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Extension > Agriculture > Nutrient Management > Phosphorus > Understanding phosphorus fertilizers

Understanding phosphorus fertilizers

By George Rehm, Michael Schmitt, John Lamb, Gyes Randall, and Lowell Busman

Copyright © 2002 Regents of the University of Minnesota. All rights reserved.

Several phosphate fertilizers can be used to meet the phosphorus (P) requirements of Minnesota's crops. This extension folder is designed to:

Manufacturing commercial phosphate fertilizer

Rock phosphate is the raw material used in the manufacture of most commercial phosphate fertilizers on the market. In the past, ground rock phosphate itself has been used as a source of P for acid soils. However, due to low availability of P in this native material, high transportation costs, and small crop responses, very little rock phosphate is currently used in agriculture.

Figure 1. The process used in the manufacture of various phosphate fertilizers.

The manufacture of most commercial phosphate fertilizers begins with the production of phosphoric acid. A generalized diagram showing the various steps used in the manufacture of various phosphate fertilizers is provided in Figure 1. Phosphoric acid is produced by either a dry or wet process. In the dry process, rock phosphate is treated in an electric furnace. This treatment produces a very pure and more expensive phosphoric acid (frequently called white or furnace acid) used primarily in the food and chemical industry. Fertilizers that use white phosphoric acid as the P source are generally more expensive because of the costly treatment process.

The wet process involves treatment of the rock phosphate with acid producing phosphoric acid (also called green or black acid) and gypsum which is removed as a by-product. The impurities which give the acid its color have not been a problem in the production of dry fertilizers. Either treatment process (wet or dry) produces orthophosphoric acid— the phosphate form that is taken up by plants.

The phosphoric acid produced by either the wet or the dry process is frequently heated, driving off water and producing a superphosphoric acid. The phosphate concentration in superphosphoric acid usually varies from 72 to 76%. The P in this acid is present as both orthophosphate and polyphosphate. Polyphosphates consist of a series of orthophosphates that have been chemically joined together. Upon contact with soils, polyphosphates revert back to orthophosphates.

Ammonia can be added to the superphosphoric acid to create liquid or dry materials containing both nitrogen (N) and P. The liquid, 10-34-0, is the most common product. The 10-34-0 can be mixed with finely ground potash (0-0-62), water, and urea-ammonium nitrate solution (28-0-0) to form 7-21-7 and related grades. The P in these products is present in both the orthophosphate and polyphosphate form.

When ammonia is added to the phosphoric acid that has not been heated, monoammonium phosphate (11-52-0) or diammonium phosphate (18-46-0) is produced depending on the ratio of the mixture. The P present in these two fertilizers is present in the orthophosphate form.

The cost of converting rock phosphate to the individual phosphate fertilizers varies with the process used. More importantly, the processes used have no effect on the availability of P to plants.

Phosphate Fertilizer Terminology

Because of the number of products on the market, the selection of a phosphate fertilizer can be confusing. An explanation of some terminology may help to avoid some of the confusion. Some important terms are:

Water-Soluble–

Fertilizer samples analyzed by a control laboratory are first placed in water and the percentage of the total phosphate that dissolves is measured. This percentage is referred to as water-soluble phosphate.

Citrate-Soluble –

The fertilizer material that is not dissolved in water is then placed in an ammonium citrate solution. The amount of P dissolved in this solution is measured and expressed as a percentage of the total in the fertilizer material. Phosphate measured with this analytical procedure is referred to as citrate-soluble.

Available –

The sum of the water-soluble and citrate-soluble phosphates is considered to be the percentage that is available to plants and is the amount guaranteed on the fertilizer label. Usually, the citrate- soluble component is less than the water-soluble component.

Table 1. Percentages of water-soluble and available phosphate in several common fertilizer sources.

P2O5
P2O5 Source N (%) Total (%) Available (%) Water soluble* (%)
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 38 0
*Water-soluble data are a percent of the total P2O5
Source: Ohio Agronomy Guide. Ohio Cooperative Extension Service Bull.472.

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. From a fertilizer/nutrient management perspective, the major differentiating factor is the availability of P. As with any of the fertilizer products, especially those with varying analysis, chemical analysis should be done on these products. Then an availability coefficient should be used to determine the available P as a portion of the reported total P.

Phosphorus from manure or sludge should be comparable to P from inorganic fertilizer. Therefore, if a producer has a P recommendation for 30 lbs/A of P2O5 , applying approximately 65 lbs of 18-46-0 (DAP) or 6 tons of 11-6-9 (manure, 80% available P coefficient) should provide equivalent results.

