Their activity in the soil offers many benefits: increased nutrient availability, better drainage and a more stable soil structure, all of which help improve farm productivity.
Worms feed on plant debris (dead roots, leaves, grasses, manure) and soil.
Their digestive system concentrates the organic and mineral constituents in the food they eat, so their casts are richer in available nutrients than the soil around them.
Nitrogen in the casts is readily available to plants. Worm bodies decompose rapidly, further contributing to the nitrogen content of soil.
New Zealand research shows worm casts release four times more phosphorus than does surface soil.
Worms often leave their nutrient-rich casts in their tunnels, providing a favourable environment for plant root growth.
The tunnels also allow roots to penetrate deeper into the soil, where they can reach extra moisture and nutrients.
Earthworm tunnelling can help incorporate surface-applied lime and fertiliser into the soil.
Better soil drainage
The extensive channelling and burrowing by earthworms loosens and aerates the soil and improves soil drainage.
Soils with earthworms drain up to 10 times faster than soils without earthworms.
In zero-till soils, where worm populations are high, water infiltration can be up to six times higher than in cultivated soils.
Stable soil structure
Earthworm casts cement soil particles together in water-stable aggregates. These are able to store moisture without dispersing.
Research has shown earthworms that leave their casts on the soil surface rebuild topsoil.
In favourable conditions they can bring up about 50 tonne/ha annually, enough to form a layer 5mm deep.
One trial found worms built an 18cm thick topsoil in 30 years.
Improved productivity
Research into earthworms in New Zealand and Tasmania found earthworms introduced to worm-free perennial pastures produced an initial increase of 70 to 80 per cent in pasture growth, with a long-term 25 per cent increase, which raised stock carrying capacity.
Researchers also found the most productive pastures in the worm trials had up to seven million worms per hectare, weighing 2.4 tonnes.
There was a close correlation between pasture productivity and total worm weight, with some 170kg of worms for every tonne of annual dry matter production.
