The easiest and most simplistic way to assess the health of soil is to dig a hole and observe the number of worms present. This gives a great indication of the structure, fertility and nutrient content of the soil. Earthworms are an essential part of soil health, often being referred to as ‘nature’s plough’; they undertake a similar function but at a much slower rate.
There are many different species of earthworms, each have a different function, different characteristics and exist at different depths of the soil. There are three main categories of worms in the UK: Epigeic, Endogenic, and Anecic. Epigeic worms live at the surface of the soil and digest root matter and dung pats. They are small and do not have burrows. Endogeic worms are slightly bigger and live between 0-50cm deep into the soil. They eat a huge amount of soil and are responsible for making aggregates within the soil. They create branching, horizontal channels. Anecic worms are the largest and they dig large, vertical, unbranching channels (up to two meters) from the topsoil to the waterbed., meaning they are very beneficial in terms of drainage. They forage for food and digest straw.
Improving the Soil
Their main function is to improve soil fertility by digesting organic matter into castings. These castings are extremely nutrient dense and are in forms which can be utilised by the plant and held in the soil. They also contain many microorganisms which aid the soil ecosystem and are vital in soil fertility. Through their actions, they release much nitrogen. As earthworms are active in warm and damp conditions, this is when the most nitrogen is made available by them, and this coincides with the time the plant is also most able to uptake nitrogen.
One of the major benefits earthworms provide is improving the structure of the soil. As they move through the soil, they create channels which allow for drainage, allowing the soil to aerate and improving soil structure. Not only do these channels aid air flow, but they also help water movement. The water holding capacity of the soil is improved, reducing run-off and water erosion in other places.
Getting the Benefits
However, in order to obtain the benefits they provide, earthworms must be looked after and fed. Compacted soils (e.g. from machinery, livestock and prolonged wet weather) are difficult for earthworms to move through, and therefore these soils will contain few worms. This is why it is important to aerate soils to prevent a compacted soil.
Earthworms are adverse to high levels of heavy metals, such as copper. In soils such as these, earthworm counts will be low and action should be taken to remove the metals to boost the earthworm population.
As well as heavy metals, ploughing is very disruptive to earthworms. The process destroys earthworm channels, moves organic matter into deeper layers of the soil, as well as physically damaging the earthworms and moving them to a depth of soil which is not part of their habitat. This means that populations are very slow to recolonise after ploughing, and so in soil which is earthworm rich, minimum cultivations should be considered.