[adrian_b]

Sea water contains huge amounts of potassium that is easy to extract.

The most abundant rocks also contain huge amounts of potassium, but which is expensive to extract.

Neither in nature nor in agriculture potassium is ever a limiting nutrient like nitrogen and phosphorus.

[Mistletoe]

I have to object to the last sentence. I’ve gotten lots of soil tests from various locations in soil and greenhouse media where I need more nitrogen (almost always) and potassium (sometimes), but never phosphorus.

And I even live in one of the blue states with lower P in the soil.

https://pubs.usgs.gov/sir/2017/5118/sir20175118_element.php?...

[pfdietz]

There's a gardening channel on youtube I watch (RED Gardens) where the guy does all sorts of small scale experiments with organic gardening. He was a bit surprised to discover his soils have excess potassium, from the various organic materials he had been using (like wood chips for mulch; pot ash, right?) The soils were also sodium deficient, so he was thinking of spreading measured amounts of salt!

He also discovered commercial compost can be crap, since it hasn't broken down enough and soaks up nitrogen as it finishes its decomposition in the ground. So if your potatoes are struggling in something like that, slather on some urea and they'll perk (and green) right up.

[kragen]

if potassium was never a limiting nutrient it wouldn't be sold as a fertilizer at all, much less be one of the three primary fertilizer nutrients

[adrian_b]

You are right that I did not express that well, because as said it is true only in natural places, not in artificial cultures.

For modern cultivated lands potassium must be indeed used as a fertilizer, because much of what exists in the soil is removed by remaining in the plant parts that are harvested then transported elsewhere. Another part of the potassium may be washed away by intensive irrigation.

So the potassium that is taken away depending on the cultivation methods that are used must be replenished. However its abundance is such that this will never be a problem.