There are three types of potassium phosphate, namely, monopotassium phosphate (KH2PO4), dipotassium phosphate (K2HPO4), and tripotassium phosphate (K3PO4). All three are salts of phosphoric acid (H3PO4) and, like other potassium compounds as a general rule, are soluble in water.
Monopotassium phosphate is weakly acidic, meaning it gives hydrogen ions away to water molecules when dissolved. Dipotassium phosphate is weakly basic, meaning that it tends to take up hydrogen ions from water, and tripotassium phosphate is more strongly basic. When dissolved in water, these compounds can act as buffers, meaning that they help to buffer the solution against changes in pH.
Dipotassium phosphate is often dissolved in water then sprayed on leaves both to help control fungal diseases in certain ornamental plants and act as a fertilizer. Monopotassium and tripotassium phosphate are used in a similar fashion, although the pH of the water is different in each case. Potassium phosphates are most suitable as fertilizers for plants in cases where it's necessary to limit nitrogen input since all three are nitrogen-free fertilizers.