The digging done by earthworms leads to aeration and drainage of the soil. In addition, plant nutrients are released from harvest residues that have passed through the earthworms’ intestines. Farmers can encourage earthworms by supplying large amounts of organic material and by using less disruptive soil tillage.
In a normal soil on an arable farm, there can be between 100,000 and 1 million earthworms, with a combined weight of between 100 and 1000kg, within one hectare. These earthworms play a very important role in the soil when they carry out their work.
Through their activities they increase the drainage and aeration of soil when they open up channels for water and air down to the subsoil.
In addition to their effect on drainage and aeration, earthworms also affect other soil physical properties of the soil. The number of pores increases and soil dry bulk density decreases when worms dig their way through the darkness of the soil. The soil tillage work carried out by the worms therefore greatly increases the number of macropores (diameter > 0.5mm) and creates a network of channels and spaces in the soil. This network can amount to 4000-5000km per hectare and the tunnels can extend to 2-3m depth. The tunnels act as ‘motorways’ for the roots in the soil. In a few years, earthworms move tens of tons of soil per hectare up to the soil surface as worm casts.
Soil biology is also improved, since the activity of earthworms stimulates microorganisms and actively spreads fungi and bacteria in the soil profile. This ultimately affects soil chemistry, since the availability of practically all nutrients is improved when organic material passes through the earthworm gut. For example, the concentration of nitrate is 8 times higher in worm casts than in the surrounding soil. These worm casts act as ‘glue’ between the soil particles, which improves aggregate stability and the soil structure.
Earthworms are sensitive to many components of modern agriculture, such as pesticides and soil compaction. Soil tillage is a sensitive issue, since it disturbs the worms and destroys their tunnel system. This is particularly true in September and October, when worm reproduction occurs. Soil tillage can be ranked according to the damage it causes to earthworms in the order: Direct drilling < tine cultivation < stubble cultivation < ploughing < rotavation.
The effect of the plough on worms is the subject of frequent debate. One study found that ploughing carried 10% of the total mass of earthworms in the soil up to the surface. Once there, birds ate around one-third, while two-thirds managed to escape and found their way back into the soil.
To encourage earthworms it is important to feed them regularly. The best way to do this is to include a grass-clover ley in the crop rotation. However, any measure that increases the amount of organic material in the soil is positive for the earthworm population. Green manuring and intercrops are therefore excellent worm feed.
In just a few years of active fallow with a ley consisting of clover and grass instead of winter wheat, the number of earthworms in the following crop can be increased by 100%. Earthworms are therefore a good indicator of soil fertility. Where earthworms are thriving, crops will also thrive.
Subsoil = is that part of the soil profile that comes directly under the topsoil and is often unaffected by normal soil tillage to ploughing depth, but is sometimes tilled by deep loosening. The boundary between topsoil and subsoil is often clearly visible in ploughed soil as a plough pan, where plough shares and tyre slip have compacted the soil
Pores = are the spaces, channels and cracks in the soil, which are filled with either water or air depending on the actual water content of the soil.
Dry bulk density = is also called volume weight and refers to the weight of the soil in relation to its volume, including the air-filled spaces, once the soil has been dried to 105°C.
Worm casts = faeces/waste from the intestines, which in the case of earthworms is often visible as small heaps around the opening of worm channels at the soil surface.
Nitrate = plants normally take up the majority of the nitrogen they require as nitrate, NO3- which is a form of nitrogen that is found in the soil and also in mineral fertiliser. In the soil, specialist bacteria transform ammonium, NH4+, via nitrite, NO2- into nitrate. This process is called nitrification.
