Casting no doubt on earthworms’ good work
The twists and turns of the humble worm have a major positive effect on the chemical, physical and biological properties of farm soils.
This impact is a combination of the worms’ burrowing, feeding, digestion and castings.
Waikato Regional Council’s soil quality reports have revealed only 34 per cent of sites sampled in the region have fully met national soil quality targets.
One key indicator of soil biological health and condition is the number of earthworms found in it.
Worm population density and species are affected by soil properties and management practices.
Promoting good soil health and limiting soil damage in winter are important factors in sustaining earthworm population.
Overall, there are nearly 200 species of native and introduced earthworms in New Zealand but only a few introduced species are actually beneficial to agriculture. The most commonly found species arrived in New Zealand with early European settlers.
Earthworms shed and decompose plant residue converting it to organic matter and mineral nutrients.
Compared with uningested soil, these earthworm ‘‘casts’’ can contain five times as much plant-available nitrogen, three to seven times as much phosphorus, 11 times as much potassium and three times as much magnesium.
They may also contain more calcium and plant available molybdenum, and have higher ph, organic matter and moisture content.
The breakdown of organic matter by worm digestion also results in increased fertility as it provides for faster nutrient recycling and makes soil nutrients more available for plants. It assists in a significant increase in pastoral productivity, faster decomposition of organic matter, increased plant growth, and better mixing of nutrients, fertiliser and lime through the soil profile.
Worm movement through, and feeding on, soils also results in improved soil structure. It provides for better mixing of soil layers, creates aeration and drainage holes, and allows for better plant root development. Less water run-off, less erosion and better drainage and water holding capacity are other benefits.
The disposal of organic wastes from domestic, agricultural and industrial sources is causing increasing environmental and economic problems. Earthworms can be used to process these wastes.
Most of these wastes contain considerable amounts of inorganic and organic contaminants such as heavy metals, pesticides, aromatic hydrocarbons and sulphur compounds, as well as human and animal pathogens.
There are three types of earthworms, epigeic, endogeic and anecic. Epigeic earthworms (Lumbricus rubellus) feed on organic matter on the soil surface and do not form permanent burrows. Endogeic earthworms (Aporrectodea caliginosa) ingest topsoil and its associated organic matter, forming semi-permanent burrows.
Anecic earthworms (Aporrectodea longa) take organic matter from the soil surface into their deep, permanent burrows to feed on. In an ideal soil all three types may be found.
Precise identification of earthworm species is difficult but for most purposes it is only necessary to be able to distinguish between native and introduced species. When matured, most introduced worms have a visible ‘‘collar’’, called the clitellum, about one third of the distance along the body from the head, and often a pair of white glandular swellings on the underside between the head and the clitellum.
In the native worms, this collar is close to the head, and there are no glandular swellings on the underside. Immature worms do not have a clitellum and are therefore harder to identify.
The introduced earthworms mentioned are essential to the development of fertile productive soils.
Soils without earthworms are usually poorly structured and tend to develop a turf mat or thatch of slowly decomposing peatlike material at the surface. They are also less productive, having a harsh unhealthy appearance with old dung and dead plant material lying on the surface.
Other features of such soils can include the fact that lower producing grasses are often more evident than ryegrass, and pasture growth is slow to start in spring and stops early in autumn. Plant nutrients can remain locked in the organic layer and there is no mixing of applied fertilisers. Plant roots are relatively shallow and therefore pastures are susceptible to drought. Water tends to run off the pasture rather than being absorbed into the soil, thereby increasing water quality problems.