Soil – our carbon store
Soil has a great capacity to store carbon and as farmers we have the ability to divert CO2 back in to our soil through plants. Plants absorb CO2 from the atmosphere during photosynthesis which is converted to sugars building plant biomass above and below ground, returning carbon to the soil as they decompose. Simply by growing our crops in a carefully managed sustainable system we can have a positive influence on climate change mitigation. Since we started tilling the earth and burning fossil fuels, we have upset natures carbon balance by exposing and oxidising carbon which was previously locked away underground. We all know that combustion engines and other types of historic carbon burning activities have contributed to our warming planet. But now our fields are where the battle lines are currently being drawn in an effort to restore some balance to the carbon cycle. Soil respiration releases CO2 naturally however intensive soil cultivation liberates vast amounts of carbon from top soil. Field studies demonstrated ploughing to a depth of 280mm could release as much as 162g / CO2 / m2 while only 11g / CO2 / m2 was released from the uncultivated surface during the first 24 hours following soil disturbance, the volume of CO2 released increases exponentially with depth of cultivation.
The soil carbon cycle
Reducing tillage
Reducing our reliance on excessive soil cultivations and maximising the time we maintain actively growing crops on the land will help restore the balance of carbon in the system. A healthy soil ecosystem relies upon complex essential symbiotic relationships between plants, fungi and soil microbes which consume carbon rich root exudates and in turn provide plant essential nutrients in an available form. Five grams of healthy soil contains as many microbes as the total human population and to feed that microbial population we need a constant supply of plant material. Soil microbes are also an essential part of soil humification the process by which plant and animal material containing carbon is converted to stable organic compounds which enhance many soil qualities.
Soil health
Good soil health is fundamental to the sustainability of our agricultural systems. Healthy soils will grow healthy crops, the definition of a healthy soil is “a continued capacity to function as a vital living ecosystem that sustains plants, animals, and humans”. But building and maintaining soil health is a long-term investment, soil needs the correct balance of chemistry, diverse biology, and a continual source of organic material to feed the biology and in turn the soil biology will provide the elements plants require for growth and create and build a well-structured soil. No single piece of high tech expertly designed cultivation machinery can repair soil structure, we only disguise our problems and destroy natures work in the process, plant roots and soil biology create natural soil structure. In order to achieve well-structured soils, we should endeavour to keep soils alive by retaining growing crops and living roots in our fields for the maximum time possible in our rotations.
The healthy functioning soil food web
Cover crops
The potential to use cover crops between annual cash crops as a tool to sequester carbon from the atmosphere is a growing debate. There are often periods between cash crops where arable soils are left bare and if managed correctly may provide the potential to successfully produce carbon capturing soil restoring crops. Many countries are offering incentives to farmers to adopt cropping and management practices which aid carbon capture, U.K. farmers can currently source funding via the countryside stewardship scheme. Cover crops which produce large volumes of high C:N residues will obviously sequester more carbon however a balance must be struck when we consider other reasons why we may grow cover crops such as pest, weed or disease suppression, nutrient capture or environmental protection. We have to maximise the amount of carbon added to the soil system without detriment to the following cash crop through either nitrogen immobilisation or allelopathic activity and without adding excess nitrogen which could become a potential pollutant.
Diverse species mixes can accumulate ~20% greater carbon than single species cover crops
Carbon: Nitrogen
Residues with a C:N ratio of around 24:1 have the ideal balance which complement the soil environment, soil microorganisms have a C:N ratio of around 8:1 and as they assist in the decomposition and recycling of residues they consume a proportion of the carbon as an energy source this is typically around 16 parts. Therefore, residues with a carbon content greater than 24:1 will create a temporary nitrogen immobilization whereas residues with a lower carbon content will result in a net nitrogen excess. Generally speaking continuous cereal rotations creating high C:N residues will benefit from cover crops with lower C:N ratios being incorporated in the rotation, whereas intensive vegetable / horticultural situations where there are likely to be crop residues which rapidly decompose and mineralize excess nitrogen will benefit from the inclusion of cereal type cover crops where microbes can utilise this free nitrogen as they process the CC high carbon residue. The longer we can keep cover crops in situ enables accumulation of increased biomass and where possible by sowing the following cash crop directly into the cover allows residues to decompose on the soil surface slowly and more akin to the natural processes which reduces the oxidation of organic material retaining more carbon in the soil.
Retaining surface crop residues allows slower decomposition and increases carbon capture
Diversity rather than mono-cultures
Diverse cover crop species mixes can accumulate ~20% greater carbon than single species cover crops mainly due to an increased biomass influenced by the varied plant structure of the growing crop. Different crop species will support more diverse soil micro-organism communities owing to their alternative root exudates and contrasting root architecture which will also have a positive effect on soil structure formation. Diversity of crop species also provides enhanced habitats supporting beneficial insects and pollinators.
| Crop species | Carbon: Nitrogen ratio |
| Rye straw | 82:1 |
| Wheat straw | 80:1 |
| Oat straw | 70:1 |
| Rye cover crop (Anthesis) | 37:1 |
| Daikon radish cover crop | 35:1 |
| Oilseed radish cover crop | 30:1 |
| White mustard cover crop | 30:1 |
| Pea straw | 29:1 |
| Rye cover crop (Vegetative) | 26:1 |
| Black oat cover crop (Vegetative) | 25:1 |
| Rotted FYM | 20:1 |
| Phacelia cover crop | 20:1 |
| Berseem clover cover crop | 14:1 |
| Hairy vetch cover crop | 11:1 |
Average C:N ratios of some common arable crops and cover crop species