Hi all,
Akadama is a clay, and clay minerals have the ability to hold on to and exchange ions. All salts (products of the neutralization reaction of an acid and a base), when they dissolve, go into solution as cations and anions, in the case of NaCl (sodium chloride or salt, hence "salts") this is Na+ the sodium ion a cation, and Cl-, the chloride ion, an anion.
The ability of any compound to hold on to and exchange cations is called its Cation Exchange Capacity or CEC (it will also have an Anion Exchange Capacity - AEC).
Sand has no CEC, but clay minerals have high CECs, this is why we like them for the planted tank, in the case of Akadama it is clay from an area which is naturally poor in "bases" or alkaline ions such as calcium and magnesium. The technical term is that it has a low "percent base saturation", and it means that most of the exchange sites naturally have an H+ ion. When you add Akadama to a solution containing K+, Ca2+, Mg2+ ions (your tank water), the H+ ion will be exchanged for one of the other cations, and the solution may become more acid. If you soak the Akadama in a solution of calcium chloride or magnesium sulphate, the H+ ions will be exchanges for Ca2+ or Mg2+ ions before it gets in the tank. The dGH we measure is the amount of 2+ ions, therefore solutions of calcium chloride and/or magnesium sulphate are the "strong GH" solutions it talks about.
Monovalent ions (like K+) will also replace H+ ions, but they don't add any dGH. This is actually a 2 way process and those ions can be replaced by H+ ions etc and become available to the plant as the concentration of the ions change around the negatively - charged exchange sites of the clay mineral. This is the "LYOTROPIC SERIES":
most tightly bound H+ = Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+ least tightly bound.
Adding large amounts of one cation will replace others, regardless of their position in the lyotropic series. For example Na+ could replace Ca2+ on the mineral's exchange sites if sufficient concentration of Na+ existed in tank water.
cheers Darrel