Orthophosphate vs polyphosphate fertilizers

Kolby R. Grint, Northwest Regional Specialist, NPM program, UW-Madison

Orthophosphate vs polyphosphate fertilizers, does it matter?

When assessing phosphorus fertilizer options, some information sources may recommend using fertilizers with phosphorus available primarily in the orthophosphate form (i.e. monoammonium phosphate, diammonium phosphate, triple super phosphate) over fertilizers with phosphorus in the polyphosphate form (i.e. ammonium polyphosphate, urea ammonium phosphate). The argument is often that phosphorus is more immediately plant available in the orthophosphate form. However, decades of research work has shown, in most environments, there is little agronomic difference between the two chemical forms of phosphorus fertilizers.

Commercial phosphate fertilizers that are used in agriculture are produced by a series of processes that result in phosphoric acid. These phosphoric acid fertilizers contain phosphorus as orthophosphates, polyphosphates, or a mixture of the two. Chemically, orthophosphates only contain only one phosphate group, whereas polyphosphates are chains of multiple phosphate groups. The process for creating fertilizer sources with polyphosphates involves the conversion of orthophosphates into the polyphosphate structure and the process typically doesn’t convert all orthophosphates into polyphosphates, meaning products identified as polyphosphate phosphorous fertilizers also contain orthophosphates. Once applied to the soil, orthophosphates often quickly react with water to form plant-available H2PO4- or HPO42- ions. Polyphosphates fertilizers must react with water and enzymes to convert over into these plant-available forms, a process often completed within a few days to a couple of weeks during the growing season, depending on soil temperature. This process occurs faster at higher temperatures. While the initial release of plant-available phosphorus is a little slower with poly vs orthophosphates, phosphorus supply is typically sufficient to meet the nutrient needs of the growing crop. There is little scientific evidence the use of orthophosphate fertilizers improves crop productivity compared to the use of polyphosphates.

While phosphorus form should be of minimal concern in most circumstances, due to the differences in manufacturing processes and inputs between liquid phosphorus fertilizers, poly vs orthophosphate products may have different salt indexes, a consideration when using the products as a pop-up starter.

For additional information about phosphorus fertilizers check out the University of Minnesota resource, Understanding Phosphorus Fertilizers.