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The nature of phosphorus in soils

Paulo H Pagliari, Daniel E Kaiser, Carl J Rosen, and John A Lamb

Phosphorus facts for Minnesota

  • Phosphorus chemistry of soils is very complex. Soil pH is one of the most important factors to be known to influence decisions regarding phosphorus management.
  • In Minnesota, the two most widely used soil tests to determine available P for plants are the Bray-1 and Olsen. The Bray-1 test is used when soil pH is less than or equal to 7.4, while the Olsen test used when soil pH is greater than 7.4.
  • Although soil has both organic and inorganic P forms, plants can only utilize inorganic P. Mineralization of organic P is the pathway used by plants to convert organic into inorganic P so that it can be used for growth.
  • Phosphorus in the soil solution is found in relatively low concentrations; however, soil particles can have a significant amount of active P that can cause environmental problems if runoff occurs.

Phosphorus (P) is an essential element classified as a macronutrient because of the relatively large amounts of P required by plants. Phosphorus is one of the three nutrients generally added to soil as fertilizer. One of the main roles of P in living organisms is in the transfer of energy. Organic compounds that contain P are used to transfer energy from one reaction to drive another reaction within cells. Adequate P availability for plants stimulates early plant growth and hastens maturity.

Although P is essential for plant growth, mismanagement of soil P can pose a threat to water quality. The concentration of P is usually sufficiently low in fresh water so that algae growth is limited. When lakes and rivers receive amounts of P that exceed their background levels, excessive growth of algae often occurs. Increased levels of algae reduce water clarity and can lead to decreases in available dissolved oxygen as the algae decay. These conditions can be very detrimental too much of the aquatic life and can limit the recreational use of lakes such as game fishing and other water activities.

the phosphorous cycle of an agricultural area

The phosphorus cycle

The P cycle is similar to several other mineral nutrient cycles in that P exists in soils and minerals, living organisms, and water. Phosphorous has a unique purpose to serve wherever it is in the cycle. Rain and weathering of rocks cause the release of phosphate ions, these are then distributed in soils and water. Plants use the P to grow. The plants are then eaten by animals and used for maintenence. After the animal dies the P is returned to the soil. Bacteria in the soils may use the phosphorous to grow. The phosphorous can end up in waterways and is then incorporated into sediments.

Forms of phosphorus in soils

In soils, P exists in many different forms. In practical terms, however, P in soils can be thought of existing in three pools:

Fate of phosphorus added to soils

The phosphate in fertilizers and manure is initially quite soluble and available. Most phosphate fertilizers have been manufactured by treating rock phosphate with acid to make it more soluble. Manure contains soluble, organic, and inorganic phosphate compounds that are highly available. When fertilizer or manure phosphate contacts soil, various reactions begin occurring that make the phosphate less soluble and less available. The rates and products of these reactions depend on such soil conditions as pH, clay content and type of clay, moisture content, temperature, and the P minerals already present in the soil.

Predicting the availability of phosphorus in soils

To determine the need for supplemental P, soil tests are often used to estimate how much phosphate will be available for a crop. The two most common types of soil tests in Minnesota are the Bray-1 and the Olsen test. Researchers have calibrated the Bray-1 P soil test for acidic soils and is the preferred test when soil pH is less than or equal to 7.4. The Olsen test is used when the pH of a soil samples is 7.4 or greater.

Soil tests like the Bray-1 P and the Olsen are an availability index, which have the goal of estimating the amount of P from both the solution and active pools that the plant can use. Calibration studies have been done to correlate crop response to fertilizer additions in soils with various soil test levels of P. Using the calibration data, recommendations can be made as to the amounts of phosphate fertilizer that will most likely give optimum yields.

Soil phosphorus and water quality

Phosphorus is a somewhat unique pollutant in that it is an essential element, has low solubility, and is not toxic itself, but may have detrimental effects on water quality at quite low concentrations. Phosphorus is typically the limiting nutrient for algal growth in freshwater ecosystems. The addition of P to freshwater increases algal growth. When algae die and begin to decompose, anoxic conditions are created as O2 is depleted from the water, a process called eutrophication. Because of eutrophication, there is considerable concern about P being lost from soils and transported to nearby streams and lakes. Several chemical properties of soil P have important implications for the potential loss of P to surface water.

Phosphate in soils is associated more with fine particles than coarse particles. When soil erosion occurs, more fine particles are removed than coarse particles, causing sediment leaving the soil through erosion to be enriched in P. When those soil particles are carried to a river or lake, P will be contained in this sediment.

In instances when sediment is a source of P to the water, eutrophication can potentially impair the water system. Therefore, responsible P management in agricultural and industrial systems should be sought to reduce potential negative impacts on the environment.

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