Phosphorus Fertilizer Forms: Orthophosphate or Polyphosphate (Ortho-P or Poly-P)

This article originally appeared in the AGVISE Laboratories Spring 2024 Newsletter.

Each spring, we get questions about what form of phosphate fertilizer is better. The debate over orthophosphate and polyphosphate has raged for decades. Just when we think we have put the issue to bed, it comes up again! These questions originate from the simple fact that orthophosphate (ortho-P) is the form that plant roots are able to uptake. This simple fact, however, is a simplistic understanding of phosphorus fertilizer forms and chemistry in soil. The truth is either form of phosphate fertilizer form (ortho-P or poly-P) will provide the same crop yield response when applied at the same rate of phosphorus. With “new” fertilizer products and additives on the market, it is time to revisit the basics of phosphorus fertilizer materials and their behavior in soil.

All dry phosphate fertilizers are orthophosphate forms; this includes monoammonium phosphate (MAP, 11-52-0), diammonium phosphate (18-46-0), and triple superphosphate (TSP, 0-46-0). Yes, this means that all dry phosphate fertilizers are orthophosphate forms. Liquid phosphate fertilizers are usually some blend of orthophosphate and polyphosphate of varying proportions. There are pure liquid ortho-P fertilizers available, but the liquid poly-P fertilizers will also contain some smaller proportion of ortho-P in solution too. Either way, all phosphorus fertilizer materials, dry or liquid, ortho-P or poly-P, will act the same once applied to soil.

In fertilizer manufacturing, all products start as phosphoric acid derived from rock phosphate. You could use liquid phosphoric acid as a phosphorus fertilizer material itself, but it is corrosive and difficult to handle. The next step is converting phosphoric acid to a more stable product that is easier to handle. If you want to make a high concentration liquid phosphorus fertilizer, you must also remove some water from the phosphoric acidic. The dehydration process (removal of water) produces linked chains of orthophosphate, which are called polyphosphate chains, resulting in a denser and more concentrated liquid phosphate material. The name “polyphosphate” literally means “many phosphate.” The dehydration process is never 100% efficient, so some orthophosphate always remains in solution. In most polyphosphate fertilizers, like 10-34-0, the breakdown is around 75% polyphosphate and 25% orthophosphate. This results in a higher concentration liquid phosphorus fertilizer with more pounds of phosphorus per gallon, meaning that you have to haul less material to the field to achieve the same phosphorus rate.

As a fertilizer applied to soil, does this mean polyphosphate is less available than orthophosphate to plant roots? When polyphosphate is applied to soil, the fertilizer reacts quickly with water in soil and breaks into orthophosphate again. The “many phosphate” becomes normal orthophosphate again, and the rehydration step is very fast. Even at cool soil temperatures (40 deg F), over 40% of polyphosphate converts to orthophosphate within 72 hours. Within one to two weeks, the conversion is complete, leaving all plant-available orthophosphate for crop uptake.

In the end, it all comes back to crop yield, right? In studies across the Midwest, Great Plains, and Canadian Prairies, the performance of orthophosphate and polyphosphate have been equal, as long as you apply the same rate of phosphorus. As stated earlier, all phosphorus fertilizer materials, dry or liquid, ortho-P or poly-P, will act the same once applied to soil.

BONUS A unique property of ammonium polyphosphate (APP, 10-34-0) is the ability to chelate or sequester metal cations, such as micronutrients like zinc, in the polyphosphate molecule. This allows polyphosphate solutions to maintain a higher concentration of micronutrients in solution than pure orthophosphate. Ammonium polyphosphate can maintain 2% Zn in solution, compared to only 0.05% Zn with pure orthophosphate.