What can I do to repair soil in a yard that is occupied/visited by several neighborhood cats (to help eliminate the smell and to make the soil better for plant growth)?
Cats are territorial and often use urine as a way to mark their properties. Cat urine contains pheromones, which is a substance that cats and other animals use for communicating. Pheromones are much like fingerprints with humans, as they are used to identify the cat to other animals. When a cat sprays, it is his way of letting other cats know that this is his territory.
If you can identify the areas most frequented, you can repair the soil by removing the compromised material and adding new soil appropriate for the plants you are growing. Cat urine is highly acidic. Some resources suggest neutralizing any remaining acidity still in the soil with hydrated lime and thoroughly mixing it into the soil.
Once you remove the urine-marked soil, the cats will return and want to refresh their territory. Therefore, you need a multi-pronged approach to keep them away. Go for odor repellant and the element of surprise. Purchase a commercial product to spray on your plants and soil surfaces that will repel them from re-staking their claim to your garden. A product that contains the scent of a predator such as coyote or fox, effective against rabbits and squirrels, is also effective with cats. They will go someplace else rather than take the chance of encountering a predator. Secondly, a motion-activated water sprayer has demonstrated that its unexpected “attack” can keep the more persistent offenders away. Using other motion-activated devices that emit unexpected noises, like a dog barking, can also be effective when used in combination. Cats are quick learners despite their often aloof attitude. Once you have established your gardens are no longer a feline restroom, they will seek out less threatening facilities.