Understanding phosphorus in Minnesota soils
Paulo H. Pagliari, Daniel E. Kaiser, Carl J. Rosen, and John A. Lamb — Extension Specialists in Nutrient Management
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Phosphorus (P) is essential for crop production. It stimulates early plant growth giving a healthy and vigorous start. In Minnesota, most agricultural soils contain from 100 to about 4,000 lb. of total P per acre.
Stimulated by economic as well as environmental concerns, the efficient use of phosphorus is becoming more and more important. This Fact Sheet provides a discussion of:
- Phosphorus reactions in soils
- Uptake of phosphorus by plants
- Crop response to phosphate fertilizers
- Predicting the need for phosphate fertilizers
- Management of phosphate fertilizers
Phosphorus reactions in soils
Phosphorus exists in soils in both organic and inorganic forms. Organic forms are found in humus and other organic material. The phosphorus in organic materials is released for plant uptake by a process called mineralization that occurs when microorganisms break down soil organic matter. Although the microorganisms do not break down organic phosphorus directly, they release phosphatase enzymes that are responsible for breaking organic P into the phosphate form, which is then used by plants for growth. The activity of the microorganisms and the enzymes released is highly influenced by soil temperature and soil moisture. The process is most rapid when soils are warm and well drained.
Figure 1. The availability of phosphorus is affected by soil pH.
Inorganic phosphorus occurs in a variety of combinations with iron, aluminum and calcium. When P reacts with these elements, the products formed are not very soluble and the P in the insoluble product is considered to be fixed or tied up. The solubility of the various inorganic P compounds has a direct impact on the availability of P for crop growth. The P solubility is highly related to soil pH Figure 1.
In general, tie up of P as calcium phosphates is a concern when the soil pH exceeds 7.3. Soils if not limed, over time will become more acid. With the decrease in pH, the availability of P will change. As illustrated in Figure 1, when the pH of soils range between 4.8-5.5 the form of P is more reactive with aluminum in the soil and is tied up as aluminum phosphates that are not available to the plants. Liming of the soil can help to increase P availability from Fe and Al bound forms.
The P that is tied up is not measured by routine soil test procedures. Some P that is tied up can be returned back to plant available forms depending on the solubility of the phosphate compound formed in the soil. Calcium bound forms of P vary in their solubility with dicalcium phosphate dihydrate being the most soluble followed by (in order of decreasing solubility) dicalcium phosphate, octocalcium phosphate, tricalcium phosphate, and primary phosphate containing minerals such as apatite. Acidification to release Ca bound P forms is not feasible in Minnesota. Changing management of P fertilizers, such as banding, is the most effective way to deal with tie up of P in Ca bound forms.
Soils throughout most of western Minnesota usually have natively low levels of available P because of the materials the soils where formed in. Therefore, appropriate management of phosphate fertilizers is a major concern for these soils. On the other hand, soils in southeastern, central, and east-central Minnesota usually have a natively high level of available phosphorus. For these regions, phosphate is usually not needed in large quantities in any fertilizer program.
Uptake of phosphorus by plants
Nearly all of the P absorbed by plants is taken up as two ions called phosphate. Phosphorus is not absorbed in an organic form. One phosphate ion form is HPO4-2. This form is the most abundant in calcareous soils and the form of P absorbed when crops are grown on these soils. The second form in H2PO4-. This form is most abundant in acid soils and the dominant form of P absorbed by plants when the soil pH is less than 7.0.
Figure 2. Equilibrium of phosphorus in the soil system.
For most soils in Minnesota, the amount of phosphate dissolved in the soil solution and accessible for crop uptake is around a pound per acre. However, in fields where a large concentration of P (e.g greater than 40 ppm Bray P1) is present as a result of heavy manure or fertilizer applications over the years, the amount of P in the soil solution can be greater than 10 pounds per acre. The amount of P that is dissolved and accessible in the soil solution is in equilibrium with the P in the solid phase Figure 2. This solid phase P is both organic and inorganic. The phosphorus present in the solid phase is the P that is chemically bound to calcium in calcareous soils and to aluminum and iron in acidic soils.
During the growing season, plants need more P than is dissolved in the soil solution at any one time, therefore, the solution P must be replenished many times during the growing season. The ability of a soil to maintain adequate levels of P in the solution phase is the key to the plant available P status of the soil.
Crop response to phosphate fertilizers
Figure 3. Kaiser 2016
If the level of available P in soil is not adequate for optimum crop growth, phosphate fertilizers must be used to insure that there are adequate amounts of this nutrient in the solution phase. 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 ppm in the Bray-1 test (Figure 3) or 11 ppm in the Olsen test.
The yield data in Table 1 provide an example of the response of corn to phosphate fertilization. In Table 1, a medium testing soil is fertilized with P using various management strategies including crop removal and the current University of Minnesota broadcast and banded P suggestions for corn. Crop removal is common in many areas of the state. In the example in Table 1, banding the P at a lower rate resulted in the same yield as a crop removal based application of P. This illustrates the effect that banding P can have on reducing the overall P requirement for the corn crop.
The ability of the banded fertilizer application to supply all a crop’s 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. Rates used when banding on the seed need to be low due to potential reduction in emergence due to high salts or ammonia formation near the seed. The example in Figure 4 shows that a small rate of phosphate banded with the seed can provide maximum yield for corn for a medium soil test but is not enough to maximize yield when soil P test low. In contrast, recent data has shown that a small rate of fertilizer banded with the seed it is better than higher rates of broadcast P for sugarbeet (Figure 5).
