I bought a 40 lb bag of STS at the local Farm & Fleet store for $4.50.
The attached photo shows the test I'm running in my 55g tank. Only a tiny corner of the tank was converted to STS. The black stuff is black Flourite. I've been really disappointed in the growth I've been getting from the Flourite. I had previously used Hi-Dri as a substrate and plant growth was phenomenal. I only changed to Flourite because I got tired of a beige substrate. It just seemed that Hi-Dri did better at growing plants than Flourite did so I'm testing STS in parallel with Flourite.
The STS is just planted with a few micro chain swords. Normally they grow like crazy and take over the tank but I didnt see that with Flourite. I use both root tabs (aquariumplants.com tabs) and EI dosing. There are 4 tabs buried under that patch of STS.
I'll know in a few weeks how well the STS works.