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Phosphorus is an essential crop nutrient. Soil phosphorus does not exist in elemental form, but in chemical combination with complex minerals and organic materials that cannot be taken up by plant roots. The total soil phosphorus content can range from 200 pounds per acre in very sandy soils to more than 2000 pounds per acre in very fine textured soils.
At any given moment, only a tiny fraction of this phosphorus (less than an ounce per acre) is found as the orthophosphate ion (either HPO4-2 or H2PO4) - the form that can be taken up by plant roots. Phosphorus gradually becomes available to plant roots over the growing season as the mineral and organic phosphorus materials decompose or weather down to a water soluble or “plant available” form. Plants growth may be governed by the speed at which water soluble phosphorus becomes available from these mineral or organic sources.
Scientists have been using soil testing in one form or another for over 150 years. Modern soil testing was developed in the 1940's and continues to improve with new technology and research.
Agronomic soil tests are used to estimate the comparative rate at which in a particular soil will supply plant available phosphorus to a growing plant. Plants can grow slowly in soils with low phosphorus levels, but may need supplemental fertilizer phosphorus to meet crop production goals. One of the most useful soil test values is the “critical concentration”. This is the value above which we do not expect to see a yield increase when we add fertilizer.
Agronomic soil tests are used to help identify both when additional fertilizer may be needed and how much may be needed. The proper fertilizer rate depends on many factors, but soil phosphorus tests provide the first step to help identify that rate.
Each soil test method has three parts: 1) a specific chemical extractant, 2) a specific laboratory determination method, and 3) an interpretation of the results.
Soil phosphorus is found in various combinations with other elements, predominately calcium phosphates (Ca-P), aluminum phosphates (Al-P), and iron phosphates(Fe-P). The soil organic matter also contains phosphorus combined with various cellulosic, lignin, and humic materials. The exact makeup of these mineral and organic phosphorus sources varies between geographic regions, between soil types, across fields, and with soil depth.
Soil test extractants are specific chemical solutions used to remove a fraction of the total phosphorus from the soil sample. The liquid extractant is mixed with the soil sample at a certain ratio for a certain time. Then the mixture is filtered, the liquid extractant is captured, and the remaining soil sample is thrown away. Thus we do not actually test the soil, but instead test the extractant that has been in contact with the soil.
The “determination” step identifies the phosphorus concentration in the filtered extractant. Phosphorus is determined in two ways: colorimetrically and spectroscopically. We determine phosphorus colorimetrically by adding additional chemicals to the filtered extractant solution and developing a color, usually blue. The instrument reads the intensity of the color in the solution and relates it to the concentration of phosphorus.
We can also determine phosphorus spectroscopically by using the inductively coupled plasma (ICP) method. The filtered extractant solution is drawn into a plasma torch. The instrument reads various wavelengths from the light emitted by the ICP torch. The phosphorus concentration is determined by reading the intensity of light at the specific wavelengths.
Most laboratories report these concentrations as “parts per million (ppm)”. A few laboratories report concentrations as “pounds per acre (lb/ac)”. “lb/ac” is calculated by multiplying the concentration as “ppm” by a factor of “2". “lb/ac” is equivalent to “parts per two million (pp2m)”.
Successful soil test methods provide a good correlation between the phosphorus concentration removed during the laboratory procedure to the soil phosphorus “release rate” in the field and to the amount of phosphorus taken up by the particular crop during the growing season. Soils are then classified as “low”, “medium”, or “high” testing according to the results of field plot research.
There is no single soil test method that works on every soil. If used on the same soil sample, different extracting solutions may remove phosphorus from different fractions of the various phosphate minerals and organic materials. This can be affected by many factors, including soil pH, type and amount of organic matter, original soil parent materials, and others. Different soil test methods must be interpreted differently.
Soil test methods are usually named after the scientist who first developed the procedure. The following discussion includes the major agronomic soil test methods used in the United States for determining “plant available” soil phosphorus.
The Bray-1 soil test method was developed in 1945 for Illinois soils. Sometimes referred to as the "weak Bray", it uses a weak solution of hydrochloric acid (HCl) and ammonium fluoride (NH4F) at pH 2.6 to solubilize and extract soil phosphorus. It was also used for available potassium extraction. Bray-1 is designed to extract forms of soluble phosphorus, primarily calcium phosphates (other than apatite) and a portion of the iron/aluminum phosphates.
