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Minnesota Crop News > 2001-2008 Archives
March 7, 2002
Orthophosphate and Polyphosphate: Is There a Difference?George Rehm, Extension Soil ScientistThere's no question. Everyone understands that phosphorus (P) is essential for crop growth. When the P supplied by the soil is low, use of phosphate fertilizers are essential for optimum crop production. In modern fertilizers, P exists as either orthophosphate or polyphosphate or a combination of both. The fact that P in fertilizers is present in either form, of course, leads to a variety of "sales" claims. Exposed to these claims, the grower is frequently confused and asks if there is any difference between orthophosphate and polyphosphate. In general, P in dry fertilizers such as 18-46-0 and 11-52-0 is present as orthophosphate. In contrast, the P in common fluid fertilizers such as 10-34-0 and 7-21-7 is present as both orthophosphate and polyphosphate. In these fertilizers, approximately 2/3 of the P is present in the polyphosphate form. The remainder is orthophosphate. Some fluid fertilizers such as the 9-18-9 materials, however, have all of the P present in the orthophosphate form. The effect of both forms of P on crop production has been evaluated in various field trials throughout the Corn Belt. The results from a study with corn in Nebraska are summarized in the following table. The effect of polyphosphate and orthophosphate on corn yield
ave of 5 sites; low P test The increase in yield produced by the use of both phosphate fertilizers is obvious. This would be expected when soil test values for P are in the low range. It's also obvious that the source of P had no effect on corn production. The fact that both sources of P have an equal effect on crop production should not be surprising if there is some understanding of the processes that take place when both sources of P are added to soils. To begin, it's important to understand that plants absorb or take up P in the orthophosphate form. A unit of orthophosphate consists of phosphorus surrounded by oxygen (O) and hydrogen (H). If water is removed when two units of orthophosphate come together, pyrophosphate is formed. This linkage of two orthophosphate units is the least complicated of all polyphosphates. It is possible to link many orthophosphate units together producing a complex mixture of various polyphosphates. When pyrophosphate or any other polyphosphate is added to
soil, the linkage of orthophosphate units is broken and the
orthophosphate units are formed. This breakdown is referred
to as hydrolysis. This breakdown is both a biological and chemical
process and is complete in about 16 days for all conditions.
The effect of soil temperature of the hydrolysis of polyphosphate
fertilizers added to two soils is illustrated in the following
figure supplied by Dr. Raun Lohry of Nutra-Flo Fertilizer in
Sioux City, Iowa. The rate of breakdown (hydrolysis) for two polyphosphate materials is shown. The pyrophosphate form (curves A and B) is not complex. The hydrolysis of a more complex polyphosphate (curves C and D) is also shown. When the polyphosphate fertilizers were applied to both soils, The rate of hydrolysis (breakdown) increased as soil temperature increased. It's also apparent that the majority of the hydrolysis was completed before 48 hours after application to the soils. Therefore, P in either 10-34-0, 7-21-7, etc. should be hydrolyzed to orthophosphate within a week after application. With the speed of hydrolysis of polyphosphates understood, it's an obvious conclusion that the availability of P in 10-34-0, 7-21-7, 9-18-9, 18-46-0, and 11-52-0 should be equal. So, performance of 9-18-9 should be equal to the performance of 7-21-7 or 10-34-0. Use of liquid and dry fertilizers as equivalent materials applied at the same rates should have an equal effect on crop production. Results of various field trials have shown that this is the case. Therefore, Minnesota crop producers should not base choice of fertilizers on orthophosphate or polyphosphate claims. |
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