practice exam

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given of a modern bank, where the colloids act as "storage facilities" of cations, which will slowly be "deposited" or "withdrawn" as the soil continuously tries to maintain an equilibrium between the colloids and the soil solution.