Understanding phosphorus fertilizers (2024)

Rock phosphate is the raw material used to manufacture 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, very little rock phosphate is currently used in agriculture due to low availability of P in this native material, high transportation costs and small crop responses.

Most commercial phosphate fertilizer manufacturing begins by producing phosphoric acid.

The generalized diagram in Figure 1 shows the steps taken to manufacture various phosphate fertilizers. Phosphoric acid is produced by either a dry or wet process.

Dry vs. wet process

In the dry process, an electric furnace treats rock phosphate. This treatment produces a very pure and more expensive phosphoric acid – frequently called white or furnace acid – primarily used 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 treating 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 that give the acid its color haven’t been a problem in the production of dry fertilizers.

Orthophosphoric acid

Both the wet and dry treatment processes produce orthophosphoric acid, the phosphate form that’s taken up by plants.

The phosphoric acid produced by either the wet or 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 percent.

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.

Adding ammonia

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 phosphoric acid that hasn’t been heated, it produces monoammonium phosphate (11-52-0) or diammonium phosphate (18-46-0), depending on the ratio of the mixture. The P present in these two fertilizers is in the orthophosphate form.

Cost and outcome

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

The fertilizer material that isn’t 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.

From a fertilizer/nutrient management perspective, the major differentiating factor is the availability of P.

As with any fertilizer products, especially those with varying analysis, do a chemical analysis. Then use an availability coefficient 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. So, if a producer has a P recommendation for 30 pounds per acre of P₂O₅, applying approximately 65 pounds of 18-46-0 (DAP) or 6 tons of 11-6-9 (manure; 80 percent available P coefficient) should provide equivalent results.

The P contained in organic P sources combines inorganic and organic P. Essentially, all inorganic P is in the orthophosphate form, which is the form taken up by growing plants.

Chemical makeup

Diet fed to the animal has some control over this chemical makeup. Consider P feed supplements and the fact that many could be considered P fertilizers as well. Generally, 45 to 70 percent of manure-P is inorganic P. Organic P constitutes the remaining total P.

Decomposition

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.

Available P

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 to 80 percent 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, expect lesser amounts of plant-available P compared to total P.

Optimal soil test levels

Numerous research projects have demonstrated that agronomic crops will respond to phosphate fertilization if soil test levels are in the very low, low and medium ranges, or below 15 parts per million (ppm) in the Bray-1 test (Figure 2) or 11 ppm in the Olsen test.

Crop removal

Crop removal is common in many areas of the state. In the example in Table 2, banding the P at a lower rate resulted in the same yield as those based on crop removal recommendations.This illustrates the effect that banding P can have on reducing the corn crop’s overall P requirements.

Application method

Table 2: How banded starter and broadcast phosphate affect corn yield

Shows the effect of banded starter (two inches beside and below the seed) and broadcast phosphate on corn yield when soil test levels for phosphorus are medium. Data are an average of two years of data collected at the West Central Research and Outreach Center in Morris.

The ability of the banded fertilizer application to supply a crop’s entire P requirement can depend on the type of band used and the soil test. Banding liquid fertilizer on the seed is common for corn and sugarbeet.

When banding on the seed, use a low rate. This is because there’s potential to reduce emergence due to high salts or ammonia forming near the seed.

The example in Figure 3 shows that a small rate of phosphate banded with the seed can provide maximum yield for corn with medium soil test P levels. However, it’s not enough to maximize yield with low soil test P levels.

In contrast, recent data has shown that a small rate of fertilizer banded with the seed is better than higher rates of broadcast P for sugarbeet (Figure 4).

The ability of the banded fertilizer application to supply a crop’s entire P requirement can depend on the type of band used and the soil test. Banding liquid fertilizer on the seed is common for corn and sugarbeet.

When banding on the seed, use a low rate. This is because there’s potential to reduce emergence due to high salts or ammonia forming near the seed.

The example in Figure 3 shows that a small rate of phosphate banded with the seed can provide maximum yield for corn with medium soil test P levels. However, it’s not enough to maximize yield with low soil test P levels.

In contrast, recent data has shown that a small rate of fertilizer banded with the seed is better than higher rates of broadcast P for sugarbeet (Figure 4).

Table 3 shows corn and soybean plants’ response to using phosphate.

Soil test values

This example illustrates the effect that starting soil test level, soil type and crop can have on the response to P.

Corn grain yield responded to P at two of the locations, Lamberton and Morris, while soybean only responded at Morris, which had the lowest starting soil test value for P. Understanding which crops respond better at which soil test values is important to ensure maximum return on investment when applying P.

Application method

Crop response to P application varies:

• Alfalfa: Will respond to levels up to 40 ppm (soil test Bray P-1 value).

• Wheat and soybean: Will only respond up to 10 to 15 ppm (soil test Bray P-1 value).

• Corn: Will respond to levels up to 15 to 20 ppm.

• Potato: Will respond to levels above 30 ppm. However, response is more likely when soil test P is below 30 ppm.

Table 4 summarizes recent data on corn response to P in Minnesota. For various starting soil test values, Table 4 gives:

• The percentage of times that applying P resulted in a measurable increase in corn yield.

• The average yield achieved when no P was applied.

It’s important to note that there’s always a possibility that applying P will increase the crop’s yield. As shown in Table 4, applying P in the high and very high categories increased corn grain yield 14 and 9 percent of the time, respectively.

