Never heard anything like that. Sounds like it could end up getting nasty though, imagine milk sitting out in the heat (bright light)...gets gross pretty quick. Dunno if it would do the same in soil or not. But then again, people mix bat shit, cow shit, and earthworm shit, etc in their soil, along with god knows what else, so I don't think milk could hurt lol. Let us know how it works out if you decide to try it. Maybe try it in on one plant and try another one side by side with it without milk and see if you can see a difference.