Exchangeable Potassium

Lucas, I would not mix concepts - although in this example both are related to exchange reactions.

1) active acidity - due to H⁺ and Al³⁺ ions in the soil solution; Exchangeable acidity - associated with H⁺ and Al³⁺ ions that are easily exchanged by other cations in the soil solution; and Reserve acidity - associated with H⁺ and Al³⁺ ions that are bound (non-exchangeable) on soil particles.

2) K cycle - the main sources of K are: minerals such as micas and K-feldspar (weathering releases K); SOM; and potash fertilizer (KCl a soluble salt). There is a gradation between exchangeable and "fixed" K. K⁺ electrostatically attracted to negative charges on soil colloids (e.g. clay or SOM) is exchangeable. The non-exchangeable pool contains less available K+ ions that are trapped in between the structure of the clays and are released slowly by weathering. The fixed or mineral pool contains K held within the mineral and so is very slowly by the weathering.

Regarding the role of K in drought tolerance it is my understanding that K plays a role in guard cell regulation and thus in the opening and closing of stomata.

Thanks Sandra!