/N ratio organic matter:Avoid adding large quantities of high C/N ratio materials, such as cereal residues or plant waste materials with high lignin content (sawdust, straw...), without balancing them with nitrogen sources. Decomposition of these materials can temporarily deplete plant-available nitrogen.
- Controlling the use of agricultural residues with high C/N ratio:If you use agricultural residues as amendments, be sure to handle them properly to avoid imbalances in the C/N ratio.
- Apply mycorrhizae:Mycorrhizae are beneficial fungi that form symbiotic associations with plant roots and can increase the efficiency of nutrient uptake, including nitrogen.
- Managing irrigation:Keep watering adequate and avoid over-watering, as over-watering can encourage slow decomposition of organic matter and increase the C/N ratio.
It is important to perform regular soil or substrate analyses to monitor the C/N ratio and take measures to correct it if necessary. Maintaining a balanced C/N ratio will support nutrient availability and promote optimal plant health and sustainability of the soil or substrate ecosystem.
One way to improve soil fertility, in the case of depleted soils with low organic matter content and low C/N ratio, is to use mixed production systems using cocopeat fiber substrate. These systems combine the benefits of hydroponics and soil production, allowing the medium to increase its organic matter content over time and to produce more efficiently.