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Understanding phosphorus in Minnesota soils

George Rehm, Extension Soil Scientist
Michael Schmitt, Extension Soil Scientist

Phosphorus (P) is essential for crop production. It stimulates early plant growth giving a good and vigorous start and, thereby, hastens maturity. 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

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 growth by a process called mineralization which occurs as microorganisms break down soil organic matter. The activity of the microorganisms 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.

The inorganic phosphorus occurs in a variety of combinations with iron (Fe), aluminum (Al) and calcium (Ca). When phosphorus reacts with these elements the products formed are not very soluble and the phosphorus in the insoluble product is considered to be fixed or tied up. The solubility of the various inorganic phosphorus compounds has a direct impact on the availability of phosphorus for crop growth. The phosphorus solubility is highly related to soil pH Figure 1.

In Minnesota, the pH of most soils is not low enough to enhance the formation of iron and aluminum phosphates. In general, fixation of phosphorus as calcium phosphates is a concern when the soil pH exceeds 7.3. The phosphorus that is fixed is not measured by the routine soil test procedures that are used.

Consequently, soils throughout most of western Minnesota usually have natively low levels of available phosphorus. 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 amounts in any fertilizer program.

Uptake of phosphorus by plants

Nearly all of the phosphorus absorbed by plants is taken up as two ions. Phosphorus is not absorbed in an organic form. The HPO4-ion dominates in calcareous soils and is the form of phosphorus absorbed when crops are grown on these soils. The H2PO4-ion dominates in acid soils and is the ion absorbed 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 these ions dissolved and accessible for crop uptake is a fraction of a pound per acre. The amount of phosphorus that is dissolved and accessible is in equilibrium with the phosphorus in the solid phase Figure 2. This solid phase phosphorus is both organic and inorganic.

Crops need more phosphorus than is dissolved in the soil solution at any one time, therefore, this phosphorus in the solution phase must be replenished many times during the growing season. The ability of a soil to maintain adequate levels of phosphorus in the solution phase is the key to the plant available phosphorus status of the soil.

Crop response to phosphate fertilizers


Figure 3. Response of wheat to phosphate fertilization. Source: Univ. of Nebraska Soil Science News Vol 15: No. 11.

If the level of available phosphorus 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.

The yields in Table 1 provide an example of the response of corn to phosphate fertilization. A response of soybeans to phosphate use is shown in Table 2. The impact of phosphate fertilization on small grain production is shown in Figure 3. This figure also illustrates the importance of soil test level in determining the rate of phosphate fertilizer needed. The optimum rate was lower when the soil test level was in the medium instead of the low range.

Table 1. The effect of banded and broadcast phosphate on corn yield when soil test levels for P are low.

Phosphate Applied
lb P2O5/acre
Placement Yield
0 - 122
40 starter 146
100 broadcast 148
Soil test P (Bray and Kurtz #1) = 6 ppm; study conducted
at the Southwest Experiment Station, Lamberton

Soil test P (Bray and Kurtz #1) = 6 ppm; study conducted at the Southwest Experiment Station, Lamberton

Table 2. Soybean yields as affected by the rate of phosphate applied (1986-1988 ave.)

Phosphate Applied Waseca Lamberton
lb./acre - - - - - bu./acre - - - - -
0 31 27
23 33 29
46 35 30
69 36 30
92 37 32
Soil Test P (ppm): 5 3.5
(Bray and Kurtz #1)

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 index of P in soils that is related to the phosphate fertilizer needs of the crop. The relationship between the P index determined by a soil test and the phosphate fertilizer requirements is developed from the results of numerous research trials where various rates of phosphate are applied and yields are measured.

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 higher. The Bray procedure is used when the soil pH is less than 7.4. 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 related to the crop response to phosphate fertilizer in Minnesota and are not useful in predicting the amount of phosphate fertilizer to apply.


Figure 4. Phosphorus deficient corn plant.

There are several situations where the soil pH is higher than 7.4 and the P value from the Bray procedure is higher than the P value from the Olsen procedure. When soil samples are analyzed by both the Olsen and Bray procedures, research data indicates that phosphate fertilizer recommendations should be based on the higher 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. For corn, a severe P deficiency inhibits the translocation of carbohydrates within the plant. This leads to a purple color on the margin of the leaves. The purpling is usually most evident in young corn plants because there is a higher demand for P early in the growing season. A P deficient corn plant is shown in Figure 4.

Some hybrids have a purple appearance early in the growing season regardless of the P supply in the soil. This hybrid characteristic should not be confused with P deficiency.

When plant analysis is used as a management tool, it's 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 3.

Table 3. Interpretation of plant analysis values for phosphorus.

Crop Plant part and sampling stage deficient low sufficient high
- - - - - - - - - % P- - - - - - - - -
corn whole plant; 12 in. height - - .40 > .80
ear leaf at tasseling < .16 .16 .25 > .50
small grain Top leaves, boot stage < .16 .16 .21 > .50
alfalfa upper 1/3 of plant, early bloom < .16 .16 .25 > .70
sunflowers 5th leaf at flowering < .15 .15 .30 > .50
Adapted from Soil Testing and Plant Analysis, Soil Science Society of America (1973). Values subject to modification as research develops.

In general, plant analysis should not be used as the only tool for making phosphate fertilizer recommendations. Plant analysis works best when used in combination with soil testing.

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 soil test level for P.

For corn and small grain production, the phosphate fertilizer needed can be either broadcast and incorporated before planting or applied in a band as a starter fertilizer at planting. With small grains, the amount of phosphate needed can be applied with the drill at planting. The 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 use can be reduced by one-half if the phosphate is applied in a band for these crops.

Research trials with soybeans have shown that higher yields are produced if the needed phosphate is broadcast and incorporated before planting. 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 other row crops, there is not enough research information to suggest that there is a preferred method of phosphate placement.

Alfalfa and other forage crops, both 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.

The preferred management program for phosphate fertilizer for these crops should be to broadcast and incorporate the recommended rate before planting, followed by an annual topdress application at rates that are needed to produce optimum yields.

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

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.

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