NUTRIENT MANAGEMENT FO-6795-C (REVISED 2017) The Nature of Phosphorus in Soils Paulo H Pagliari, Daniel E Kaiser, Carl J Rosen, and John A Lamb: Extension Specialists in Nutrient Management 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 to much of the aquatic life and can limit the recreational use of lakes such as game fishing and other water activities. THE PHOSPHORUS CYCLE The P cycle is similar to several other mineral nutrient cycles in that P exists in soils as components of primary and secondary minerals, living and decaying organisms, and the soil solution. Although P is widely distributed in nature, P is not found by itself in elemental form. Elemental P is extremely reactive and combines with oxygen when exposed to the air. In natural systems like soil and water, P will exist as phosphate, a chemical form in which each P atom is surrounded by 4 oxygen (O) atoms. Orthophosphate, the simplest phosphate, has the chemical formula PO 4 3-. In the soil solution, orthophosphate mostly exists as H 2PO 4 - in acidic conditions or as HPO 4 2- in alkaline conditions. Phosphate is taken up by plants from soils, utilized by animals that consume plants, and returned to soils as organic residues decay in soils (Figure 1). Much of the phosphate used by living organisms becomes incorporated into organic compounds. When plant materials are returned to the soil, this organic phosphate will slowly be released as inorganic phosphate or be incorporated into more stable organic materials and become part of the soil organic matter. The conversion of organic phosphate to inorganic phosphate is called mineralization. Mineralization is caused by phosphatase enzymes, produced by microorganisms and plant roots, which are responsible for breaking down organic compounds in the soil. The activity of microorganisms and enzymes is controlled by soil temperature and moisture content. The process is most rapid when soils are warm (above 50 degrees) and moist (at about 60% to 80% of field capacity) but well drained. Phosphate can potentially be lost through soil erosion and to a lesser extent to water flowing through the soil carrying soil particles loaded with P.
Figure 1. The phosphorus cycle. (courtesy of IPNI) The most stable forms of P found in soil ecosystems are relatively insoluble in water. Soil water and surface water (rivers and lakes) usually contain relatively low concentrations of dissolved (or soluble) P. In most cases, the total dissolved P concentration is at or below 0.05 mg L -1 (or 0.05 ppm) in healthy soil ecosystems depending on the types of minerals in the area. Surface water usually contains less than 0.01 mg L -1 (or 10 ppb) of total dissolved P as orthophosphate. Water bodies may also contain organic P and phosphate attached to small particles of sediment. Total P in water is all of the P in solution regardless of its form and is often the form reported in water quality studies. Algal available or bioavailable P is P that is estimated to be available to
organisms like algae present in a lake or river. This is usually estimated by a chemical test designed to measure easily available dissolved P and particulate P. Particulate P includes organic P that is easily broken down by phosphatase enzymes and also loosely bound clay or oxide inorganic P. This is a P form of immediate concern to water quality. The word phosphorus or P refers to the element and is also used as a general term when a particular chemical form of P is not being designated. For example, the total P content of a soil or plant material is usually expressed as percent P. However, fertilizer analyses are usually reported as percent P 2O 5. The form P 2O 5 is a chemical term used to express the analysis of all phosphate containing fertilizers, but this form does not exist in either fertilizers or soils. To convert elemental P to P 2O 5 use the following equation: %P x 2.29. 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": Solution P Active P Fixed P THE SOLUTION P POOL is very small and usually contains less than a pound of P per acre. The solution P contains a mix of inorganic P and organic P with the inorganic P representing the majority of the soil solution P. Plants take up P primarily in the orthophosphate form. Solution P is important because it is the pool from which plants take up P and is the only pool that has any measurable mobility. Most of the P taken up by a crop during a growing season will have moved only a few millimeters or less through the soil to the roots. A growing crop would quickly deplete the P in the soluble P pool if the pool were not being continuously replenished by the active and fixed pools. THE ACTIVE P POOL is P in the solid phase that is more readily released to the soil solution, the water surrounding soil particles. As plants take up phosphate, the concentration of phosphate in solution is decreased and some phosphate from the active P pool is released. Because the solution P pool is very small, the active P pool is the main source of available P for crops. The ability of the active P pool to replenish the soil solution P pool in a soil is what makes a soil fertile with respect to phosphate. An acre of land may contain several pounds to a few hundred pounds of P in the active P pool. The active P pool will contain inorganic phosphate that is attached (or adsorbed) to small particles in the soil, phosphate that reacted with elements such as calcium or aluminum to form somewhat soluble solids, and organic P that is easily mineralized. Adsorbed