Are you referring to what happens in coco coir when it hasn't been properly produced? Because the K (along with the Na) takes up all the ionic positions available leaving the Mg (and Ca) to float away with the next watering? (ohyay and I now understand why too! it's because the K and Na only have one electron in their outer orbital, making them way more likely to share that than the Ca and Mg with their outermost sp-orbital complete! So in the ionic musical chairs game, K and Na are always going to be quicker, so we don't want much of that around ) This is very reassuring I've been conundering over this with regard to hi-K amendments, and whether the K then accumulates to the point of toxicity in the soil (nods to @MustangStudFarm ) if we, say, keep feeding our no-tills with comfrey or borage (which both bring along high amounts of potassium, if the patchy data I could collect from the web is anything near correct) over long periods of time. So there goes that conundrum to the back burner, not quite gone, but less urgent now Cheers!