Earthworms have food preferences…they’re pickier than you might think. What exactly they want to eat when is a topic of active research (http://news.bbc.co.uk/earth/hi/earth_news/newsid_8694000/8694377.stm) but they generally appear to prefer dead stuff to live stuff, to the point that scientists were surprised when they found out they will eat live seeds & plants.
So if you stick dead ginger in your soil the worms will go to town (there are Reddits about earthworm husbandry, have a ball). But a live growing ginger root (or a potato) seems to be less appealing unless they don’t have anything else around that they like better.
I imagine it’s also a lot harder for them to eat a living root with intact skin, since earthworms don’t have teeth and a living root has it’s own defense mechanisms to decay so the worm doesn’t have anything to breach the surface.
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