I missed the first postings on this discussion so apologies if I have got the context wrong. In the UK quite a few bulb growers are using cat litter as an essential part of their potting mix and especially so by growers of Cypripedium and Pleione. One particularly good grower of Cypripedium uses a mix based on cat litter plus 10% each of loam and fine or composted bark. Cat litter is said to be calcinated moler clay. Certainly in the UK it is widely available in bags from supermarkets and garden centres with a pets section. Tesco Premium quality cat litter is said to be the best available and it is recognised that some cheaper brands break down in the potting mix and cause, rather than prevent, drainage problems. A search of the Scottish Rock Garden Club Forum on "cat litter" will bring up a host of information. David Nicholson in Devon, UK. Zone 9b