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University of Minnesota Extension

Understanding phosphorus fertilizers

When producers pay special attention to managing phosphorus (P), it can lead to profitable crop production. The best way to use fertilizers to meet P requirements changes with crop, soil properties and environmental conditions.

Finding the best P source

Inorganic commercial P fertilizers have evolved over the last several decades into a refined, predictable product. Plus, there are the organic P sources closely associated with livestock operations or with proximity to major metropolitan areas.

There should be no difference in P fertilizer sources, as long as nutrient analysis differences are taken into account. While there are certain situations where one product performs better, phosphorus fertilizer recommendations are the same regardless of the phosphate fertilizer source.

How commercial phosphate fertilizer is manufactured


Phosphate fertilizer terminology and sources

Selecting a phosphate fertilizer can be confusing due to all the products on the market. Understanding the terminology may help avoid some of the confusion.


Comparison chart: Common fertilizer sources

Table 1: Percentages of water-soluble and available phosphate in several common fertilizer source

P2O5 source N Total Available P2O5 Water soluble* P2O5
Superphosphate (OSP) 0% 21% 20% 85%
Concentrated Superphosphate (CSP) 0% 45% 45% 85%
Monoammonium Phosphate (MAP) 11% 49% 48% 82%
Diammonium Phosphate (DAP) 18% 47% 46% 90%
Ammonium Polyphosphate (APP) 10% 34% 34% 100%
Rock Phosphate 0% 34% 3% 0%
*Water-soluble data are a percent of the total P2O5.
Source: Ohio Cooperative Extension Service.

Organic phosphorus sources

Organic P fertilizers have been used for centuries as the P source for crops. Even with the advent of P fertilizer technology processes, organic P sources from animal manures – including composts – and sewage sludge are still very important.


How crops respond to phosphate fertilizers

If the level of available P in the soil isn’t adequate for optimum crop growth, use phosphate fertilizers to ensure adequate amounts of this nutrient in the solution phase.


Predicting the need for phosphate fertilizer

Phosphorus soil tests measure soil’s ability to supply P to the soil solution for plant use, but do not measure the total quantity of available P. These tests provide an availability index of P in soils that relates to the phosphate fertilizer’s ability to provide an economically optimal increase in yield.

The relationship between the P determined by a soil test and the phosphate fertilizer requirements are developed from the results of numerous research trials that measured various rates of applied phosphate and yields.


Managing phosphate fertilizers

Because P isn’t mobile in soils, placing phosphate fertilizers is a major management decision in crop production systems. There’s no special placement that’s ideal for all crops. Decisions about placing phosphate fertilizers are primarily affected by the intended crop and P soil test level.


Frequently asked questions: Phosphate fertilizers


Daniel E. Kaiser, Extension nutrient management specialist and Paulo Pagliari, Extension soil scientist


Partial funding for this content was provided by the Metropolitan Council and the Minnesota Board of Water and Soil Resources.

Reviewed in 2018

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