Are you sure you using the right spell? The "plow" spell removes an existing plot, it doesn't create one.
To create a soil plot you need a soil spell. Its been a while since I trained them, but if memory serves Farley in the golem court does NOT sell soil spells. There are other gardening trainers scattered across the spiral. I think most of the soil spells are available from Charley in Krokotopia (he stands next to the pyramid of the sun). A couple of the more advanced ones have to be bought from Marley in Mooshu.