Nutrient Extraction and Chemical Analysis
C. Owen Plank
Associate Professor, University of Georgia
The process of selecting a soil test laboratory to provide analytical services is an important step with which all turf mangers are confronted. This process could determine to a great extent the success or failure of a turf nutritional program. Therefore, it is essential to check with laboratories to make sure that the services provided are applicable to your region and meet your needs.
A soil test requires an extractant to determine the amount of plant nutrients in the soil. An extractant is a solution made of water and a certain concentration of a chemical and/or chemicals. A known amount of the extractant is added to a fixed amount of soil and the two are shaken together for a prescribed length of time. This mixture is then separated by filtering through filter paper. The soil is retained on the filter paper, and the extractant, now containing the dissolved plant nutrients, is caught in a vial. The amounts of nutrients in the extractant are determined using the appropriate laboratory instruments.
Some laboratories offer multiple extractants and the one selected for use on a sample is determined by the geographical region from which the sample originated. This is a practice that is used by many laboratories that provide services over a wide variety of geographical regions. However, some laboratories offer only one extractant and using the extractant on soils for which it was not designed or soils for which it is not calibrated can result in unreliable analytical results and recommendations. The ensuing section provides information on the nature and properties of some of the more commonly used extractants and the soil conditions for which they are best suited.
A number of extractants have been developed by soil chemists to assess the relative nutrient status of soils and to serve as the basis for making nutrient recommendations. The extractants are designed to remove (extract) a portion of a soil nutrient that can be correlated with some plant growth factor such as dry matter production, quality, etc. Thus, the total quantity of a particular nutrient is not extracted. The portion that is extracted represents only a small fraction of the total amount of a nutrient present in a soil and is related to the amount of the nutrient that may be potentially utilized by the plant. Therefore, the quantity an element extracted may be thought of as an INDEX VALUE because the amount of a nutrient taken up by a plant may be more or less than the amount extracted from the soil. These INDEX VALUES are used by researchers for correlation and calibration purposes to determine fertilizer needs. Common chemical extractants are noted in Table 1 for P and Table 2 for K, Ca, Mg, S, and micronutrients.
Extractants used for a particular nutrient are normally well correlated with one another, but each extracting solution may extract different quantities of a nutrient (Westerman, 1990). This is due to the fact that extractants are designed to extract different nutrients from different compounds or granules in the soil. Consequently, the chemical properties of extracting solutions are determined largely by the chemical and mineralogical properties of the soils for which they were developed. When extractants are used on soils for which they were not designed, attempting to interpret the analytical results can often be futile.
Turf managers will sometimes send split-samples to two different laboratories and may receive soil test reports with widely different values for some nutrients. Such discrepancies are most often noted with phosphorus and micronutrients and are generally the result of the use of different chemical extractants by the laboratories. However, when tests are properly calibrated for a region or a soil group, the categorical ranking (such as low, medium, high, etc.) should be similar regardless of the extractant. Table 3 contains a comparison of common extractants used for soils with extractable nutrient levels and category rankings.
Extractants for Phosphorus (P)
Most extractants for P were developed to estimate the capacity of the soil to supply P. They were designed to extract some fraction of the labile P and thus provide an index of the availability of P to plants over the growing season. The compounds that control the availability of P differ with soil conditions in different geographical regions of the country and different extractants have been developed for these situations. For example, P chemistry of soils in the northeast (Wolf and Beegle, 1995) and southeast primarily involves factors affecting the availability of aluminum and iron phosphates. Therefore, extractants in these regions consist of a dilute acid solution to dissolve portions of these minerals and extract P. The Mehlich 1 extractant, which is composed of HCl and H2SO4, and the Mehlich 3 extractant, which contains HNO3, are both strong acid extractants. The Morgan extractant, which is used by some states in the northeast and Pacific northwest and contains acetic acid, is a buffered weak acid extractant. Both of the Mehlich solutions extract more P from soils than the Morgan extractant. The Mehlich 3 extractant removes about 10 times more P from soils than the weak acid-based Morgan extractant (Wolf & Beegle, 1995). On coarse textured Georgia Coastal Plain soils, Gascho et al. (1990) found that Mehlich 3 extracts about 1.46 times more P than Mehlich 1. However, on finer textured soils the Mehlich 3 extractant removes about 1.6 times more P than the Mehlich 1 (Personal Communication, Dr. Ray Tucker, North Carolina Department of Agriculture). The ability of the Mehlich 3 to extract more P from soils is due to the presence of the F- ion (as NH4F), similar to the Bray P extractants used in the Midwest.
The extractant used predominantly in the Midwest is the Bray-Kurtz, commonly referred to as Bray P1. It is composed of HCl and NH4F. The extraction of phosphorus by this procedure is based upon the solubilization effect of the H+ on soil phosphorus and the ability of the F- to lower the activity of A13+ and to a lesser extent that of Ca2+ and Fe2+ in the extraction system. Clay soils with a moderately high degree of base saturation or silty clay loam soils that are calcareous or have a very high degree of base saturation will lessen the ability of the extractant to solubilize P. Consequently, the method should normally be limited to soils with pHw (soil pH determined in water) values less than 6.8 when the texture is silty clay loam or finer. Calcareous soils, or high pH, fine textured soils may be tested by this method provided the soil:solution ratio is changed from 1:10 to 1:50. However, it should be noted that the Olsen extractant is used most extensively for calcareous soils and would be a better choice (Anonymous, 1992, Knudsen and Beegle, 1988).
