Sorry Rex, your conclusions are just erroneous. Just following your assumption further, in heavy water column fertilization stem plants would have NO need to grow any roots if they were only seeking out nutrient!??!
original kuhli is correct.
A large root mass means that the plants are searching for nutrients. So when you pull plants out of a substrate and see a small mass of roots that means they are not having to search for nutrients in the substrate. Plants don't waste energy growing roots they don't need.
Roots serve several functions in addition to nutrient uptake. Roots also serve to help ancor a plant in place and provide support to above ground portions. For some species by spreading their roots they also serve another important biological function: reproduction. Plants like Cryptocoryne
can produce new plants from their expanding root mass. Plants also store "food" in the form of starches in their root system and can use these at later times; some even regrow entirely from the root when the above ground stems and leaves are damaged (due to weather extremes or predation).
Two, small root systems can indicate quite the opposite of a plant living a happy, luxurious life finding all nutrients easily; small, puny roots can sometimes indicate very bad conditions and unhappy plants. Think: soil compaction and anerobic conditions.
Thirdly, per original kuhli:
I'd suggest that the fluorite has less root growth due to higher nutrient levels, and that the Soilmaster develops more roots to access the relatively (but adequate) scarce nutrients.
Again, the opposite is true: Soilmaster has a greater nutrient breakdown than Flourite. Soilmaster also has a CEC that by far exceeds Flourite; I'm not a plant scientist but I wonder if some of the better root growth may be due to the greater CEC?
Here is a link to an article which has a nutrient analysis chart for over a dozen different substrates (the chart is near the end of the page).
Substrates for the Planted Aquarium
While Soilmaster itself is not listed, I believe it to be VERY close in make up to both Profile and Turface.
Just a few comparisons:
Nutrient ---- Profile ---- Flourite
Ca ---- 12800 ---- 520
Fe ---- 13200 ---- 9610
K ---- 3920 ---- 1710
Mg ---- 4220 ---- 1490
33.2% ---- 1.7%
Once again, I LOVE my inexpensive Soilmaster!