Cover crops build soil organic matter and affect soil moisture

Cover crops can be used as green manures to add fresh organic material to the soil.

Regular use of cover crops can raise soil organic matter content.

How much organic matter do cover crops add to the soil?

The impact of cover crops on total soil organic matter content is highly variable and can be challenging to detect, particularly in short-term studies and with low-yielding cover crops.

Several studies have reported no change in soil organic matter, whereas other studies have reported increases, ranging from 0.3% to 42% (relative to no cover crops).

Notably, no studies have reported a decline in soil organic matter as a result of using cover crops.

• Crops destroyed later – with more stem and a deeper, denser root system – are more likely to increase soil organic matter
• Crops with a low C:N ratio break down more quickly and have less of a long-term impact on soil organic matter
• A well-developed cover crop can return 1–3 t/ha of organic matter (compared with 4–5 t/ha for a typical application of organic manure)
• More carbon is likely to be retained in the soil from organic manures (particularly compost) than from cover crops

Soil organic matter enhances soil biology

Adding fresh organic material to the soil can enhance the populations and activity of key soil organisms, such as earthworms, mesofauna and microorganisms.

Surveys show that cover crops can increase the presence of anecic (deep burrowing) earthworms.

However, the effect varies according to soil type and the species of cover crop.

Some cover crop species are less beneficial to soil biology. For example, mustards do not increase earthworm populations.

Cover crops influence soil moisture

As cover crops grow, they can help dry out and warm the soil in spring. This may aid cultivation but could also hinder the germination of subsequent crops.

In wet winters, removing excess water via cover crop growth can be beneficial.

Deep-rooting cover crops, such as fodder radish, plantain and stubble turnips, have been shown to improve water infiltration threefold.

However, in dry years, the removal of water by the cover crop may reduce the available water supply for the following crops.

It is important to consider soil type, rainfall and cover crop management to optimise soil moisture.

Further information