By Jake Freestone, Tewkesbury farmer and founding member of the Green Farm Collective
As a farmer very focused on the soil I and my team nurture, you can’t help but have an interest in the earthworms that flourish in our soils. Over time you probably become slightly obsessed in their ability to improve the soil and a sense of satisfaction from seeing them flourish because we don’t disturb their homes with cultivation or ploughing. We feed them with crop residue (straw and leaves), cover crops and compost. When you get your eye in, evidence of their presence on the soil surface is very easy to spot.
Worming your way through the different species
In the United Kingdom there are 27 species of earthworm, but the most common ones that we find in our arable (crop land) soils are Epigeic, from the Greek translation – ‘on the earth’, as they live in the organic or leaf litter on the soil surface; Endogeic, which translates from the Greek- ‘in the earth’ as they have burrows in the top layers of the soil and rarely come to the surface; and, the final type, (my favourite) are the Anecic (deep burrowing) - from the Greek, ‘out of the earth’. This is an odd description as they can burrow down to a depth of 2m but they come to the surface to feed. The evidence of their activity is seen by the earthworm middens on the soil surface. These middens are small piles of crop material, effectively a fridge for the earthworm to feed on when hungry. During damp weather the worms will come up from their burrows to gather food or to mate.
Earthworms have 4 main functions in the soil:
Improve soil nutrition
Earthworms feed on decomposing organic matter, such as leaves, dead plant roots, and manure. These nutrients become concentrated in the worm’s digestive system and released back into the soil in the excreted earthworms cast. These casts are rich in nutrients, notably nitrogen and phosphorus, and they are often left in the worm burrows. This means that burrows beneath the soil’s surface contain nutrients that are readily available to the roots of plants. Some studies have shown that soil that has been digested and excreted by earthworms is five times richer in nitrogen than soil that has not been digested. 50% of our soil passes through our worms every year!
Organic matter decomposition
Worms feed on decaying and rotting organic matter, and this process helps to break down the materials, enabling them to be fed on by bacteria and fungi. These micro life forms are important in helping organic matter to decompose, and they are also an important part of our ecosystem. Bacteria and fungi thrive on nutrients released by earthworms, and they themselves are an important food source for other creatures that inhabit the soil. A soil rich in worm casts is likely to have 1000 times more beneficial bacteria than soil that is not home to worms.
Improve soil structure
Earthworms improve soil structure by creating burrows and opening up spaces within the soil. This physical alteration means that water and soluble nutrients can travel more easily down to the plant roots. Research has shown that worm burrowing improves water filtration by up to ten times. For this reason, soil that is inhabited with worms is much less likely to experience flash flooding than that that is free of worms. The tunnels made by worms, which can go very deep into the soil, vastly improve drainage, which helps to protect plant roots against rotting. The burrows not only help with water drainage and filtration, but they also create airways that help with soil aeration. Good soil aeration is important to allow ventilation for plant roots and other living organisms that inhabit the soil.
Predators food source
While keeping worm populations alive is great for our soil and plants, we should also remember that worms are an important food source for many predators, such as farmland birds. This is evident with work undertaken by our West Midland Bird Ringing Group who regularly see farmland birds feeding on our arable fields.
With worms playing such a vital role in the functionality of healthy soil, it is imperative that we nurture their populations with the best possible management. A target population of 400 worms/m2 has shown (meta-analysis by Jan Willem van Groenigen) to have a positive yield infect so it's in our interest as well as theirs.