Phosphorous is a chemical element. It is a metal that bursts into flame in air. Obviously not good as a fertilizer. When chemically combined with other elements, it becomes an easy to handle salt. Phosphate is a chemical compound made up of one atom of phosphorous and 4 atoms of oxygen, which leaves it lacking 3 electrons so it isn't a stable compound, but an ion, which cannot exist by itself. So it has to combine with another element, or elements that provides the missing electrons. The chemical compound we generally use is made up of one atom of potassium, two atoms of hydrogen, and one phosphate combination, KH2PO4. In water, that separates into ions, one of which is a phosphate ion, PO4---, and that ion is the provider of phosphorous for plants. I'm not a chemist, so that is a barely correct answer to your question.