Josh~ Superphosphate is used instead of just phosphate because superphosphate is a compound whereas phosphate is an ion. This means that phosphate must attach itself with a cation in order to give a plant its phosphorus supplement, it could do this by attaching itself to 3 calcium to make tribasic calcium phosphate but that is insoluble in water and Plants can only absorb materials when they are dissolved in water. In chemistry ‘like dissolves like’ so nonpolar solutes can only dissolve in nonpolar solvents, water is polar and tribasic calcium phosphate is nonpolar so phosphate cannot dissolve in water. This is why Superphosphate is so great because it has phosphorus and it can provide it because it is soluble.
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Ions are always dissolved in their solution. Anything that has charge is an ion. If phosphate ions are present in a solution it is because they came from a soluble parent molecule such as phosphoric acid.