The Bray-1 is a standard soil test phosphorus method that is widely used in the Corn Belt states. It is best adapted for testing soils with a neutral to acidic pH and a texture of silty clay loam or finer. The Bray-1 test should be avoided on high CEC soils, calcareous soils, or soils with relatively large amounts of calcium phosphates.
Soils containing more than about 2% carbonates (i.e. soils with "medium" to "high" excess lime) present problems for the Bray-1 extractant. The hydrochloric acid in the Bray-1 can be neutralized by the soil carbonates (> 2%) and may not properly extract phosphorus. In effect, the neutralized Bray-1 behaves like water as an extractant. This can result in "low" soil test phosphorus results which underestimate the true phosphorus availability, leading to high phosphate fertilizer recommendations.
The Bray-1 extractant has probably been the most widely used soil phosphorus extractant, but has been replaced by the Mehlich-3 extractant in a number of areas. Typical Bray-1 phosphorus results are about 90% those obtained by Mehlich-3 extraction from neutral to acidic soils. Mehlich-3 phosphorus results from alkaline to calcareous soils are roughly 2 to 21⁄2 those obtained using an Olsen bicarbonate extraction.
The Bray-2 extractant was also developed in 1945 and is often called the “strong” Bray. It is similar to the Bray-1, but uses a stronger hydrochloric acid concentration than the Bray-1 It doubles the amount of extract solution and uses the same concentration of ammonium fluoride.
The Bray-2 was adopted by some states during the 1950's when unprocessed rock phosphate was a major fertilizer source. Less than 1% of the available phosphorus in rock phosphate is water soluble. Rock phosphate must be applied to an acidic soil, so it can dissolve and become available for crop uptake. This process may take several years. It is not an effective fertilizer phosphate source for neutral to alkaline soils because it does not dissolve.
The higher acid concentration in the Bray-2 extracted more phosphorus from rock phosphate than did the Bray-1. Bray-2 results were considered superior for estimating fertilizer requirements in those soils that had been previously fertilized with unprocessed rock phosphate. Current research shows the Bray-2 can overestimate the phosphorus available from rock phosphate as compared with water soluble phosphorus.
The Bray-2 extractant can also be neutralized by the carbonates found in calcareous soils, but it may take more excess lime to accomplish neutralization than does the Bray-1. The point at which this occurs has not been documented in research literature. The Bray-2 extractant is used in countries like Malaysia and Colombia for acid, tropical soils.
The sodium bicarbonate method (NaHCO3, pH 8.5) was developed by Colorado State University as an extractant for alkaline Colorado soils. This extractant solution tends to extract the more soluble calcium phosphate forms, aluminum and iron phosphates, and adsorbed phosphates. The phosphorus concentration found in the filtered extracting solution is usually less than that from acidic extractions.
The Olsen bicarbonate extractant is best adapted to calcareous soils with medium to fine texture, particularly those with over 2% calcium carbonate, but has been shown in some research to be reasonably effective for acidic soils. It is used widely as the primary extractant by state and private soil testing laboratories in the western Mountain states and the Pacific Coast states where alkaline and calcareous soils dominate.
The Mehlich-1 method was developed in 1953 by North Carolina State University as one of the first multi- element extractants. It is commonly referred to as the “double-acid” method because it uses hydrochloric acid (HCl) and sulfuric acid (H2SO4) to extract phosphorus, potassium, calcium, magnesium, and zinc. Mehlich-1 is best suited for sandy, acid soils with low cation exchange capacity (CEC, less than 10 meq/100g) and organic matter levels less than 5%. It is a standard method commonly used in the southeast U.S. and Coastal Plains.
Mehlich-1 Is not suited for alkaline soils because it tends to overestimate phosphorus supply by calcium phosphates in the soil. The Mehlich-3 extractant is replacing the Mehlich-1 in a number of states to accommodate situations where the soil pH is above 6.5 and may have free carbonates. Phosphorus results from Mehlich-3 extraction are roughly 11⁄2 times those from Mehlich-1 extraction.
