Improve water-holding capacity.
By fragmenting organic matter, and increasing soil porosity and aggregation, earthworms can significantly increase the water-holding capacity of soils.
( gronkpan , a friend, Thu 30 Aug 2007, 11:46, archived)

they improve drainage AND increase water retention?
They are truly magical.
( Debbilgrrrrl is kinda back, Thu 30 Aug 2007, 11:47, archived)