NZ Farmer - Earthworms play a vital role in improving New Zealand soils

The humble earthworm is worth its weight in gold when it comes to the soil productivity of farms and improving soil health and water quality.

Common earthworms introduced from Europe in the 1800s improve the general condition of soils, reduce surface run off of contaminants from pasture and prevent soil erosion generally.

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.

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. These factors can naturally inhibit pasture and crop production.

Lower producing grasses are often more evident than ryegrass on these types of soils as well. Pasture growth is slow to start in spring and stops early in autumn.

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, as indicated earlier, water runs off this type of pasture more easily rather than being absorbed into the soil, increasing water quality problems.

To help avoid these types of problems, soils should have a good diversity of relevant earthworm species.

The most common introduced earthworm in New Zealand is Aporrectodea calignosa, a topsoil dweller. This earthworm grows up to 90mm long and may vary in colour from grey to pink or cream.

Another common introduced earthworm is Lumbricus rubellus, a surface dweller. Often found under cow pats, this earthworm will grow up to 150mm long. It is reddish-brown or reddish-purple with a pale underside and flattened tail.

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

Undertaking an earthworm count will let farmers know if they have enough of the right type. Counts should preferably be done late winter to early spring when soil moisture and temperature conditions are ideal.

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

Farmers can increase their worm populations by ensuring soil calcium levels are near 7 as calcium promotes earthworm reproduction, maintaining a soil pH between 5.8 and 6.3, limit the use of fumigants and other pesticides and reduce ammonium-based fertilisers because they made soils acidic.

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 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.