Figure 4. Response of corn to a combination of broadcast phosphorus and in-furrow (on seed) banded phosphorus as 10-34-0. Source: Kaiser 2015.
Figure 5. Response of sugarbeet to a combination of broadcast phosphorus and three gallons per acre in-furrow (on seed) banded phosphorus as 10-34-0. Source: Sims 2010.
A response of corn and soybean to phosphate use is shown in Table 2. This example illustrates the effect that starting soil test, 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 from the application of P.Crop response to P application varies. For example, alfalfa response to P application up to a soil test Bray P 1 values of 40 ppm, while the response in wheat and soybean yield to P application occurs up to only 10 to 15 ppm Bray P-1. Corn will respond up to levels of 15-20 ppm. Potato can respond to levels above 30 ppm, but response is more likely when soil test P is below 30 ppm.
Predicting the need for phosphate fertilizer
Phosphorus soil tests measure the ability of the soil 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 are related to the phosphate fertilizer ability to provide an economic 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 where various rates of phosphate are applied and yields are measured. Table 3 summarizes recent data on corn response to P in Minnesota. Given in Table 3 is the percentage of times within a given soil test category that the application of P resulting in a measurable increase in corn yield and the average yield achieved when no P was applied based on the starting soil test value.
Figure 6. Reduced plant growth in potato due a deficiency of phosphorus.
It is important to note that there is always a possibility that application of P will increase the yield of the crop. From Table 3, the application of P in the high and very high categories increased corn grain yield 14 and 9% of the time, respectively. However, the average yield produced in those categories was within 1% of the maximum of maximum. Maintenance of 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.
Two laboratory procedures are used to measure the P status of soils in Minnesota. The Olsen procedure is preferred when the soil pH is 7.4 or greater. The Bray-1 procedure is used 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. The Bray P-2 results have not been correlated and calibrated to the crop response to phosphate fertilizer in Minnesota and are not useful in predicting the amount of phosphate fertilizer to apply. The Mehlich-3 soil test is used by several states in the Corn Belt but is not recommended for use in Minnesota. The Mehlich-3 soil test will typically result in soil P test levels 0-5% 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.
Figure 7. Purpling on the edge of corn leaves due to phosphorus deficiency.
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. Plant analysis can also be used as an aid in determining the availability of P in soils. Symptoms of P deficiency are not obvious or easily identified for most crops in Minnesota. For most crops, a shortage of P reduces plant size. Figure 6 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. 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 is a greater demand for P early in the growing season. A P deficient corn plant is shown in Figure 7.
Some hybrids have a purple appearance early in the growing season regardless of the P supply in the soil. This purple appearance can be a genetic response to stress caused by cold temperatures. This hybrid characteristic should not be confused with P deficiency.
When plant analysis can be used as a management tool
It is important to relate the interpretation of the analytical results to the stage of growth. The concentration of P in plant tissue usually decreases as the plant matures. Some interpretations of P concentrations for several crops are summarized in Table 4.
Management of phosphate fertilizers
Since P is not mobile in soils, placement of phosphate fertilizers is a major management decision in crop production systems. There is no special placement that is ideal for all crops. Decisions about placement of phosphate fertilizers are affected primarily by the intended crop and P soil test level.
For corn and small grain production, the phosphate fertilizer needed can be either broadcast and incorporated before planting, applied in a band away from the seed row as a starter fertilizer at planting, or if small amounts are needed directly on the seed at planting. With small grains, the amount of phosphate needed can be applied with the drill or air seeder at planting. Starter fertilizer for corn is usually separated from the seed by approximately 1 inch of soil. The banded application is a very efficient way to use phosphate fertilizer. The rates that are recommended for broadcast application can be reduced by one-half if the phosphate is applied in a band for these crops. Results suggest that a small amount of fertilizer can be placed directly on the corn seed with the planter but the rate applied may not satisfy the amount needed for corn if soil test phosphorus is low.
Research trials with soybean 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 to placement is opposite to the response of corn and small grain and may best be explained by differences in the development of the respective root systems.
For sugar beet, current research suggests that seed row placement of 15 lbs of phosphate will produce similar yield as 45-60 lbs of phosphate broadcast to the soil.
For other row crops, there is not enough research information to suggest that there is a preferred method of phosphate placement.
Application of 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. Therefore, these crops are capable of absorbing phosphate fertilizers that are broadcast annually to established stands if additional fertilizer is required.
The rate of phosphate fertilizer for the various placements varies with the yield goal of the intended crop and the soil test level for P. These rate suggestions for the major crops are provided in the following publications:
- Fertilizing Alfalfa in Minnesota
- Fertilizing Barley in Minnesota
- Fertilizing Corn in Minnesota
- Fertilizing Soybeans in Minnesota
- Fertilizing Wheat in Minnesota
- Banding Fertilizer with the Corn Seed
- Plant Analysis Sampling and Interpretation
The importance of P for crop production is well documented. The management of fertilizers to meet the requirement for this nutrient changes with crop, soil properties, and environmental conditions. The chemistry of P in crops and soils is complex. Special attention to the management of this nutrient, however, can lead to profitable crop production.