phosphate ions are held on active sites on the surfaces of soil particles. The amount of phosphate adsorbed by soil increases as the amount of phosphate in solution increases and vice versa (Figure 2) until a sorption limit is found. In most soils when the solution concentration reaches between 60 to 90mg L -1 (or 90 ppm), the soil can no longer hold any more P as most of the sorption sites are occupied. Soil particles can act either as a source (desorption) or a sink (adsorption) of phosphate to the surrounding water depending on conditions. Soil particles with low levels of adsorbed P that are eroded into a body of water with relatively high levels of dissolved phosphate may adsorb phosphate from the water, and vice versa. THE FIXED P POOL of phosphate will contain inorganic phosphate compounds that are very insoluble and organic compounds that are resistant to mineralization by soil microorganisms. Phosphate in this pool may remain in soils for years without being made available to plants and may have very little
impact on the fertility of a soil. The inorganic phosphate compounds in this fixed P pool are more crystalline in their structure and less soluble than those compounds considered to be in the active P pool. Some slow conversion between the fixed P pool and the active P pool does occur in soils. Figure 2. Relationship between P absorbed by soil and P in solution. This data illustrates that a soils capacity to hold P decreases once all P sorption sites are saturated by the P in solution. 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 (the phosphate-bearing mineral that is mined) with acid to make it more soluble. Manure contains soluble phosphate, organic phosphate, 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. As a granule of P fertilizer contacts soil, moisture from the soil will begin to move towards the fertilizer granules, which will help with the dissolution of the granules. However, the water also carries with it other cations such as calcium (Ca 2+ ), magnesium (Mg 2+ ), aluminum (Al 3+ ), and iron (Fe 3+ ) and these cations will counteract the rate of dissolution and the movement of P away from the granule. As the fertilizer granules dissolves, it increases the soluble phosphate in the soil solution and allows the dissolved phosphate to move a short distance away from the fertilizer particle. Movement is slow, but may be increased by rainfall or irrigation water flowing through the soil. As phosphate ions in solution slowly migrate away from the fertilizer particle, most of the phosphate will react with the minerals within the soil. Phosphate ions generally react by adsorbing to soil particles or by combining
with the cations that moved with the water and precipitate forming solid compounds. The adsorbed phosphate and the newly formed solids are relatively available to meet crop needs. Gradually, reactions occur in which the adsorbed phosphate and the easily dissolved compounds of phosphate form more insoluble compounds that cause the phosphate to be become fixed and unavailable. Over time, this results in a decrease in soil test P. The mechanisms for the changes in phosphate are complex and involve a variety of compounds. In alkaline soils (soil ph greater than 7), Ca 2+ is the dominant cation (positive ion) that will react with phosphate. A general sequence of reactions in alkaline soils is the formation of dibasic calcium phosphate dihydrate, octocalcium phosphate, tricalcium phosphate, and lastly hydroxyapatite. The formation of each product results in a decrease in solubility and availability of phosphate. In acidic soils (especially with soil ph less than 5.5), Al is the dominant ion that will react with phosphate. In these soils, the first products formed would be amorphous Al and Fe phosphates, as well as some Ca phosphates. In these amorphous Al and Fe phosphates, one of the O atoms from the phosphate will bind to the Al or Fe cation forming what is called a monodentate bond. Figure 3. The availability and form of phosphorus is affected by soil ph. The amorphous Al and Fe phosphates gradually change into compounds that resemble crystalline variscite (an Al phosphate) and strengite (an Fe phosphate). During this change to a more crystalline compound, another O atom from the phosphate bonds to the Al or Fe forming a bidentate bond. The bidentate bond can be on the same Al or Fe cation where the first O is bound or it can be on a neighboring atom. The solubility of this newly created mineral will depend on where the second O atom is bound, and is generally lower when the same Al or Fe cation is bound to more than one O
atom from the same phosphate. Each of these reactions will result in very insoluble compounds of phosphate that are generally not available to plants. Reactions that reduce P availability occur in all soil ph ranges, but can be very pronounced in alkaline soils (ph > 7.0) and in acidic soils (ph < 5.5). Maintaining soil ph between 5.5 and 7 will generally result in the most efficient use of phosphate (Figure 3). BECKER, MN TOTAL P - 329 PPM Readily Available Pi Moderately Available Pi Moderately Available Po Unavailable Pi Readily Available Po CROOKSTON, MN TOTAL P: 592 PPM Moderately Available Po Readily Available Pi Moderately Available Pi Readily Available Po Unavailable Pi Figure 4. Relative fraction of pools of inorganic (P i) and organic (P o) phosphorus in the top three inches for two soils considered low testing by the Olsen classification collected near Becker and Crookston, MN.
