I don't intend for this to be simply argumentative; we are here to exchange knowledge and opinions, so in that spirit I must make a couple of corrections.
First on the light spectrum/wavelengths. It is true that plants use light in the red and blue wavelengths for photosynthesis. But there is also the scientific evidence that aquatic plants grow best when green/yellow light is predominant. A study [K. Richards, The Effects of Different Spectrum Fluorescent Bulbs on the Photosynthesis of Aquatic Plants
, Freshwater & Marine Aquarium, July 1987, pp. 16-20] determined that plants photosynthesized best [= more oxygen was produced] under a combination of Vita-Lite [which is rich in red and blue] and cool white [produces mainly green/yellow], and second best under straight cool white. Third best (poorest growth) was under the straight Vita Lite. At this point I will cite from Diana Walstad's book since she has opinioned why this might be the case.
The fact that plants did very well with Cool-white, which produces mostly green-yellow light was an unexpected result of this study. One would have expected the plants to do better with Vita-Lite. This is because Vita-Lite was designed for growing plants; its spectrum, which is rich in red and blue light, matches the light absorption of plant chlorophyll much better than Cool-white and many other fluorescent bulbs.
Cool-white was found to give off 13% more photosynthetic light than Vita-Lite. Perhaps Cool-white's slightly higher light intensity explains its better performance? However, I would also argue that green-yellow light is what many submerged aquatic plants encounter in their natural environment. Aquatic light is not like terrestrial light where the blue and red wavelengths predominate. Aquatic light is unique. This is because the water itself absorbs red light, while DOC [dissolved organic carbon] absorbs blue light. What's left for plant photosynthesis is mainly green-yellow light. Aquatic plants may have adapted their photosynthetic machinery (over the course of evolution) to use green-yellow light fairly efficiently. Thus, the assumption that aquatic plants grow best with full-spectrum light may not be valid.
At this point, I would refer members to all written articles by acknowledged planted tank sources which maintain that light around 6000K to 7000K is best for aquarium plant growth. Walstad in another section of her book mentions that the combination of red/blue and green/yellow that is found in tubes having either a K rating between 5000K and 7000K or a CRI of 80-100 is the closest match to the best light for plants. I carried out a minor study on this and found it to be so.
Moving on to the substrate issue. As organics in the substrate decompose, CO2 is released and plants take this up. The largest source of CO2 in an aquarium comes from the substrate, not from respiration of fish/plants/bacteria. This is in fact the only real benefit of using soil; the initial release of CO2 from the organics in soil provide a significant source of carbon for plants in a new aquarium. But as Walstad [who is pro-soil substrate exclusively] admits, any sand or gravel substrate will become equivalent after organics have been allowed to settle into it. Removing significant amounts of organics from the substrate is counter-productive in a natural method planted tank.
Fine gravel or coarse sand has been determined to be the best substrate with respect to plants. I use mainly play sand, and as the photos of my tanks under the "Aquariums" tab below my name on the left illustrate, the plants are certainly thriving.
In most of these tanks, I never touch the substrate. There seems to be sufficient CO2 being produced naturally to balance a light period of 8 hours; going beyond this, algae becomes a nuisance, due I believe to the insufficient carbon which is the only nutrient [apart from oxygen, hydrogen and nitrogen] I am not adding artificially.
The issue of de-nitrification in the substrate is one that is largely mis-understood. Anaerobic activity is actually a vital aspect of a healthy aquarium. This post is getting long as it is, so rather than go into all this, I would refer members to my article on bacteria: http://www.tropicalfishkeeping.com/f...quarium-74891/
I realize I reference Walstad frequently, but simply because she [a microbiologist] has probably done the most research into all this. Her book, Ecology of the Planted Aquarium
, is a solid source of much scientific evidence. Given time, I could pull out articles by other aknowledged authorities to substantiate what is above.