Cats appreciate the feel of most any kind of garden soil under their paws, so long as it isn't muddy. Soil that works for growing plants that cats favor, such as catmint, catnip and clumping grasses (that cats can hide among before pouncing) is rich in organic matter, including compost, crumbles well and is spongy under the paws. If your cat garden has soil like this, both your plants and your cat can enjoy it.
Cats like to dig and need a place to go to the bathroom. Creating an area in the garden with sand just for these activities is ideal. The sand is different enough in texture from the garden soil that the cat won't mistake one area for the other, and gives easily for digging and burying. The sandy area should be well enough away from salt-sensitive plants and their feeder roots that the digging and waste from the cat won't damage the plants.
If potted plants are included near or inside the cat garden, the cat may dig in them, uprooting the plant. The soil in plant pots needs to be rich potting soil with good drainage, but the top layer should be something to discourage digging. Pea gravel, pebbles or river rock allow water to filter down, but won't invite pawing around. One to 2 inches of rocks are sufficient.
If the plants in the cat garden get mushed, clawed, rolled around on or otherwise made into a mess, this is a good thing: it's less time the cat is doing these things to the favorable parts of your own garden. However, if the cat won't stop digging around the roots of the plants in the cat garden, you may soon not have plants to distract your cat. Applying the same stone layer around the sensitive root areas in your cat garden will help protect the valuable soil and plant roots from damage.