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Abstracts - Soil Methods

Elrashidi, M.A. 2001. Selection of an appropriate phosphorus test for soils. USDA/NRCS, National Soil Survey Center, Lincoln, NE.


Phosphorus and Eutrophication

Phosphorus (P) is an essential element for plant growth and is often applied to agricultural land to increase crop production. Animal waste generally has a high concentration of P. Livestock feedlots and cattle grazing on grassland can introduce substantial amounts of P-rich manure to the environment. Nonpoint sources of P, such as surface runoff and subsurface leaching from agricultural land and livestock operations, are major contributors to eutrophication in freshwater bodies. Eutrophication has been linked to a variety of ecological and health problems, ranging from increased growth of undesirable algae and aquatic weeds to fishkills and human illness.

Phosphorus Loss From Soil

Phosphorus is lost from agricultural land to surface water bodies in sediment-bound and dissolved forms. Sediment-bound P includes P associated with minerals and organic matter. Dissolved P constitutes 10 to 40 percent of the P transported from most cultivated soils to water bodies through runoff and seepage (Sharpley et al., 1992). Surface runoff from grassland, forest, and uncultivated soils carries little sediment and carries dominantly dissolved forms of P. Unlike sediment-bound P, dissolved P is readily bioavailable and thus is the main cause of eutrophication. A concentration of P above 0.02 ppm in lake water generally accelerates eutrophication (Sharpley et al., 1999). This concentration is much less than the P concentration in soil solution of cultivated soils and leads us to an important question regarding the relationship between P in soil and surface runoff. Selection of an appropriate soil test is essential for understanding this relationship and for identifying nonpoint sources of P contamination from agricultural land.

Soil Phosphorus Tests

Many chemical solutions have been proposed to extract potential forms of P in soils. Water probably was the first extractant that researchers applied to measure P in soils. The small amounts of soil P extracted by water (mainly P in dissolved forms) and difficulties related to chemical analysis limit the use of water as an extractant.

Bray and Kurtz (1945) suggested a combination of HCl and NH4F to remove easily acid soluble P forms, largely Al- and Fe-phosphates. In 1953, Mehlich introduced a combination of HCl and H2SO4 acids (Mehlich 1) to extract P from soils in the north-central region of the U.S. Sulfate ions in this acid solution can dissolve Al- and Fe-phosphates in addition to P adsorbed on colloidal surfaces in soils. In the early 1980s, Mehlich modified his initial soil test and developed a multi-element extractant (Mehlich 3) which is suitable for removing P and other elements in acid and neutral soils. Mehlich 3 extractant (Mehlich, 1984) is a combination of acids (acetic [HOAc] and nitric [HNO3]), salts (ammonium fluoride [NH4F] and ammonium nitrate [NH4NO3]), and the chelating agent ethylenediaminetetraacetic acid (EDTA).

Olsen et al. (1954) introduced 0.5 M sodium bicarbonate (NaHCO3) solution at a pH of 8.5 to extract P from calcareous, alkaline, and neutral soils. This extractant decreases calcium in solution (through precipitation of calcium carbonate), and this decrease enhances the dissolution of Ca-phosphates. Moreover, this extracting solution removes dissolved and adsorbed P on calcium carbonate and Fe-oxide surfaces.

The concept of P-sink was applied to measure the amount of soil P which can be released in response to such sink. An anion exchange resin (AER) and Fe-oxide impregnated paper (IIP) were used (in a water matrix) as a P-sink to determine available P in a wide range of soils. Recent publications describe AER (Sharpley, 2000) and IIP (Chardon, 2000) methods.

Selecting an Appropriate Test

When extracting solution is added to soil, there are four basic reactions by which P is removed from the solid phase: 1) dissolving action of acids, 2) anion replacement to enhance desorption, 3) complexing of cations binding P, and 4) hydrolysis of cations binding P. Therefore, the selection of a P soil test depends on the chemical forms of P in the soil.

One can conclude that for acid and neutral soils, Al- and Fe-phosphates are the primary source of P. A soil extractant that removes these minerals along with dissolved and adsorbed forms should be a good choice. Either Bray 1 or Mehlich 3 can be used successfully. Mehlich 3 may be preferable, since it can also remove available forms of macronutrients (Ca, Mg, K, and Na) and micronutrients (Cu, Zn, Fe, and Mn) for analyses of these soils.

Calcium phosphates are the main P minerals in alkaline and calcareous soils, whereas neutral and slightly acid soils (pH 6 to 7) may contain both Ca- and Al-phosphates. The NaHCO3 extractant (Olsen et al., 1954) can remove Ca-phosphates and phosphate adsorbed on surfaces of calcium and magnesium carbonates along with Al-phosphates and is considered the most suitable P test for these soils.

A water extract removes dissolved forms of P but very little of the adsorbed and mineral forms. It is suitable for both acid and calcareous soils. The amount of P extracted is small for most soils, and may not reflect all forms of labile P. A P-sink in a water matrix can remove more P from soil than water extract alone. As an alternative to water, either the AER or IIP method can be used to measure bioavailable P in soils.

