With the materials you have on hand you can sure get the hang of dosing, and balancing the amounts of each needed by the plants. Just add some source of potassium. As noted by Anthony, you will not want to dose the KH2PO4 at high enough levels to supply the K the plants need. That would way overdose the P.
Make a liquid from the KH2PO4 or dose it dry, just do not dose very much.
Seachem Nitrogen: Nitrogen
KH2PO4: Phosphate, and a little potassium, but not much in the low level of dosing that is appropriate for P.
Seachem Iron: Iron.
Seachem Comprehensive: Trace minerals, and a tiny amount of macros.
Seachem also makes a potassium, or you can read the other labels in the fish store to find a good K source.
Once you get the hang of it all you may want to switch over to all dry sources. These can be ordered on line without having to pay for shipping water. Liquid ferts are mostly water with just a little active ingredient. Why pay so much for water?