The "soft" in this case really refers to the temporary or carbonate hardness (dKH), theoretically lack of carbonate hardness (as bicarbonate ions HCO3-) can effect biological filtration. This is because the nitrification of ammonia (NH3 as NH4+) utilises both oxygen (O2) and carbonates (HCO3 in solution, CO3 as a buffer).
To balance the equation you get:
55NH4+ + 76O2 + 109HCO3- = > C5H7O2N + 54NO2-+ 57H2O + 104H2CO3
400NO2- + NH4+ + 4H2CO3 + HCO3- + 195O2 => C5H7O2N + 3H2O + 400 NO3-
Which approximates to 4.3mg O2 & 8.6mg of HCO3- are consumed for every 1mg NH3 oxidised to NO3.
I say theoretically, because in the planted tank this isn't really that relevant and potentially you can have very low dKH values. Soft water gives you the potential to grow plants like Tonina, that doesn't do well in harder water, and to keep black water fish, see the excellent "Joe's Tank" <
http://www.ukaps.org/forum/viewtopic.php?f=27&t=16361&p=184861&hilit=Dicrossus#p184861>. Some other plants don't do well in carbonate depleted water, Vallisneria comes to mind, although I believe that if you use added CO2 and EI this isn't necessarily true.
The other thing you need to take into account is the carbonate <-> CO2 equilibrium:
CO2 + H2O is in equilibrium with H2CO3 is in equilibrium with HCO3- + H+
You can alter this equilibrium by adding CO2 or the other way by adding carbonate (disassociated as HCO3-) ions, and this is the buffering we measure as dKH. Details here: <
http://en.wikipedia.org/wiki/Calcium_carbonate#Solubility>
This is the reason people who add CO2 usually buffer their tank water up to 4dKH. You can do this in a number of ways: (from the excellent James' Planted Tank <
http://www.theplantedtank.co.uk/RO.htm>)