Some plants require enchanted soil in the form of plot or pot. It isn't necessary for all plants but as I rule I use enchanted pots and plots exclusively so I don't have to worry about what I plant since the non enchanted needing seeds grow just fine in enchanted soil.
When you mouse over some seeds in your backpack, they will say "Requires (whatever size) enchanted plot. If it doesn't say this, there's no point in spending extra gold on an enchanted pot. Hunter Spellstone Lvl 42 Master