Bala Tikkisetty: Earthworms indicators of soil health

Only a few introduced earthworm species are beneficial to agriculture.

There are nearly 200 species of earthworms in New Zealand but only a few introduced species are beneficial to agriculture.

There are many species of native earthworms, but these seldom occur in developed soils.

The most commonly found species in such soils originated in Europe and arrived with early Pakeha settlers.

These introduced earthworms are, in fact, essential to the development of fertile productive soil. They act as biological aerators and physical conditioners of the soil, improve soil porosity, structure, aggregate stability and water retention.
Earthworms also increase the population, activity and diversity of soil microbes, such as mycorrhizal fungi. These microbes play a vital role in the supply of nutrients to pasture unlocking certain nutrients.
Soils without enough of the right type of earthworms are usually poorly structured and tend to develop a turf mat or thatch of slowly decomposing peat-like material at the surface. Old dung and dead plant material lie about the surface.
Plant nutrients tend to remain locked in the organic layer and there is poor absorption of applied fertiliser.
Plants roots in such soils are relatively shallow and pastures are therefore susceptible to drought.
And water runs off this type of pasture more easily rather than being absorbed into the soil, increasing water quality problems.
These factors can naturally inhibit pasture and crop production.
To help avoid these types of problems, soils should have a good diversity of relevant earthworm species.
The most common species in New Zealand is Aporrectodea calignosa, a topsoil dweller. This earthworm grows up to 90 millimetres long and may vary in colour from grey to pink or cream.

Another very commonly introduced earthworm is Lumbricus rubellus, a surface dweller.

Often found under cow pats, this earthworm will grow up to 150 mm long.

It is reddish-brown or reddish-purple colouring with a pale underside and flattened tail.

Aporrectodea longa live in burrows as deep as 2-3 metres below the surface.

Earthworm functional groups are; Epigeic earthworms feed on organic matter on the soil surface and do not form permanent burrows.

Endogeic earthworms ingest topsoil and its associated organic matter, forming semi-permanent burrows.

Anecic earthworms draw organic matter from the soil surface into their deep, permanent burrows to feed on.

Undertaking an earthworm count will let farmers know if they have enough of the right type.

Counts can be done by taking out a 20 centimetre cube of soil with a spade. Aim to have an earthworm number of between 30 and 35 in that cube.

If soils are scoring low, then there are ways to increase the populations:

• Ensure soil calcium levels are near 7 as calcium promotes earthworm reproduction.
• Maintain soil pH between 5.8 - 6.3.
• Limit use of fumigants and other pesticides.
• Reduce ammonium-based fertilisers as they make soils acidic.

Also, moist soils promote earthworm spread and activity and more will remain active in topsoil during summer under irrigation.

Direct drilling and no tillage cultivation methods is another way to promote help earthworm numbers. Use a mould board or disc plough rather than a rotary hoe.

Cropping farms should include a phase of pasture in their cropping rotation to increase organic matter returns.

Ideally the weight of earthworms below healthy productive pastures is equivalent to the weight of animals grazing above ground.