Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Plants will, sooner or later, use up all the nutrients in the soil.
Sooner: with high light, CO2 they will use up nutrients that:
~are already in short supply
~are used the most
~are not being replenished.
Later: with lower light, less CO2 they will not use up the nutrients so fast, but eventually they will.
So it is better to start supplementing the tank earlier rather than later. The substrate will latch onto many of the nutrients and hold them in reserve for the plants.
Sources of nutrients may include fish food (N, P, most traces, but not K, Ca, Mg or Fe)
fertilizer tablets, ferts dosed to the water column, water changes (especially Ca and Mg)
If you wait until deficiencies show up, the plants growth will already have slowed before the discolored leaves etc. show up. Algae will already have the signal (whatever it is!) and be growing.