You don't want carbon in a planted tank because it DOES absorb the nitrates, phosphates, and trace elements the plants need to thrive.
If you have more than 1.5watts per gallon of water in the tank, then you should be addnig CO2. Think of CO2, light, and fertilizer as the 'holy trinity' of a successful planted tank. As soon as you get light cranked up above 2 wpg, CO2 is mandatory, and without it you'll end up with more algae, and weak looking plants.
Believe me, carbon on a planted tank is a no-no.