let's see, technically earthworms should eat anything that classifies as "Any plant" in recipes which have that as an ingredient.
http://salemwiki.info/index.php/Any_Plant Should have a complete list, so check that out.
You need to look at the elemental composition of the worms and the plants you feed them, just feeding them 3-4% food won't matter much if it's all of different elemental composition. Purity increases when one element gains more and more value than the others, so you need to pick an element, find plants with that element and keep feeding the worm plants with a higher value of that element than it already has. For example say you find autumn grass with 20/20/20/40 composition and put it next to a 25/25/25/25 worm inside a box/chest. 12 hours later the worm will eat the plant and its elemental composition will become 24.5/24.5/24.5/26.5 and the more you increase that lead value the sooner your worm will reach the various percentages of purity. Remember that you need to place food adjacent to the worms, left, right, above or below them.
Exactly 6.022 x 10^23 worth of Lach molecules.