practice exam

Active acidity is the relative concentration of H+ and Al3+ ions in the soil solution, whereas Exchangeable Acidity, as the name suggests, are the H+ and Al3+ ions that are loosely held by the soil colloids and can be replaced (exchanged) by the addition of a salt solution. This is exactly what we did in the lab, when we mixed the soil sample with calcium chloride to knock "acid ions" off of the exchange complexes of the soil solids.

i see your point, but what is the distinction related to the plant growth?

a few thoughts: the ions IN SOLUTION are what is of immediate consequence for plants, ie. nutrients available for uptake, and exposure to potentially toxic or damaging compounds. we've talked about buffering capacity in relation to pH, so a soil with a lot of colloids helps to "protect" plant roots from exposure to acid cations by adsorbing them onto the solid particle, where they would be held in exchangeable form (exchangeable acidity) and thus, not directly affecting the plant roots. since most plants like a neutral pH a soil that can buffer against the addition of acid forming ions will help to maintain an ideal environment. plant growth also depends on nutrients, so remember the analogy we were given of a modern bank, where the colloids act as "storage facilities" of cations, which will slowly be "deposited" or "withdrawn" as the soil continuously tries to maintain an equilibrium between the colloids and the soil solution.