The Mehlich-2 method was developed at North Carolina State University in 1978, designed for use on a wide range of soil type. It was an improvement to overcome problems with the earlier Mehlich-1, "double acid" method. It uses hydrochloric acid (HCl), acetic acid (HOAc), ammonium chloride (NH4Cl), and ammonium fluoride (NH4F) at pH 2.5. The solution is buffered to maintain a pH of at least 2.9, so is not neutralized by the excess lime levels found in most calcareous soils. It is one of several "multi-element" extractants and can be used to test soils for phosphorus, potassium, calcium, magnesium, sodium, zinc, and manganese.
The Servi-Tech Laboratory used the Mehlich-2 from 1979 to 1993 as a routine soil phosphorus test method. Soil phosphorus results from neutral to acidic soils were nearly identical to those obtained using Bray-1. Mehlich-2 phosphorus results were about double those by Olsen bicarbonate extraction from alkaline and calcareous soils.
The Mehlich-3 extractant was developed in 1984 by North Carolina State University as an improvement on the Mehlich-2 extractant, both to include available copper extraction and minimize instrument corrosion problems. It is a multi-element extractant that uses acetic acid (HOAc), ammonium nitrate (NH4NO3), ammonium fluoride (NH4F), nitric acid (HNO3), and EDTA, adjusted to pH 2.6. It is used for extracting phosphorus, potassium, calcium, magnesium, sodium, zinc, copper, and manganese.
The Mehlich-3 method is suitable for use as a phosphorus extractant on a wide range of soils. Phosphorus can be determined colorimetrically or by ICP. The Mehlich-3 has replaced the Bray-1, Mehlich- 1, AA-EDTA, and Morgan extractants in many states.
Texas A&M University developed an extractant in 1980 derived from the Morgan extractant. It includes ammonium acetate (NH4-OAc), hydrochloric acid (HCl), and EDTA1, buffered to a pH of 4.2. It is a multi- element extractant used not only for phosphorus, but for potassium, calcium, magnesium, and sodium. EDTA is a chelate included to help extract organic and metal-bound phosphates.
Subsequent research found that this extractant tended to overestimate plant available phosphorus in certain production areas of Texas. The Mehlich-3 method replaced the acidic ammonium-acetate-EDTA as the phosphorus extractant of choice in 2004.
The ammonium-bicarbonate-DTPA (AB-DTPA) multi- element extractant was developed in 1977 by Colorado State University (CSU). It has a pH of 7.6 and is used to analyze nitrate, phosphorus, potassium, zinc, iron, manganese, and copper in alkaline soils. DTPA2 is a chelating compound used primarily for micronutrient extraction.
AB-DTPA extractable phosphorus is strongly related to the calcium phosphate in soil. This method provides soil test phosphorus results that are about half those obtained from using the Olsen sodium bicarbonate extraction. AB-DTPA is primary method used by CSU for available soil phosphorus determination.
The Morgan extraction procedure was developed at the University of Connecticut in the 1930's for use on acidic New England soils and was later modified. It is a weak acid extractant that uses sodium acetate (NaOAc) or sodium hydroxide (NaOH) and acetic acid (CH3COOH), buffered to pH 4.8.
Most New England state universities and Cornell University still use a modified Morgan extract as multi- element extractant for phosphorus, potassium, calcium, magnesium. and (in some cases) micronutrients. The majority of Northeast laboratories utilize ICP for determination, although several utilize a colorimetric method for phosphorus. The Mehlich-3 extraction typically provides phosphorus results that are about five to ten times that of a Morgan extraction on the same sample.
The following table compares the various methods and provides the reference information including the year the method was published.
