put tin foil over it.

If they were living outside, they are used to going in soil.  You might want
to put some in the litterbox for a while, and gradually reduce the amount
you put in to get them used to regular litter.

The first thing to do is visit the vet to rule out any underlying
medical problem.

Just make sure you keep the litter clean. Clumping litter is best since you
can get all the used litter out at each scoop. Pour a little baking soda in
the litter box about once a week and mix it in real well, that helps prevent
odor so you don't smell the box and the cats will go more cuz they won't
find the box so smelly either.

I think cats don't understand they are ONLY supposed to go in the litter
box. They can be opportunists and figure since they're next to the plant,
why not go ahead and go there? Well, put pine cones to cover the dirt and
they can't get to it. Or, you can put a few moth balls on the plant dirt,
cats don't like it, and it's not harmful to the plant or the cats. You can
water your plant like normal and leave the moth balls on the dirt. There are
many ways to prevent the cats from getting to the dirt... be creative and do
what works for you.

You can also use positive/negative reinforcement. Whenever you WITNESS them
scratching the plant dirt just say a firm "no".  Of course, use the name of
the cat that's doing it. And, whenever you WITNESS them going in the litter
box you can soothingly, with a nice tone of voice let them know they're
doing what you want with a "good kitty". Preferably use their name along
with any verbal reinforcement so they know you're talking to them. The
reinforcement has really worked for me, however, kittens are more difficult
to direct with just verbal cues, but at 2yrs old, patient use of verbal
reinforcement along with treats and whatever training methods you use, will
solidly direct your cats' behavior.