Is Orthophosphate Better Than Polyphosphate?
To answer this question, it’s important to understand the difference between these two forms of phosphorus. The phosphorus in the phosphoric acid used to make most dry phosphate fertilizers as well as a few liquids is in the orthophosphate form. If ordinary phosphoric acid is heated, water is removed and the orthophosphate ions combine to form a polyphosphate. This process does not convert 100% of the orthophosphate ions into the polyphosphate form. Most polyphosphate fertilizers will have 40 to 60% of the phosphorus remaining in the orthophosphate form. In the soil, polyphosphate ions readily convert to orthophosphate ions in the presence of soil water. This conversion is rapid and, with normal soil temperatures, can be complete in days or less. This conversion process is enhanced by an enzyme called pyrophosphatase, which is abundant in most soils. Polyphosphates are usually marketed as liquid ammonium polyphosphate fertilizers. Because water is removed in the manufacturing process,
