I stumbled upon this article posted to the Chicago Aquatic Plant Society's facebook page, and found it interesting.
What particularly interests me is this quote: "Substrates can also hold nutrients in an available form for plants through their cation exchange capacity (CEC) and bacteria activity; deeper substrate with slightly anaerobic conditions can reduce Fe3+ into the more usable Fe2+ format."
I have always kept low tech tanks (no CO2) with medium lighting, and worked to balance the fish load and plant load, to create as self-sustaining ecosystem as possible and dosing as needed (but not often) with ferts. I've also always used play sand (yes, the $3 per 50 lbs dirty play sand at Lowe's) for my planted tanks. There are plenty of comments around aquarium forums that the sand gets sucked up into the filter, or that it can't be gravel vacced, or that it can lead to anaerobic bacteria if too deep / not agitated frequently. But, my experience has always been great using this substrate. Especially the longer the tank is set up. My 75 gallon after about 1.5 years was pretty much self sustaining, and housed angelfish, which are sensitive to fluctuations.
Those of you who "science", can you explain this concept - what about anaerobic conditions would cause the chemical change from Fe3+ to Fe2+?