Elrashidi, M.A., M.D. Mays, and C.W. Lee. 2003. Assessment of Mehlich3 and ammonium bicarbonate-DTPA extraction for simultaneous measurement of fifteen elements in soils. Commun. Soil Sci. Plant Anal. 34: 2817–2838.


Few existing extractions such as Mehlich3 and ammonium bicarbonate-DTPA (ABDTPA) can be used as a multielement soil test. A multielement extraction is attractive to scientists and soil testing laboratories because it eliminates the need for multiple extractions and allows simultaneous measurement of elements by using the Inductively Coupled Plasma (ICP).The objective of this study was to evaluate Mehlich3 and ABDTPA for simultaneous measurement of 15 elements in 30 acidic and 20 alkaline U.S. soils from 21 states. Widely accepted and conventional soil tests (Bray1 and Olsen for phosphorus (P); NH4OAc for calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na); and diluted HCl and DTPA for aluminum (Al), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn)) were employed for the evaluation. Single and multiple regression analyses were applied to investigate the relationship between Mehlich3 or ABDTPA and the respective soil test. The results can be summarized as follows: (i) Mehlich3 provided a good measurement for labile P in all soils, whereas ABDTPA provided reliable results for alkaline soils; (ii) Mehlich3 was a suitable extract for Ca, Mg, K, and Na for all soils, whereas ABDTPA could not be used for Ca; (iii) Mehlich3 or ABDTPA was an appropriate extract for Al, Cd, Cu, Fe, Mn, Ni, Pb, and Zn in all soils. The fact that most soils investigated had trace amounts of Co and Cr hindered their evaluation. Accordingly, Mehlich3 extraction could be recommended for simultaneous measurement of at least 13 elements in soils.

Elrashidi, M.A., M.D. Mays, and P.E. Jones. 2003. A technique to estimate release characteristics and runoff phosphorus for agricultural land. Commun. Soil Sci. Plant Anal. 34:1759–1790.


Using soil tests to estimate P released from agricultural soil by runoff had limited success because P loss is a function of source and transport parameters. There are good procedures applying these parameters, but they are lengthy and expensive and demand numerous laboratory and field data. The objective is to develop reliable exploratory techniques to estimate runoff P for agricultural land. Various forms of P, like moisture, are held by soil particles at different energy levels. Kinetic energy exerted by raindrops on surface soil plays a major role in releasing P. The Soil Survey Laboratory suggests an anion exchange resin (AER) method to determine P release characteristics (PRC) for soils. In this method, different levels of energy are applied by water on soil particles when a soil suspension is shaken for various periods. Understanding the relationship between shaking and rainfall energy enabled us to use the AER method to predict P released by rainfall. The USDA/NRCS (SCS) runoff equation is applied to determine the relationship between rainfall and runoff for agricultural watersheds. Soil hydrology, rainfall, and type of vegetation are parameters utilized by the runoff model. We propose a technique implementing the AER method and runoff equation to estimate runoff P for agricultural land. The estimated runoff P for 24 soils investigated ranged between 0 and 8.3 kg P/ha/y (fallow), 0 and 7.5 kg P/ha/y (cropland), and 0 and 6.0 kg P/ha/y (grassland). Field studies on different benchmark soils of the United States are in progress to estimate runoff P by using rainfall simulators. These data could be used to verify and calibrate the technique.

Elrashidi, M.A., D. Hammer, C. Seybold, R.J. Engel, R. Burt, and P. Jones. 2007. Application of equivalent gypsum content to estimate potential subsidence of gypsiferous soils. Soil Sci. 172:209–224.


The extensive application of surface irrigation for farmland throughout arid and semiarid areas may pose severe engineering problem for gypsiferous soils. Subsidence of gypsiferous soils is attributed to the dissolution and removal of gypsum by water from the gypsic horizon. Gypsum content is usually used to estimate subsidence of these soils. However, a number of water-soluble minerals may occur in association with gypsum in gypsiferous soils. Thus, subsidence should be attributed to the dissolution and removal of both gypsum and other water-soluble minerals in soils. The objectives were (i) to develop a laboratory method (equivalent gypsum content, or EGC) to estimate both gypsum and other water-soluble minerals in gypsiferous soils and (ii) to apply the EGC method to estimate subsidence of these soils. We used the relationship between dissolved minerals and electrical conductivity in soil/water solutions under equilibrium to estimate the EGC for soils. The EGC is defined as the quantity of both gypsum and other water-soluble minerals expressed as gypsum% (by weight) in soils. We measured the EGC for 92 gypsum-rich soil samples collected from different arid and semiarid areas in the United States. A highly significant correlation (r=0.97**) was found between the EGC and gypsum determined by the standard acetone method. The EGC was greater than gypsum content for almost all soils where the average of EGC was 26.3% compared to 20.2% for gypsum. Using gypsum% would underestimate subsidence of these soils. We suggest the application of EGC instead of gypsum content to estimate subsidence of gypsiferous soils. Further, the EGC could be applied to provide a reasonable estimation of gypsum% for soils.

Equivalent gypsum content, gypsiferous soils, subsidence, gypsic horizon, electrical conductivity.