Found at
http://www.ycy63.dial.pipex.com/amphsmollusc/e arthworms.html
Worm casts contain higher levels of nitrogen, phosphorous and potassium than the surrounding soil - the products of digested organic matter and trace elements brought up from deeper levels. Calcium carbonate is also added and this can make acid or alkaline soils more neutral. This 'fertilizer' is distributed throughout the soil and research has shown that each worm can produce about 150g per year - much better than a dry or granular fast-acting chemical fertilizer which has no humus, leaches away and may repel the worms.