I assume you've already had the notice from Professor Wu to talk to Farley in Golem Court and finished that quest? If you haven't, then all the TRAIN spells can be seen, but will be gray ed out and there will not be a TRAIN button, only a CLOSE button. The reason for that is that even the lowest gardening spells require you to be at Gardening Level 1, and Farley will raise you up to that level when you complete his quest to learn about gardening.
If you did do his quest, if you put your cursor over the TRAIN button, does it say anything? You won't be able to buy something if you don't have enough gold, of course. But also if you're not a high-enough level ( I'm pretty sure Farley has a Rank 1 soil spell, though).
One other reason I can think of why you can't buy a spell, if you DO get the TRAIN button ...perhaps you already purchased it? When you're in your house/dorm and you click on the Gardening icon, you don't see any soil spells when you click on the Soil Spells icon (3rd tab from the left) ?