Elfie, I think organic material is going to do much better for you as a soil amendment than cat litter. All that's ever happened when I sprinkled cat litter outside is it becomes a sticky mess in the humidity and takes forever to dissipate into the soil.
Have you tried amending with alfalfa pellets? (horse food) Also available at Tractor Supply, about $15 for a 50lb. bag. They are fantastic, in combination with a good amount of compost as they seem to have a lot of the 'natural' biological components and the plants just jump up and grow. You can't just use straight alfalfa as an amendment, because it's pretty high in nitrogen and might burn your plants if they get too much at once.
