Your best bet for a good start in the spring would be something that has a high middle number on the box, like 6-24-12. Phosphorous (phosphate) promotes vigorous root growth and gives the plants a good foundation. Most of this stuff will even be marked 'root starter' & Peters' makes the one I use when transplanting to the fields. If the cats are using it quite a bit, you might want to consider using some 'miracid' in the soil. Cat urine is quite full of ammonia, which is pretty basic (pH >8). pH testing your soil will tell you of the need or not. They will likely have taken care of your nitrogen needs for the season too, so make sure your fertilizer isn't heavy on the nitrogen or you'll be over doing it. Hope this helps! -Jim C Mild to Wild