¶ Table 1. Comparison of Phosphorus Testing Methods |
||||||||||
| Method | Bray-1 | Bray-2 | Mehlich-3 | Mehlich-2 | Mehlich-1 | Olsen | AB-DTPA | AA_EDTA | Morgan | Modified Morgan |
| Reference (year) | 1. (1942) 2. (1945) |
2. (1945) | 3. (1984) | 4. (1978) | 5. (1953) | 6. (1954) | 7. (1977) | 8. (1980) | 9. (1941) | 10. (1950) |
| Adaptability Limits | Neutral to slightly acidic soils, moderate CEC |
Acidic soils with previous rock phosphate application |
Wide range of soils | Wide range of soils | Acid sandy soils, low CEC (< 10 meq/100g) |
Neutral to alkaline soils | Neutral to alkaline soils | Neutral to acidic soils | acidic soils with CEC < 20 meq/100g |
acidic soils with CEC < 20 meq/100g |
| Sample Size | 2 g | 2 g | 2 g | 2 g | 5 g | 2 g | 10 g | 1 g | 12.5 g | 5 g |
| Extractant Volume | 20 ml | 20 ml | 20 ml | 20 ml | 25 ml | 40 ml | 20 g | 20 ml | 50 ml | 20 ml |
| Extractant pH | 2.6 | 2.5 | 2.5 | 2.5 | 1.25 | 8.5 | 7.6 | 4.2 | 4.8 | 4.8 |
| Extracting Solution | 0.025 N HCl 0.03 N NH4F | 0.10 N HCl 0.03 N NH4F | 0.013 M HNO3 0.015 M NH4F 0.025 M NH4OAc 0.25 M NH4NO3 |
0.12 N HCl 0.015 N NH4F 0.2 N HOAc 0.2 N NH4Cl |
0.05 N HCl 0.025 N H2SO4 |
0.5 N NaHCO3 |
1 M NH4CO3 0.005 M DTPA |
1.4 M NH4OAc 1.0 M HCi 0.225 M EDTA |
0.72 N NaOAc 0.52 N CH3COOH |
0.62 N NaOAc 1.25 N CH3COOH |
| Shaking Time | 5 min. | 5 min. | 5 min. | 5 min. | 5 min. | 30 min, | 15 min. | 60 min. | 15 min. | 15 min. |
Footnotes:
1 EDTA = ethylenediaminetetraacetic acid
2 DTPA = diethylene triamine pentaacetic acid
1. Bray, R. 1942. Rapid tests for measuring and differentiating between the adsorbed and acid-soluble forms of phosphate in soils. Illinois Ag. Expt. Station Agronomy Dept. Pamphlet AG 1028.
2. Bray, R., and L. Kurtz. 1945. Determination of total, organic, and available forms of phosphorus in soils. Soil Science 59:39-45.
3. Mehlich, A. 1984. Mehlich 3 soil test extractant: A modification of the Mehlich 2 extractant. Comm. in Soil Sci. and Plant Analysis 15(12):1409-1416.
4. Mehlich, A. 1978. New extractant for soil test evaluation of phosphorus, potassium, magnesium, calcium, sodium, manganese, and zinc. Comm. in Soil Sci. and Plant Analysis 9(6):477-492.
5. Mehlich, A. 1953. Determination of P, Ca, Mg, K, Na, and NH4. North Carolina Soil Test Division mimeo.
6. Olsen, S.R., C.V. Cole, F.S. Watanabe, L.A. Dean. 1954. Estimation of available phosphorus in soils by extraction with sodium bicarbonate. USDA Circular No. 939.
7. Soltanpour, P.N., and A.P. Schwab. 1977. A new soil test for simultaneous extraction of macro- and micro-nutrients in alkaline soils. Commun. Soil Sci. Plant Anal. 7:797-821.
8. Texas Agric. Expt. Station. 1980. Soil Testing Procedures. Texas Agric Ext. Serv., College Station, Texas.
9. Morgan, M.F. 1941. Chemical soil diagnosis by the universal soil testing system. Connecticut Ag. Expt. Station Bull. 450.
10. Lunt, H.A., C.L.W. Swanson , H.G.M. Jacobson. 1950. The Morgan soil testing system. Connecticut Ag. Expt. Station Bull. 541.
11. Zapata, F & R.N. Roy, ed. 2004. FERTILIZER AND PLANT NUTRITION BULL. 13. Use of Phosphate Rocks for Sustainable Agriculture. A joint publication of the FAO Land and Water Development Division and the International Atomic Energy Agency, Rome. 12. Laboratory Extractant Change. 2004. http://soiltesting.tamu.edu/webpages/extractant.html