This has kind of been prompted by a recent thread elsewhere, but I thought I'd put it here. What I wanted to ask about is what limits growth. As far as I can see, macro and micro nutrients don't limit growth as such, otherwise we wouldn't see deficiencies - growth would just slow down, but it would still be healthy. Similarly, light isn't a limiter as plants still grow, but they are pale and spindly. So, is CO2/carbon a limiter? Do you see carbon deficiency? What do people think?