A few days ago, I found a tiny (1cm in length) earthworm inside my pot. In general, earthworms are beneficial in improving soil fertility. Are all kinds of earthworms good for the soil?
Are all kinds of earthworms good for the soil?
The answer depends on a number of factors.
In general for gardening and agricultural purposes, worms benefit the soil by providing aeration and decomposition and improving soil structure.
Some people find earthworms to be a pest in lawns because they deposit too many castings on the surface which is ugly and can make a lumpy surface texture. Also, some moles feed on earthworms, and moles can be a major lawn problem, so too many earthworms could be encouraging a mole problem.
From the Wikipedia earthworm article:
When worms decompose that leaf layer, the ecology may shift making the habitat unsurvivable for certain species of trees, ferns and wildflowers.
There is a web site at the University of Minnesota that discusses the adverse impact that earthworms have on forest soils.
A bit further down in the Wikipedia article, there's a note about the New Zealand flatworm that is invasive to the U.K., is predatory upon native earthworms and has no natural predator. This would be a net loss for soil health.
Lastly, "earthworm" is an imprecise term. If you want to know if the specific worm (or population) in your pot (or garden) is beneficial, you'd have to identify it and see if that species (or group, if you can't make an exact identification) provides benefits in your circumstance.