Confused about differences in phosphorus fertilizer sources? Here are the facts.

By: Dan Kaiser, Extension nutrient management specialist

For most farmers, the source of dry phosphorus (P) fertilizer is limited to what local retailers have on hand. The most common sources sold in Minnesota are mono-ammonium phosphate (MAP) and di-ammonium phosphate (DAP). Triple superphosphate (TSP) and Crystal Green (struvite) are also available in some areas. What are the differences between these sources and how should growers decide which one to use?

What are the differences between MAP and DAP?

The chemical formulas for MAP and DAP are:

• MAP: NH₄H₂PO₄
• DAP: (NH₄)₂HPO₄

Looking at the chemical formulas, the primary difference between MAP and DAP is the number of ammonia (NH₄) ions present. MAP contains one ammonium ion per phosphate versus two for DAP. An additional difference is that, when MAP is dissolved in water, it tends to produce an acidic pH while DAP is more alkaline (basic). Since MAP is acidifying, it provides more available P for high pH soils. However, most research shows little difference between the two when it comes to crop performance and there is no reason crop producers that have high pH (>7.5) soils should seek out MAP if DAP is the source that is commercially available to them.

What about triple superphosphate (TSP)?

Triple superphosphate’s chemical formula is Ca(H₂PO₄)₂. One thing you will notice is that the product is calcium dihydrogen phosphate and does not contain any nitrogen. One interesting fact is that, even with calcium present versus ammonium, triple superphosphate can produce a solution pH ranging from 1 to 3, which is lower than the solution pH of MAP (4 to 4.5).

Triple superphosphate has had a very small share of the phosphate market in Minnesota. However, the product may be available for fall application in areas of the state where fall nitrogen should not be applied. The primary reason why triple superphosphate has decreased in availability in the U.S. is because it contains less nutrients per ton of material. However, if the nitrogen in MAP and DAP is not needed or will be lost before a crop can utilize it, then a phosphorus-only fertilizer source like TSP would be more beneficial for fall applications.

I have had questions on the compatibility of triple superphosphate with other fertilizer sources, but I have never encountered an issue where TSP could not be blended or applied with other fertilizer sources containing nitrogen, potassium, or sulfur. Simply put, TSP is a good alternative fertilizer source that has a similar P₂O₅ concentration to DAP (46%). I do have studies comparing different P sources, but the fact is that I would expect similar yield results regardless of whether MAP, DAP, or TSP are used.

Are there alternatives to MAP, DAP, or TSP other than manure or biosolids?

One additional product I have had questions about is Crystal Green. Crystal Green is composed of struvite, which is magnesium ammonium phosphate (MgNH₄PO₄·6H₂O). So, the product is essentially MAP (NH₄H₂PO₄) with magnesium (Mg) but only contains 28% P₂O₅ by weight, compared to MAP which is 50-52% P₂O₅. Struvite is a by-product produced in the waste streams in factories or, most often, from wastewater treatment plants. Struvite can be granulated so it can be blended with other fertilizer sources. Struvite is less water-soluble than other commercial fertilizer sources.

One claim I have heard is that struvite’s lower water solubility makes it a better choice for P application in high pH soils because the low water solubility prevents P from reacting with calcium, and plant roots will intercept the unreacted P while the acids in the roots will help dissolve the struvite. However, most research in the North Central states seems to point to similar to slightly lower initial availability of P from struvite compared to MAP, with similar yield produced with similar rates of P applied. Simply put, struvite could provide an alternative source of P but the data does not support struvite being more effective than any other source of commercially available P fertilizer.

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