However, the average yield produced in those categories was within 1 percent of the maximum of maximum. Maintaining high to very high soil test levels will ensure maximum yield potential, but the low probability of response to P will result in a poor economic return from high rates of applied P.

Olsen and Bray-1 procedures

Two laboratory procedures are used to measure the P status of Minnesota soils:

• Olsen procedure: Use when the soil pH is 7.4 or greater.

• Bray-1 procedure: Use when the soil pH is less than 7.4.

Both soil tests have been correlated and calibrated with yield response. The phosphate recommendations in Minnesota are based on those correlation values.

Some soil testing laboratories analyze soils with both a weak Bray (P-1) and a strong Bray (P-2) procedure. Bray P-2 results have not been correlated and calibrated to the crop response to phosphate fertilizer in Minnesota and aren’t useful in predicting the amount of phosphate fertilizer to apply.

There are several situations where the soil pH is greater than 7.4 and the P value from the Bray-1 procedure is greater than the P value from the Olsen procedure.

When soil samples are analyzed by both the Olsen and Bray-1 procedures, research data indicates that phosphate fertilizer recommendations should be based on the greater value. You can also use plant analysis as an aid in determining the availability of P in soils.

Mehlich-3 soil test

Several states in the Corn Belt use the Mehlich-3 soil test, but it’s not recommended in Minnesota.

The Mehlich-3 soil test will typically result in soil P test levels 0 to 5 percent greater than the Bray-P1 test when soil pH is 7.5 or less. The Mehlich-3 test has been found to be less reliable for soils with excess carbonates and a pH greater than 7.5.

Symptoms of P deficiency aren’t obvious or easily identifiable for most crops in Minnesota. For most crops, a shortage of P reduces plant size.

Less plant growth

Figure 5 shows less plant growth due to a shortage of P in potatoes. This lack of growth is typical for crops such as potato and soybean when P is deficient.

Purpling

For corn, a severe P deficiency inhibits the translocation of carbohydrates within the plant. This leads to a purple color on the margins of the leaves.

The purpling is usually most evident in young corn plants because there’s a greater demand for P early in the growing season. Figure 6 shows a P-deficient corn plant.

Some hybrids have a purple appearance early in the growing season regardless of the P supply in the soil. This can be a genetic response to stress caused by cold temperatures. Don’t confuse this hybrid characteristic with P deficiency.

When to use plant analysis as a management tool

It’s important to relate the interpretation of the analytical results to the growth stage. The concentration of P in plant tissue usually decreases as the plant matures. Table 5 summarizes some interpretations of P concentrations for several crops.

For corn and small grain production, the needed phosphate fertilizer can be:

• Broadcast and incorporated before planting.

• Applied in a band away from the seed row as a starter fertilizer at planting.

• Directly on the seed at planting, if small amounts are needed.

With small grains, you can apply the amount of needed phosphate with a drill or air seeder at planting. Corn starter fertilizer is usually separated from the seed by approximately 1 inch of soil.

The banded application is a very efficient way to use phosphate fertilizer, as you can cut the recommended broadcast application rates in half.

Results suggest you can place a small amount of fertilizer directly on the corn seed with the planter. However, the rate applied may not satisfy the amount needed for corn if soil test phosphorus is low.

Soybean research trials have shown that greater grain yields are produced if the needed phosphate is broadcast and incorporated before planting, compared to a band application.

This response is the opposite of corn and small grain, and may best be explained by differences in the development of the respective root systems.

For other row crops, there isn’t enough research to suggest a preferred method of phosphate placement.

Applying phosphate for alfalfa and other forage crops is more efficient when done before stand establishment, when the fertilizer can be incorporated prior to seeding.

Grasses and legumes develop a large number of small roots near the soil surface. This means these crops can absorb phosphate fertilizers that are annually broadcast to established stands, if additional fertilizer is required.

The way plants use P isn’t affected by the liquid or dry property of the fertilizer. Plant nutrient use in both liquid and dry fertilizers is affected by factors such as:

• Method of application.

• Crop and root growth characteristics.

• Soil test levels.

• 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.

Base your selection of a liquid or dry P source on adaptation to your farm’s operation and economics.

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.

Process: Manufacturing and soil conversion

If ordinary phosphoric acid is heated, water is removed and the orthophosphate ions combine to form a polyphosphate.

This process does not convert 100 percent of the orthophosphate ions into the polyphosphate form. Most polyphosphate fertilizers will have 40 to 60 percent 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 completed in days or less. An enzyme called pyrophosphatase, which is abundant in most soils, enhances this conversion process.

Comparing products

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 with phosphate in the orthophosphate form.

Polyphosphate liquids are also more convenient for the fertilizer dealer to handle and allow for the formulation of blends that aren’t possible with the orthophosphate liquids.

Effect on yield

Numerous field trials have evaluated how orthophosphate and polyphosphate fertilizers affect crop production. The results shown in Table 6 are typical of the results obtained from several trials.

The yields shown in Table 6 are averages from five sites where the soil pH was more than 7.3.

It’s obvious that the form of phosphate had no effect on yield and, if there’s 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.

Soil pH should not be an important factor when 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, we can theorize 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 percent more N, which may be a potential injury risk.

However, because agronomic studies and economic data indicate no crop yield differences, we can conclude that fertilizer selection should be made on traditional factors such as nutrient content, price, availability, etc.