Adding to the active P pool through fertilization will also increase the amount of fixed P. Depleting the active pool through crop uptake may cause some of the fixed P to slowly become active P. The conversion of available P to fixed P is partially the reason for the low efficiency of P fertilizers. Most of the P fertilizer applied to the soil will not be utilized by the crop in the first season. Continued application of more P than the crops utilize increases the fertility of the soil, but much of the added P becomes fixed and unavailable. Most fine-to medium-textured soils have large capacities to hold phosphate by adsorption and precipitation. Occasionally the question of how much phosphate a soil can hold is asked, especially when high loading rates of P are expected or have occurred. Soils differ in their phosphate holding capacity. Fine-textured soils can generally hold hundreds of pounds of phosphate per acre, while coarse-textured soils can generally hold much less phosphate due to the more inert character of sand particles compared to clay particles. Figure 4 shows a comparison of the distribution of P in the top three inches for a fine-textured soil collected near Crookston, MN and a coarsetextured soil collected near Becker, MN. Soil P pools are broken down based on inorganic (P i) and organic P (P o) in readily available, moderately available, and unavailable P pools. The data shows that the percentage of readily available inorganic P is relatively small for both soils, the fine textured soil has a greater P holding capacity with more P in unavailable forms, and in coarse-textured soils, organic matter plays greater role in P sorption. The subsoil of many soils often has an even greater capacity to hold phosphate than does the corresponding surface soil. However, an important aspect of the ability of a soil to hold phosphate is that a soil cannot hold increasing amounts of phosphate in the solid phase without also increasing soil solution phosphate (Figure 2). Increased amounts of phosphate in solution will potentially cause more phosphate to be lost to water running over the soil surface or leaching through the soil. Loading soils with very high levels of phosphate will not negatively impact crops, but may result in increased phosphate movement to nearby bodies of water. 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 most common way of determining P availability is to mix a small amount of soil with an extracting solution that contains an acid and/or complexing agent that will remove some of the phosphate from the soil particles using colorimetry. The extracting solution and soil are separated by filtration and the amount of P extracted is determined per unit mass of soil. In Minnesota, research has 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 sodium bicarbonate (Olsen) extractant has been calibrated for alkaline soils and is the preferred test when soil ph is greater 7.4. The relationship between P extracted by the Bray-1 and Olsen P tests is shown in Figure 5. For soils with a ph less than 7.4, both tests are highly correlated and can be used to estimate P availability. For soils with a ph greater than 7.4 that also contain free carbonates, the acid used in the Bray-1 test is neutralized resulting in underestimation of available P. Most soil testing labs will automatically run the Olsen test when the ph of a soil samples is 7.4 or greater. It is not uncommon in Central and Western Minnesota for soil tests to range
above and below 7.4 within a field. If such a field is grid or zone sampled, the Olsen test can be run on all samples to provide a direct comparison among soil samples collected across a field. Bray P1 (ppm) Bray P1 (ppm) 120 100 80 60 40 20 0 y = 1.84x + 0.34 R² = 0.87 Soil ph <7.4 0 10 20 30 40 50 60 70 Olsen P (ppm) Soil ph >7.4 80 70 60 50 40 30 20 10 0 0 10 20 30 40 Olsen P (ppm) Figure 5. Relationship between the Bray-1 and Olsen P tests for surface soils in Minnesota which vary in ph greater than or less than 7.4. 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. test only for soils where ph is less than 7.4. Most other chemical tests correlate to the Bray-1 and Olsen results, but will extract differing amounts of P from the soil. Some laboratories use inductively coupled plasma atomic emission spectroscopy (ICP- AES) to determine the amount of P in the extraction solution, which results in higher readings because ICP-AES also measures organic P in addition to inorganic P. Interpretations for extractions determined with colorimetry should not be used for interpreting results measured with ICP-AES. Correlation and calibration must be conducted if the extraction solution or the method for determination of P in the laboratory is modified as both affect the amount of P extracted or determined in the extraction solution. 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 O 2 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. Other tests such as the Mehlich-III, Bray-2, and Haney H3A tests are being run in Minnesota and neighboring states. Research has shown that the Bray-1 P interpretations could be used for the Mehlich-III colorimetric
Most soils have a large capacity to retain P. Even large additions of P will be mostly retained by soils provided there is adequate contact with the soil. Increasing the amounts of phosphate in soils results in increased levels of phosphate in soil solution. This will generally result in small, but potentially important increases in the amounts of phosphate in water that passes over or through soils. 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 a 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. When the sediment reaches a body of water it may act as a sink or a source of P in solution. Phosphorus chemistry of soils is very complex. Soil ph is one of the most important factor to be known to influence (or affect) 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. For more information: www.extension.umn.edu/agriculture/nutrientmanagement/ 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. TAKE HOME POINTS 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. 2017, Regents of the University of Minnesota. University of Minnesota Extension is an equal opportunity educator and employer. In accordance with the Americans with Disabilities Act, this publication/material is available in alternative formats upon request. Direct requests to the Extension Store at 800-876-8636. Printed on recycled and recyclable paper with at least 10 percent postconsumer waste material.