Earthworms in Soil Health Management

It’s the early bird that gets the worm.

But, besides being a juicy reward for the more proactive of our feathered friends, worms are also an important barometer of soil health.

Environment Waikato’s recently released report on the condition of rural water and soil in the region revealed only 34 per cent of sites sampled in the region have fully met national soil quality targets.

One key indicator of the biological health and condition of soil are the earthworms found in them

Through their burrowing, feeding, digestion and casting, earthworms have a major effect on the chemical, physical and biological properties of the soil.

And worm population density and species are, in turn, affected by soil properties and management practices.

Overall, there are nearly 200 species of earthworms in New Zealand, but only a few introduced species are actually beneficial to agriculture. There are many species of native earthworms, but these seldom occur in developed soils. The most commonly found species originated in Europe and arrived with the early European settlers.

Earthworms shed and decompose plant residue converting it to organic matter and mineral nutrients. Compared with uningested soil, earthworm casts can contain five times as much plant available nitrogen, 3-7 times as much phosphorus, 11 times as much potassium and 3 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 worms through digestion also results in increased fertility as it provides for faster nutrient recycling, and soil nutrients being more available for plants. It also assists in about a 25 per cent 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.

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 I mention are in fact essential to the development of fertile productive soil in the Waikato region.

Soils without earthworms are usually poorly structured and tend to develop a turf mat or thatch of slowly decomposing peat-like material at the surface. They are also less productive, having a harsh unhealthy appearance with old dung and dead plant material lying about 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, there is no mixing of applied fertilisers.,Plants 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.

So what are some of the ways you can increase the volume of worms in your soils?

· Increase soil calcium levels to near 7 as calcium promotes earthworm reproduction

· Maintain soil pH between 5.8 – 6.2

· Limit use of fumigants and other pesticides

· Reduce ammonium-based fertilisers as they make soils acidic.

Moist soils promote earthworm spread and activity, and more earthworms will remain active in the topsoil in summer under irrigation. Use direct drilling/no-tillage rather than conventional cultivation.

Mould board or disc ploughs should be used rather than rotary hoes, while cropping farms should include a phase of pasture in their cropping rotation to increase organic matter returns.

As a general rule, the weight of earthworms below healthy productive pastures is equivalent to the weight of animals grazing above ground. So building up your worm population if necessary will not only feed those early birds – it will help support a healthy stocking regime and your bottom line.

For further information and advice, please contact Bala Tikkisetty, Sustainable Agriculture Coordinator, Environment Waikato on 0800 800 401.