How to increase soil organic matter: a guide for UK arable farmers
Taking active steps to protect and improve soil organic matter (SOM) plays a vital role in maintaining soil fertility, structure, and water retention on farmland. According to AHDB findings, increasing SOM by just 1% can improve the water-holding capacity of soils by up to 16,000 gallons per acre, highlighting its crucial role in supporting crop performance in prolonged dry periods and overall soil productivity for arable land1.
This blog will delve into practical strategies for increasing SOM, explaining the role of fertilisers for farmers striving to build the organic content of arable soils.
What does soil organic matter contain?
Soil organic matter (SOM) comprises plant and animal residues at various stages of decomposition including cells and tissues of soil organisms and substances synthesised by soil organisms.
Fresh plant residues, such as crop stubble, roots, and leaves, provide immediate energy for soil microbes.
As these residues decompose, they transition into humus, a dark, rich substance that serves as a reservoir for essential nutrients such as nitrogen, and helps stabilise soil structure.
Stable organic matter, including biochar and long-lasting humic substances, remains in the soil for extended periods, continuing to improve soil properties and support microbial life.
How does organic matter benefit arable soils?
Increasing soil organic matter across arable land can lead to numerous benefits, including improved soil structure, increased water infiltration and retention, enhanced nutrient availability, and greater biological activity.
Here is a more detailed explanation of the key functions of SOM.
1. Soil structure
The presence of humus in SOM enhances soil structure by promoting the formation of soil aggregates.
Aggregation improves soil porosity and aeration, which are crucial for root penetration and reducing soil compaction.
This is particularly beneficial for heavy clay soils, as it helps to prevent waterlogging and facilitates better root development for crops.
Read expert advice on managing crops in waterlogged fields.
2. Water infiltration and retention
High SOM content increases the soil’s capacity to absorb and retain water
This is due to the sponge-like nature of organic matter, which can hold several times its weight in water.
Improved water retention reduces soil erosion and runoff, maintaining soil moisture during dry periods, and also helping to protect water quality of any surrounding water courses.
For instance, soils with higher organic matter are better equipped to support crops like wheat and barley during prolonged dry spells by retaining rainfall better within the soil, ensuring consistent growth and yield.
Learn more about maximising crop nutrition to help improve resilience during dry growing conditions.
3. Biological activity
SOM supports a diverse range of soil biota, including bacteria, fungi, and earthworms, which are essential for nutrient cycling and organic matter decomposition.
For example, mycorrhizal fungi form symbiotic relationships with plant roots, extending the root system and improving nutrient uptake.
Earthworms, another key component of soil biota, enhance soil structure and fertility by breaking down organic matter and creating channels that improve soil aeration and water infiltration.
These biological processes are vital for maintaining soil health and fertility, supporting the productivity of all arable crops.
4. Carbon sequestration
Soils with high organic content act as carbon sinks, reducing atmospheric CO2 levels.
It is widely understood that organic matter in the soil is composed of approximately 58% carbon, making it an effective medium for long-term carbon storage.
For arable farmers, practices such as incorporating cover crops, applying compost and manure, incorporating green manures and reducing tillage can enhance carbon sequestration as well as SOM content.
Incorporating cover crops, like clover or vetch, not only adds biomass to the soil but also captures carbon through photosynthesis which is then stored in the soil as organic matter.
Read more on cover cropping options available through the SFI 2024 offering.
How to increase soil organic matter following harvest
Autumn is an optimal time for incorporating soil organic matter on cereal stubble fields following harvest.
The average soil temperatures and rainfall levels in autumn support microbial activity, aiding the decomposition of organic matter, ready to benefit winter crops like wheat and barley, as well as ensuring a gradual release of nutrients for spring crops.
Farmers can adopt several strategies to increase soil organic matter following harvest, including:
- Cover crops: Planting cover crops such as clover, vetch, and radishes immediately after harvest provide ground cover, reduce soil erosion, and add organic material to the soil. They also improve soil structure and nutrient cycling.
- Incorporating of crop residues: Instead of removing stubble and other crop residues, farmers could incorporate these materials back into the soil. This can be done through shallow tillage or by using mulching techniques, which help in retaining moisture and adding organic content as the residues decompose.
- Applying organic matter: Adding compost or well-rotted manure to stubble fields enhances organic matter levels. Compost provides a balanced mix of nutrients and improves soil microbial activity. Manure adds nutrients and organic material but should be well-composted to avoid nitrogen loss.
- Reduced tillage: Minimising soil disturbance through practices such as no-till or minimum tillage preserve soil structure and promote the buildup of organic matter. This approach also supports soil microbial life and reduces erosion.
- Green manures: Sowing green manure crops like legumes can help with adding substantial organic material to the soil. Ploughing these crops under, before they flower, incorporates the biomass stored by these plants back into the soil, increasing SOM and releasing readily available nutrients.
The role of fertilisers in increasing organic matter
While organic practices are crucial, fertilisers also play a significant role in enhancing SOM by promoting plant growth, which in turn helps incorporate organic material into the soil.
Here is how specific nutrients contribute to building SOM:
Nitrogen (N)
Nitrogen is essential for vegetative growth, enhancing the biomass that ultimately contributes to SOM. However, autumn application must be carefully managed to prevent environmental impact.
Products like Dynamon combine nitrogen with sulphur, improving nitrogen use efficiency and helping to reduce runoff. To avoid leaching, apply nitrogen in minimal amounts during autumn, reserving the majority for the spring growth phase.
Phosphorus (P)
Phosphorus is critical for root development and energy transfer, supporting early growth stages and robust root systems that contribute to SOM.
Autumn phosphorous application ensures that young plants have sufficient nutrients for root establishment and early growth. This can lead to stronger plants that add more organic material to the soil.
Potassium (K)
Potassium enhances disease resistance and drought tolerance and plays a key role in grain filling and quality.
Autumn potassium application strengthens plant tissues and improves overall plant health, leading to increased organic matter from healthier crop residues.
Read further advice on ensuring optimal crop nutrition during autumn.
Considerations and benefits of adding organic matter
According to AHDB, adding organic materials such as compost, farmyard manure, and green manures can improve soil structure, enhance soil biology, and increase nutrient availability.
For example, compost can significantly improve soil structure and nutrient content, while farmyard manure can stimulate biological activity and microbial biomass.
Improvements in SOM can take several years to manifest, but consistent management can lead to better drainage, more resilient soils, and higher crop yields, especially in spring crops during extreme weather conditions.
References
- AHDB (2024) – Adding organic matter to soil