generating wheat on grass blocks

i wanted to add wheat to plains biome for some variety, so i copied all files related to berry bushes and replaced those with wheat. i added that to the plains biome config, but unfortunately it doesnt generate, while the exact same files but with berries do. so i figured it must be because the wheat only wants to generate on farmland. how do i edit the json file so that it's able to generate on grass too? (i dont just want to make the blocks underneath become farmland)