Roksolid is THE BEST slow release fertilizer I have ever used for my cacti. It is a form of rock dust I believe. For NZ'ers (This site isn't cheap, you can find it cheaper elsewhere but it has all the necessary info): http://www.0800466464.co.nz/plant-nutrition/60-wallys-rok-solid-1-kg.html Constituents of it are: Analysis of Rok Solid (Averages)
Nitrogen [N] .15%
Phosphorus [P] .8%
Potassium {k} 1.4%
Sulphur .14%
Calcium [Ca] 4.4%
Magnesium [Mg] 5.69%
Sodium [Na] 2.95%
Silica [si] 43.0%
Iron [Fe] 39,000ppm
Manganese [Mn] 1,100ppm
Boron 55ppm
Copper [Cu] 90ppm
Zinc [Zn] 142ppm
Cobalt [Co] 59ppm
Molybdenum [Mo] 2ppm
Selenium [se] 3ppm
Plus many other trace elements.
43% Silica! You HAVE to see how healthy all my cacti and plants are because of this. Silica plays a vital role in cell development and water retention. http://www.innovationsinagriculture.com/CMSFiles/Gallery/Files/Exhibitor/1/Roles%20of%20Si%20-%20email.pdf