Q & A's on Phosphorus Fertilizers

Q & A's on Phosphorus Fertilizers

April 17, 2009

Q1. Is orthophosphate better than polyphosphate?

Polyphosphates are formed when superphosphoric acid and ammonia are combined to produce ammonium polyphosphate (10-34-0), ammonium polyphosphate (11-37-0), or urea-ammonium phosphate (34-17-0). Orthophosphate fertilizers include triple super phosphate (0-46-0), monoammonium phosphate (11-52-0), and diammonium phosphate (20-46-0). In liquid polyphosphate fertilizers, about 60%-70% of the phosphorus (P) is ortho- and 30%-40% is polyphosphate (P chains composed of two or more phosphorus ions). Once applied and in contact with soil water, the polyphosphate quickly begins reverting to orthophosphate, with most converted in two days and complete hydrolysis within a month. Therefore, plant available P from both types of fertilizers is orthophosphate. As should be expected given this information, trials conducted in the Midwest including Nebraska confirm that crop yield responses are similar for both types of phosphorus fertilizers. Neither polyphosphate nor orthophosphate ions contribute to the salt effect of fertilizers.


Q2.Are products containing Penicillium bilaii likely to boost phosphorus recovery?

Penicillium bilaii is a fungus known to produce oxalic and citric acid and increase calcium P availability in incubation studies. We ae not aware of any trials conducted in Nebraska and little has been published on Penicillium bilaii effects on crop P uptake in peer-reviewed research journals. A few articles were found through an Internet search where Penicillium bilaii application resulted in: 1) a yield decrease for field pea in Montana; 2) no lentil yield increase but a response to applied P in Manitoba; and 3) a mean yield increase of 7.6% across diverse crops on low P soils in Canada. It is most likely to work on calcareous soils that test low in available P and unlikely to be beneficial if soil pH is less than 7 or if soil test P is high. If considered for calcareous soils, apply it in strips across the field and then measure the yield of strips, such as with yield mapping or with a weigh wagon, to compare the strips with and without Penicillium bilaii applied.

Q3. Do we have efficient use of fertilizer P on Nebraska soils?

First, we need to put P recovery into proper perspective. Once phosphorus fertilizer is applied to soil, it begins a process of reverting to insoluble forms that have lower plant availability. This process occurs over several years but this P fixation results in other soil P becoming more available. Plants use both soil and fertilizer P and only a small percentage of the fertilizer P is taken up by the crop during the year of its application. However, over a period of years, we find that P recovery is high. This is demonstrated by substantial increases in soil test P when P is applied on a P removal basis for several years.


Q4. Are high soil test P levels needed for high yields?

In recent research in Minnesota, corn and soybean yields were higher with high soil test P compared with low soil test P with fertilizer P applied. If this claim were widely true, it would challenge our P sufficiency based recommendations, that is, to apply P only when soil test P is at or below the critical level. In 34 trials for high yield corn, maximum yield with P applied did not differ with soil test P which ranged from 1 to 121 ppm with an average of 23 ppm. This is evidence supporting the UNL P sufficiency based recommendations for corn; however, mean corn yields with P applied, at a site in northeast Nebraska have been higher with very high soil test P compared to medium soil test P.


Q5. Is liquid more effective than dry P fertilizer?

No, they are equally effective. Once applied to soil, mobility and chemical reactions of liquid and dry P fertilizers are the same. Consider product costs on a phosphorus per acre basis and compatibility of liquid versus dry with your equipment and operation.

Alternative Products for Soil Fertility Management

Many products are promoted for enhanced plant nutrition. Some are short-lived and others stay around. Some are conceptually sound, generally for particular situations (e.g. for areas of high leaching or situations of high volatilization potential) but many are not conceptually sound. To provide further product information, Iowa State University maintains the Compendium of Research Reports on Use of Non-Traditional Materials for Crop Production for the North Central Region at http://extension.agron.iastate.edu/compendium/index.aspx. At one count, it had information for over 500 products.

It is a difficult compendium to maintain as many of the products are short-lived and marketing is often localized; we often first learn of a product when a producer calls to ask about it. In early 2008, we contacted several companies supplying such products and offered to test the products on a fee basis, similar to UNL testing of varieties and pesticides; none of these companies requested testing.

Contributed by Charles Wortmann, Richard Ferguson, Gary Hergert and Charles Shapiro
UNL Extension Soils Specialists

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