Interesting read with the recent resurfacing of the poly vs ortho debate. Seems like every 4 years this comes up at winter meetings.
Many farmers and ag-professionals have heard the terms “poly” and “ortho” thrown about in various marketing efforts for a variety of fertiizers. By definition, “ortho” refers to “orthophosphate” and “poly” refers to “polyphosphate”. Both are forms of phosphate in a variety of fertilizers used to supply P in crop production.
Orthophosphate is a negatively charged anion and is the form of phosphorus absorbed through the root and taken up by the plant. Polyphosphate can be thought of as a string of orthophosphate anions hooked together via chemical bonding. The size or length of the string is not fixed and can be of various lengths. Thus “polyphosphate” is simply strings of “orthophosphate” ions of various lengths. Polyphosphates are not absorbed by plant roots. In general, the phosphate in dry fertilizers exists in the “orthophosphate” form. Fluid fertilizers, on the other hand, contain phosphate in a combination of the “orthophosphate” and “polyphosphate” forms. There can always be an exception. In some fluids, all of the phosphate is present in the form of orthophosphate.
Should we expect the form of phosphate applied for corn production to affect yield? This is a frequent question from anyone who has been exposed to information originating from those who market fluid fertilizers that contain contain phosphate in the orthophosphate form. We can answer this question without considering results of any field research if the chemistry of polyphosphate in soils is considered.
When polyphosphate is added to soil, there is a chemical reaction whereby the polyphosphate is converted to orthphosphate in a short period of time. This is a purely chemical reaction and no bacteria are involved. The conversion takes 7 to 10 days to complete if soil temperatures are higher than 50 degrees F. So, if both forms of phosphate were applied to soil to supply the same rate of phosphate, equal yields should be expecteed.
Comprehensive field studies conducted in Nebraska have shown that both form of phosphate have an equal effet on yield. The calcareous soils at each of the five sites had a low soil test for phosphorus. The soils at the sites used for the study summerized in the following table had a low soil test for phosphous. The yields listed in the following table are averages for those five sites.
15 124 124
30 134 134
Average of five sites: calcareous soils
There was an increase in corn yield when each rate of phosphhate was applied. That would be expected if there was a low soil test for phosphorus. It’s also obvious that yield was not affected by the form of phosphte applied.
These results lead to the conclusion that a pounf of phosphate in a dry fertilizer (orthophosphate) is equal to a pound of phosphate in a fluid fertilizer (polyphosphate and/or orthophosphate). In the common fluids such as 10-34-0 and 7-21-7, approximately 2/3 of the phosphate is present as polyphosphate. The remainder is present as orthophosphate. So, if applied to supply the same rate of phosphate, all phosphate fertilizers . should have the same effect on yield.
The marketing information from companies that promote the use of fluids where phosphorus is in the orthophosphate form tends to add some confusion to the issue. This confusion can be attributed to a lack of understanding of “availability” and “uptake” or “utilization”
When fertilizers are in the tank or the fertilizer bin, the”availability” of the phosphate is the same regardless of the source. There are analytical procedures that can be used to measure availability and this analysis shows that availability of phosphate in dry fertilizers is equivalent to the availability of phosphate in fluids. Likewise, availability of phosphate in all fluids is equal.
The chemistry of the soil (percent calcium carbonate, organic matter percentage, etc.) determines how much will be utilized. At best, approximately 30% of applied phosphate is utilized (taken up) by corn in the year of application regardless of source. In calcareous soils, this percentage is lower.
In the fertilizer industry, the phosphate fertilizers are priced over a wide range. The fluids that have all of the phosphate in the orthophosphate form usually have the highest price tag. That high price is not justified with a superior crop response. Fertilizer prices are high. So, it’s a good idea to ask questions about the fertilizer before purchasing something new and high priced.