May I ask, are the ants damaging in any way to seedlings/ crops or is it ever ok to just ignore them? I cleared a patch of church grass only to find 2 great big ants nests under the black plastic cover.
ill give you my experience after living with ants in a paved greenhouse in the first couple of years of growing.
ants will farm aphids for their honeydew, i have watched ants carry knocked off/soap sprayed greenfly back up plants from the ground. so they wont directly kill off plants but they can be a nuisance above ground as well as below. from what i remeber reading years ago they will move aphids from plant to plant too.
i don't like to kill things i dont have to so i tend to live with them but keep a watchfull eye for greenfly and such. i got them out of the greenhouse because i had to by flooding the place with water. they are stubborn little things though.
ants hate things like white vinegar solution, have used this when the little * got into the house as they will do eventually if theres ants nests nearby.