I don't buy the "not having CO2 eliminates 90% of your aquarium plants". That sounds like someone who wants to sell expensive CO2 systems.
There are quite a few tanks I've personally seen with soil capped with gravel, or substrate w/ Laterite heating, and really small amount of CO2 injection to none that have done great. You may have a bit slower growth, But not necessarily.
What I'm seeing looks either like a lack of Potassium and (or) a Pleco or Oto, or snail eating a leaf that is in less than healthy shape.
Two experiments you might give a try is picking up some Morton No Salt substitute which is KCl Potassium Chloride, a common Potassium amendment. Or if your feeling a bit more adventurous, check out the Vitamin aisle at your local store for some Potassium supplement, usually will be about 99 milligrams elemental K, in a Gluconate base with a little gelatin encapsulation. There might be a little Magnesium Stearate in the mix, all of which shouldn't hurt your tanks chemistry.
There's online calculators to figure out the tanks water volume to mix the KCl with. Consider that with elemental Potassium, you really can't over dose with. But should exercise restraint if the additional Chlorides change the water's chemistry too much.
With Potassium Gluconate you should know that Seachem's Flourish uses Iron Gluconate, and with any Gluconate, you will have the effect of adding bio-available carbon, and would boost your plant growth like adding CO2. You would have to add a lot of supplemental Potassium Gluconate to equal the same amount of KCl, almost 4 times as much. This might add too much Gluconate, I'm not sure it's a great idea, but Gluconate has the added advantage of degrading rapidly in water under biological influences. It's basically a form of blood sugar.
The key word in either case here is exercise restraint.
