I Woudl go with the soil option - although I have read you shouldn't use garden soil unless it has been baked first, I just can't remember why, and I know we had this debate on CC, and cant' remember the outcome!!! You can then gradually increase the litter - it can be hard with stubborn cats though, Ginger had lived as a stray for 3 years when he came, he took to a tray no prob for a short while, and would then prefer to hold it in all evening rather than use the tray, and then kept finding himself caught short during the night, so took to the tray again!!