AGP - Earthworms

Earthworms are extremely important part soil macro-fauna especially in the upper 15-35cm of soil where their feeding and burrowing activity affects plant growth, nutrient turnover and seedling development (Asshoff et al., 2010) and so have been described as “ecosystem engineers”.

There are more than 3,000 species of earthworms which can be separated into two distinct classes based on their feeding strategies – detritivors which feed on plant litter and mammalian dung (surface feeders) and geophages which feed in the deeper layers of soil on plant material. These two groups can be further subdivided depending on which soil horizon they feed in (Curry & Schmitt, 2007). Identification of earthworms is usually achieved through morphological comparison although DNA based methods are being developed which is proving useful in identifying juveniles (Richard et. al., 2010).

Earthworm  numbers vary widely in different soils but they prefer well aerated soils and tend to dominate soils which have  around 800 mm rainfall per year.  In arable soils, they may range from 30-300 individuals per square metre, with a biomass of 110-1100 kg/ha. They tend to prefer soils with pH 5-7 and with organic matter having a low amount of humified material. Fungal material may also impede earthworm burrowing. These types of soils tend also to have high numbers of bacteria. Soils with pH <4 such as peats are generally too acidic and too waterlogged to support earthworms. Soils with high clay content or have very coarse textured generally do not favour earthworms as the clay will have too little oxygen and coarse soils are too abrasive to the earthworms and may be too dry if the soil is freely draining. Earthworm numbers may decrease as a result of predators, ammonia fertilisers, certain insecticides and soil disturbance through tillage. Minimum tillage which leaves crop residues and mulch encourages earthworms

Most species are slow growing taking 1 - 2 years to mature, living for 1.5 - 4 years. The smallest species tend to be forest litter dwelling measuring 10-20 mm and tend also to be the shortest living species. Larger species, living longer, can reach 1-1.5 m in length and weighing about 600g, although there has been an unconfirmed report of one from South Africa measuring 7m and weighing 30 kg. They can survive temperatures from 0-35°C.

Earthworms feed by extracting nutrient from the material they are burrowing through (soil or plant debris) or by bringing plant material down from the surface into their burrows. As they tunnel, the earthworm produces casts as waste products which contain large number of bacteria and nutrients that have passed through their gut. They may ingest soil minerals such a sand grains which aid in the grinding action in their gizzard. Earthworm casts and any food (plant debris) drawn down into their burrow acts as a means to relocate valuable nutrients throughout the soil especially to the deeper soil layers. Bacteria and other microbes (Reddell & Spain, 1991) can also be spread by the mucus and also on the earthworms themselves. The casts are stable soil aggregates which are significantly higher in bacteria, organic matter and available plant nutrients than the surrounding soil. These contribute to soil formation

Most of the earthworm food lies near the surface but by creating burrows they give themselves some protection against environmental and seasonal variability. Three types of burrows are formed: a) deep vertical burrows (at least 3 m by Lumbricus terrestris) which may have several branched entrances at the surface and b) horizontal burrows with the occasional shaft into the soil. In these, the burrow is lined by mucus produced by the earthworm and last for a long time. In the third case, c) vertical burrows are made by earthworms which live near the surface. These burrows offer refuge during dry or cold periods but exist only for a short time and may terminate in chambers where the earthworm takes refuge against adverse conditions. During heavy rains burrows may become waterlogged so earthworms may climb out to stop themselves from drowning. Under very dry conditions, they may migrate down into their burrows.  The burrows serve to increase aeration, drainage and root penetration and so are of major importance in soil development and agricultural production as they enhance soil fertility and productivity through increasing availability of mineral nutrients to plants and integrating undecomposed surface residues, into the soil, hence reducing loss of nutrients, increasing the organic matter content and improving soil structure. In one year, earthworms may ingest between 22 to 450 tons/acre.

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