if I'm correct, and I'm about 85% sure that I am, you'll need to use something like one of these that does not contain Ca/Mg
30-10-10 - http://www.jrpeters.com/Products/Jack-s-Classic/Orchid-Special.html
20-20-20 - http://www.jrpeters.com/Products/Jack-s-Classic/All-Purpose.html
you can buy a...