Montmorillonite clay has a high CEC index and plants do very well in it. Currently I have been using Safe-T-Sorb #7941 in my planted tanks. It has a nice natural color, a variety of grain sizes, and plants root well in it. It is dusty and requires a good cleaning prior to first using it. It is available online at Drillspot for $9.29 per 40# bag with free shipping included.
Here is a picture of my 10 gallon, with Safe-T-Sorb substrate. This tank has no CO2, just Flourish Comprehensive and Flourish Excel. The baby Corys have been in there for over 3 months and have doubled in size.