Here is a excerpt of a talk by a wormspecialist. It's in French.
http://www.dailymotion.com/video/x1bm1rw_le-fabuleux-travail-des-vers-de-terre-marcel-bouche_news
Some key numbers and facts,
He compares earthworms to a heavy tractor.
They represent 1200 kg/hectare (Insects 60 kg/ha - Birds less than a kg)
They turn 300 tons of earth /ha/year, 63% being re-digestion of their own feces over time.
76% is organic matter that goes through their digestive system.
2,3 tons of Nitrogen go through their digestive tracts every year, a big part of it is immediately absorbed by plants.
Microorganisms live for 92% of them in the mm around their galleries.
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