I don't want my 2 cats to use my garden space soil as a litter box
If the plastic will be temporary only until the garden is planted, no adverse effects will occur. If you intend to use the plastic as a permanent fixture, you will eventually regret that decision.
That said, here are a few ideas:
- Put something on the soil surface that will discourage the cats, perhaps chicken wire laid flat on the ground.
- You can install low fencing of the kind that’s often used to edge beds. But this time, meander it through the bed so that available spaces are too small for the casts to squat. I also used such fencing after I planted, winding it in and around the young transplants; I would remove the fencing when the plants gained sufficient size to nearly cover the ground.
- Install a motion-sensor sprinkler.
- Some people try commercially available cat-repellents.
- This site lists a number of remedies that worked for various people: http://parents.berkeley.edu/advice/pets/catpoop.html
- Have a policy of cats stay indoors, then create or purchase a sturdy cat-sized enclosure somewhat similar to a screened porch. Check with pet stores and mail order catalogs.