Hi all,
potassium nitrate (KNO3) will remain in solution as K+ NO3- ions right up to when the solution is fully saturated. That is why KNO3 is used in soluble fertilisers, you can make very concentrated stock solutions that you normally dilute 1:200 to use in hydroponics or as a liquid feed etc. You can add K and N until they reach toxic levels, but if it is another element that is limiting plant growth, there will be no effect from the increased levels (until toxic levels are reached).
Iron (Fe) has to be oxidised and chelated in solutions above pH7 , or it forms insoluble iron phosphate compounds. The chelation only works because, although the in Fe EDTA is very tightly bound (it is the most tightly bound ion), it is photo-degraded and Fe then becomes available.
Phosphorus also forms insoluble calcium phosphate compounds at pH above 7, and if you have a very large reservoir of calcium carbonate (really Ca 2+ ions present), phosphate may be the limiting nutrient as it will all rapidly become insoluble and unavailable.
Magnesium may be available, but when the calcium magnesium ratio is very high, many plants find it difficult to uptake Mg, which is an important component of chlorophyll.
Last but not least 50 degrees of hardness is very hard water, each degree of hardness is equivalent to 17.85 ppm of CaCO3, and anything above 30 degrees is "very hard" according to that unimpeachable source "wikipedia".
cheers Darrel