it's Calcined Clay , here is the ditty I wrote years ago, links might be dead ?
Calcined Clay, A Well Kept Secret
I am getting ready to set up my next grow and a big part of that was looking for the perfect soilless mix to work with my style of growing. I picked SunShine Mix #4 as my base and if you read my ditty on Silica then you would know I plan on mixing in some Diatomite.
I am constantly reading and researching, as things I read lead me to more info to be researched. I recently came across and read the PH Manifesto (and advise everyone read it) it mentioned mixing Calcined Clay with SunShine Mix #4 and off I went to discover why ?
What I found was that Calcined Clay is one of Container Gardening's best kept secrets. Originally used to make golf course greens stay greener back in the 50's it was not well accepted by gardeners and never really caught on untill recently. The benefits are considerable and adding this to your medium is simply a no brainer!
I am going to share links rather than quoteing simply because some of these links are really good reads with lots of great info (quotes would require severals parts to complete the post)
a great pdf about mixing your own potting mix
http://www.docstoc.com/docs/5655752/...s-mean-better/
Ohio Edu ditty on Container Gardening
http://ohioline.osu.edu/hyg-fact/1000/1254.html
benefit if water left in trays
http://docs.lib.purdue.edu/cgi/viewc...5&context=pmag
another great post about Container mediums
http://edis.ifas.ufl.edu/cn004
experiment showing a reduction in fugas knats
http://docs.lib.purdue.edu/cgi/viewc...5&context=pmcg
Using Calcined Clay for Bonsai's
http://ezinearticles.com/?Why-Should...ts?&id=3480472
yet another good read about ammending soil
http://www.uri.edu/ce/factsheets/she...landscape.html
and finally for those of us on a budget please note that No Lump Kitty Litter is made with Calcined Clay (mentioned in one of the above links)
Enjoy