In the case of P and K, fertilizer nutrients not used the first year contribute to plant uptake in following years as a part of the soil reserve. Orthophosphates versus Polyphosphates Without going into details, there are two primary methods to produce phosphate fertilizers. This is the method used to manufacture most dry fertilizers. This form of phosphate is referred to as polyphosphate. Polyphosphates are simply long chains of orthophosphates.
Most liquid fertilizers contain polyphosphates.
When introduced into the soil, polyphosphates react with water relatively quickly, producing orthophosphates again. The majority of plant uptake of P is in the orthophosphate form. As mentioned earlier, there are good reasons to use liquid fertilizers. They're easy to handle and meter more precisely. However, they are not more available than dry forms.