The P contained in organic P sources is a combination of inorganic and organic P. Essentially, all of the inorganic P is in the orthophosphate form, which is the form taken up by growing plants. Diet fed to the animal has some control on this chemical make-up. Consider P feed supplements and the fact that many of these could be considered P fertilizers as well. Generally, 45-70% of manure-P is inorganic P. Organic P constitutes the remaining total P. Much of the organic P is easily decomposable in the soil, but factors such as temperature, soil moisture, and soil pH all have a bearing on the P mineralization rate. The final decomposition product is orthophosphate P compounds.

The combination of the organic/inorganic P ratios in the organic P sources and the soil environment affect the availability coefficient for organic P. Most animal manure research interpretations indicate that approximately 60-80% of the total P is available to crops in the first year. Due to the chemical composition of other organic P sources such as bone meal, lesser amounts of plant available P compared to total P are expected.

Phosphorus Source Issues

Some of the most frequently asked questions about phosphate fertilizer are discussed in the paragraphs that follow.

Should I Use Liquid or Dry? The utilization of P by plants is not affected by the liquid or dry property of the fertilizer. Plant nutrient use in both liquid and dry fertilizers is affected by such factors as method of application, crop and root growth characteristics, soil test levels, and climatic conditions. The amount of water in a fluid fertilizer is insignificant compared to the water already present in the soils. Therefore, P in liquid P sources is not more available than P in dry materials — even in a dry year. The selection of a liquid or dry P source should be based on adaptation to the farmer's operation and economics.

Is Orthophosphate Better Than Polyphosphate? To answer this question, it's important to understand the difference between these two forms of phosphorus. The phosphorus in the phosphoric acid used to make most dry phosphate fertilizers as well as a few liquids is in the orthophosphate form.

If ordinary phosphoric acid is heated, water is removed and the orthophosphate ions combine to form a polyphosphate. This process does not convert 100% of the orthophosphate ions into the polyphosphate form. Most polyphosphate fertilizers will have 40 to 60% of the phosphorus remaining in the orthophosphate form.

In the soil, polyphosphate ions readily convert to orthophosphate ions in the presence of soil water. This conversion is rapid and, with normal soil temperatures, can be complete in days or less. This conversion process is enhanced by an enzyme called pyrophosphatase, which is abundant in most soils.

Polyphosphates are usually marketed as liquid ammonium polyphosphate fertilizers. Because water is removed in the manufacturing process, these materials have a higher analysis than materials in which the phosphate is in the orthophosphate form. The polyphosphate liquids are also more convenient for the fertilizer dealer to handle and allow for the formulation of blends that are not possible with the orthophosphate liquids.

The effect of orthophosphate and polyphosphate fertilizers on crop production has been evaluated with numerous field trials. The results shown in Table 2 are typical of the results obtained from several trials.

Table 2. The influence of P source on corn yield.

P2O5 Applied P Source
Polyphosphate Orthophosphate
lb./acre bu./acre
15 124 124
30 134 134
45 142 142
Source: Nebraska Soil Test P: Low

The yields shown in Table 2 are averages from five sites where the soil pH was in excess of 7.3. It's obvious that the form of phosphate had no effect on yield and if there is a rapid conversion from polyphosphates to orthophosphates, these results are to be expected. Similar results from other studies have been reported throughout the Corn Belt.

Should Soil pH Influence Fertilizer P Source Selection? Soil pH should not be an important factor in selecting fertilizer P sources. From an academic perspective, monoammonium phosphates (MAP) create a more acidic zone around each fertilizer granule, whereas diammonium phosphates (DAP) create a basic zone. Thus, in high pH soils, it can be theorized that using MAP-based fertilizers should be better than DAP because the acid-producing fertilizer would offset the calcareous soils. An additional concern regarding MAP or DAP selection, aside from soil pH, is potential ammonia toxicity to germinating seeds in dry soils. In applying the recommended amount of P in a drill-row or pop-up fertilizer placement, DAP will contain approximately 60% more N, which may be a potential injury risk. However, since agronomic studies and economic data indicate no crop yield differences, it can be concluded that fertilizer selection should be made on traditional factors such as nutrient content, price, availability, etc.

Summary

There are many P-containing fertilizers to satisfy any P recommendation. While organic P sources are closely associated with livestock operations or with proximity to major metropolitan areas, inorganic commercial fertilizers are widely available for all. Inorganic P fertilizers have evolved over the last several decades into a refined, predictable product.

There should be no differences in P fertilizer sources as long as nutrient analysis differences are taken into account. Certain situations have provided certain instances where one product or the other was superior. However, phosphorus fertilizer recommendations are the same regardless of the phosphate fertilizer source.


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

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