The soil which an earthworm ingests in converted through its intestines into more plant available forms. The resulting earthworm poop has been processed several times and this results in a material which carries a higher nutrient load than the surrounding soil.
In comparison to soil, earth worms contains
5 times as much Nitrogen
7 times as much Potassium
2.5 times as much Magnesium
2 times as much Calcium