Bala Tikkisetty, a sustainable agriculture advisor at Waikato Regional Council, has gone out to bat for the humble earthworm in an article distributed to news media.
He explains that 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, Tikkisetty says.
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 2-3m below the surface.
Earthworm functional groups are: Epigeic earthworms (i.e. Lumbricus rubellus) feed on organic matter on the soil surface and do not form permanent burrows; Endogeic earthworms (i.e.Aporrectodea caliginosa) ingest topsoil and its associated organic matter, forming semi-permanent burrows; Anecic earthworms (i.e. Aporrectodea longa) draw organic matter from the soil surface into their deep, permanent burrows to feed on.
The article includes this illustration from AgResearch.
The article is designed to encourage an earthworm count to 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 a 20cm cube of soil with a spade. Aim to have an earthworm number of between 30 and 35 in that cube.