New Research Comparing Ortho/Poly-Phosphate Ratios


Blog Banner for Poly Phosphate study


  • Ortho-phosphates are 100% plant available, but a high percentage of poly-phosphates in starter fertilizers convert to ortho-phosphate within just two days after application.
  • This quick conversion from poly to ortho-phosphate suggests expensive “high” ortho starter fertilizers are not likely to result in increased corn yields compared to conventional poly-phosphate starters.
  • On-farm field studies conducted near Traer, IA and Walnut, IL from the 2016 to 2018 growing season found no statistical difference (Pr > 0.05) in corn yield between conventional and high ortho-phosphate starters.
  • High ortho starters cost more per/ac than conventional poly-phosphate starters, but do not increase corn grain yields.

Poly-phosphates Rapidly Convert to Plant available Ortho-Phosphates

Given poly-phosphates are not immediately plant available and ortho-phosphates are immediately plant available, this gives the promoters of “high” ortho-phosphate starters ample opportunity to muddy the waters. Nevertheless, the facts are, poly-phosphates are rather rapidly hydrolyzed (converted to) into ortho-phosphates once applied to soils, and this hydrolysis process generally takes just 48 hrs or so to complete.

In Sept of 2015 I posted a blog discussing some of the more technical reasons why the ratio of ortho- to poly-phosphates in starter fertilizers should have no impact on corn yields. For those that are interested in the more technical details, I encourage you to follow this link to
the Sept 2015 blog post (

While we were relatively certain that the ratio of ortho to poly-phosphates in liquid starters should have no effect
on corn yields, I decide to “test” this idea with on-farm field trials located near Traer, IA and Walnut, IL  in the 2016, 2017 and 2018 growing seasons.

tractor planting fertilizer

Picture 1. Planting starter fertilizer trials near Traer, IA in the growing season of 2016.


How the Field Trial Was Conducted

In these field trials we used two starters applied in-furrow at 6 gal/ac. Each starter had a NPK nutrient analysis of 6-24-6. The only difference between these two starters was the ratio of ortho to poly-phosphates. One of these starters contained 80% ortho-phosphate and the other contained just 50% ortho-phosphate. With the remainder of the phosphorous source in each of these two starters being poly-phosphate. At the Traer, IA locations the plots were planted with a 24-row planter (picture 1) and were nearly 2400ft long. At the walnut, IL locations the research was conducted using small plot techniques, plot dimensions there were 10 ft wide by 30 ft long. At both Traer, IA and Walnut, IL in each of the 3 growing season the experimental design used was a simple randomized complete block with 4 or 5 replications.

Figure 1. Average corn yield from field trials comparing high ortho vs conventional poly-phosphate in-furrow seed safe starter fertilizers. Yields at each location/year are averaged over 4 or 5 replications.


Figure 2. Partial profit from field trials comparing high ortho vs conventional poly-phosphate in-furrow seed safe starter fertilizers. Yields at each location/year are averaged over 4 or 5 replications. Partial profit was calculated using a grain sale price of 3.50 bu. Cost per gal used to calculate partial profit for the 6-24-6 50% ortho & 50% poly-phosphate and 6-24-6 80% ortho & 20% poly-phosphate was $2.80 and 3.20 per/gal


Field Trial Results

Averaged over the 5 site-years there was only about 1.5 bu/ac yield difference separating the high ortho and conventional poly-phosphate starter (figure 1). Moreover, this small yield difference was not statistically significant (Pr > 0.05). In addition to finding no differences in grain yield between these two starters, the high ortho starter cost about $0.50 more per/gal (so $3/ac difference in price at a 6 gal/ac rate) than the lower ortho starters. So the more expensive high ortho starter clearly did not “pay” its way in our multi-location field trials (figure 2). Lastly, our observations in these studies agree with previously published university findings (Frazen and Gerwing. 1997).


Franzen D. and J. Gerwing. 2007. Effectiveness of using low rates of plant nutrients. North Central regional research publication No. 341. (accessed 8 of Sept 2015).


– Dr. Jacob Vossenkemper (Agronomy Research Lead)