Earthworms affect soil fertility in various ways. Their burrows provide drainage channels through the soil, improve its aeration, and assist deep root penetration. The lumbricid species of New Zealand pasture lands are all topsoil dwellers but in summer, if the surface soil becomes too dry, they retreat into the subsoil and go into diapause – a state of suspended animation. The vertical channels so made remain for a short time as subsoil drainage channels but, as their thin walls are not usually firmly compacted, they soon collapse.

Earthworms increase the efficiency of the organic cycle by hastening the decomposition of forest litter, and in pastures by speeding the release of plant nutrients from dead roots and pasture litter. Their casts are generally less acid and higher in nitrogen and available phosphorus and in exchangeable calcium, magnesium, and potash than is the case in undisturbed soil. The quantity of soil deposited at the surface in the form of worm casts was found (Evans 1948), on eight fields with different management histories, to range between 1 and 25 tons per acre per year. Calculations, based on total populations and taking account of species that cast beneath the surface, showed that from 4 to 36 tons of soil per acre per year passed through the alimentary systems of earthworms and were cast at or near the surface.

Three common lumbricids of New Zealand pasture, Allolobophora terrestris, A. calliginosa, and Lumbricus rubellus, are surface-casting species and together would soon cover a field with a thick spongy layer of casts were it not for such factors as the impact of raindrops, movement of surface water, and trampling by stock. Measurements on highly fertile farms south of Auckland have shown that, under grazing conditions, the build-up of compacted cast material amounts to slightly more than 1/10 in. per year.

These topsoil earthworms play an important part in our grassland farming. They stimulate pasture growth by removing dead root material, loosening up the sod, and providing an enriched layer of cast soil in which perennial grasses and clovers are able to re-root year by year. But all is not to the advantage of the farmer. In performing these beneficial tasks, earthworms inevitably play their part in reducing the bearing strength of the topsoil by weakening the sod and casting at the surface. Moreover, there follows the resulting increased poaching of the surface soil by stock during winter months.

by Norman Hargrave Taylor, O.B.E., formerly Director, Soil Bureau, Department of Scientific and Industrial Research, Lower Hutt.

• The Earthworm Fauna of New Zealand, Lee, K. E. (1959).