Also, how is calcium phosphate not formed in waters where both dissolved Ca++ and PO4 are present?
They clearly are present in my tanks and are at ample concentrations as well as much higher levels in Hoagland's modified hydroponic solution etc.
At certain pH and heat, it will form.........
Calcium phosphate is a salt in most forms, why do not K+ and Cl- ions form solids in water?
Same sort of thing here.
Here's my tank with plenty of Ca and plenty of PO4:
About 30ppm of Ca++ and about 6-9ppm of PO4 residuals.
Dosed 15ppm of PO4 and 45ppm of Ca++ most weeks total, good size water change removes a fair amount of build up.
These are not closed systems, plants will take up the ferts, the Cl-(which is an essential nutrients, thus the statement that plants do "not like any Chloride" is false) will be taken up and sequestered or trimmed out and exported.
This chemical is not that soluble compared to other potassium salts like mono basic potassium phosphate or KCL, so I'd use those for ease of use.
Some salts can have a slight effect on pH BTW, but this is minor in these cases above. A more common type, baking soda.... is a salt, but does have an effect on aquarium pH's.
While of interest, the bottom line is just to use KCL or something for K+ and KH2PO4 for PO4 etc.
If you use KNO3, it's not likely you will need any added K+ and you also will get a little from KH2PO4.
So if you using those, for 2-3$ per lb, and they are highly soluble, easy to get etc........
No real issues or debate.