Potassium, nitrate, and certain traces like magnesium sulfate, may be dry dosed, typically because you'll need quite a bit.
But trace mixes like plantex CSM, or any phosphate should be made into a dosing solution. It would be hard to measure 1/16th tsp of
PO4, even in grams...and a slight overage would yield too much
PO4, and a possible algae bloom.
I like mixing my
PO4 so that each mL is .1ppm in my 75gal. This works out to about .37ppm in my 20gallon tanks, so while I may have to dose more mL in my 75, the solution is universal for all my tanks...so I have fewer dosing bottles laying around.
If you find you dose very little KNO3, you might make a solution for it too. I wouldn't advise making a solution of KCl or K2SO4 because you'll go through it rapidly, especially since KCl is harder to fully dissolve at room temperature without a ton of water.
I never like to totally dry dose in my tanks, mainly because my fish are pigs, and while I hear lots of people say "my fish eat the grains of KNO3 all the time, doesn't hurt em" I still just can't be sold on the idea that its 100% safe.
Thus I keep a little 10oz rubbermaid container by the tank, so I can scoop up a cup of water, add my dry ferts, shake (not stir) and dose. My trace,
PO4 and Iron are all liquids.
You can dose Iron and
PO4 at the same time, but not in the same container. Adding them to the same container will form a precipitate like Simpte mentions, which is insoluable and thus a waste of good ferts because the plants will never absorb them.
But add them to the tank from 2 separate containers, and they won't be in contact long enough to precipitate.