The Olsen extractant (NaHCO3) is quite effective for soils with medium to high CEC, high percent base saturation, moderate to high amounts of calcium phosphates, and containing free calcium carbonates (Thomas & Peaslee, 1973). It is used extensively in states such as Arizona, California, Utah, Wyoming, and Oregon. Research also indicates that the procedure works reasonably well on moderately acid soils. However, not many laboratories testing moderately acid soils use the procedure. The fact that it is not used more widely on these soils is probably due to the lack of correlation data.
Extractants for Exchangeable Cations - Potassium (K), Calcium (Ca), Magnesium (Mg), and Sodium (Na)
Exchangeable cations (i.e., cations adsorbed on to soil cation exchange sites) are determined using a variety of extractants. Soil tests for cations typically estimates the quantity of water-soluble and exchangeable forms of the cations by replacing the cations on the soil’s exchange sites with a counter ion such as Na+ (Olsen and Morgan), NH4+ (neutral ammonium acetate and Mehlich 3), or H+ (Mehlich 1 and Mehlich 3). The pH of these extracting solutions differs considerably because of their chemical composition and adjustments made in pH for the soils being analyzed. For example, acidic solutions are generally used in acidic to neutral soils and alkaline solutions for neutral to calcareous soils. Depending on the clay mineralogy, the acidic extractants may also extract some non-exchangeable K in addition to the water-soluble and exchangeable forms. Also, the ammonium-based extractants such as the neutral ammonium acetate and Mehlich 3 solutions will extract more K than the Mehlich 1 extractant or the Na-based Morgan extractant.
Extractants for Sulfur (S)
More than 20 different extracting solutions, including water, that have been used with varying degrees of success in determining plant available S levels in soils. Because of some of the factors discussed below, all laboratories do not test soils for S on a routine basis. Generally, testing soil for S is most successful in regions with low rainfall, low organic matter soils, and few industrial centers (Johnson and Fixen, 1990).
Most of the sulfur (S) in surface soils occurs in organic combination. In order for the organic forms of sulfur to become available to plants they must be mineralized to the inorganic sulfate (SO42- ) form. The sulfate form of sulfur is somewhat similar to the nitrate (NO3-) form of nitrogen in that it is easily leached from soils, particularly the coarse textured soils of the humid regions. Consequently, sulfur levels in surface soils are generally quite low and soil test summaries often indicate a very high percentage of soils in the “low” category. On many of these soils little or no crop response is observed from sulfur fertilization. This is due to plant roots being able to extract sufficient amounts of sulfur for good growth from below the sampling zone.
The sulfate form of sulfur is mobile in soils and this makes it very difficult to calibrate a sulfur soil test. In addition, predicting the amount of sulfur that will be released from the organic form to the inorganic sulfate form is not very precise because of the slow rate of mineralization by microorganisms and the numerous external factors that affect the mineralization process. For these reasons, in areas where responses are likely, it is more practical to include some sulfur in the fertilization program than to soil test for sulfur. If there is doubt as to whether the application rates are sufficient a plant tissue analysis will do a good job in revealing the sulfur status of the plants and aid in planning future applications of sulfur.
Extractants for Micronutrients - Boron (B), Copper (Cu), Iron (Fe), Manganese (Mn), and Zinc (Zn)
Several different extractants are used to evaluate micronutrient levels in soils. These include chelating agents, neutral salts, inorganic acid, or reducing agents Table 2 (Mortvedt et al., 1991). Extractanting solutions containing chelating agents, such as DTPA or EDTA, are used most widely in the U.S. Although many laboratories analyze turf samples for micronutrients, correlations of extractable micronutrient levels with turfgrass quality are less reliable than are correlations for the macronutrients (Mortvedt et al., 1991).
The two most common micronutrient deficiencies in turfgrasses are Fe and Mn. Other micronutrient deficiencies have not been widely documented under golf course or sports turf conditions. However, there is a growing concern relative to soil buildup and toxicities of Cu, B, Zn, Mo, and Ni. The application of a fertilizer with a micronutrient package once or twice a year is acceptable, but routine application of these nutrients is not recommended - even if soil tests indicate a low level. In such an instance, plant tissue analysis would provide much better information.
The analytical phase of soil testing is by far the least variable of the different phases of a soil test program. This is due to the development of highly sophisticated analytical instrumentation during the last 25 to 30 years. Major advances in electronics have allowed greater accuracy, precision, lower detection limits, simultaneous multi-element determinations, and a high speed of analysis. In addition instruments now use electronics that allow for self-control, self-evaluation, and self diagnosis. The instruments can be interfaced with computers allowing for automated-electronic capture of data, data interpretation, and computer generated recommendation report forms. These developments have reduced laboratory turnaround time and laboratory errors tremendously.
In the soil test laboratory, a chemical extractant is added to a small portion of the soil sample submitted; the sample is shaken, filtered, and an aliquot of the filtrate is collected for nutrient-element analyses. Most modern soil test laboratories use inductively-coupled plasma spectrographs(ICP) for rapid analysis. The instruments are capable of analyzing filtrates for 6 or more elements simultaneously at the rate of approximately 150 samples per hour.
Another portion of the soil sample is used for soil pH and lime requirement determinations. Known amounts of soil and distilled water are added to a suitable container (e.g. dixie cup); the suspension is stirred and allowed to equilibrate for a specified period of time (e.g. usually 30 minutes). After equilibration the suspension is re-stirred and the pH reading taken. This is often carried out using digital pH meters interfaced to a computer for data acquisition. If the pH is less than a specified value (e.g. 6.0) a buffer solution is added to the sample, shaken for a specified period of time (e.g. 15 - 30 minutes), and the pH recorded. This value (buffer pH or pHB) along with the pH value of the soil:water suspension (pHw) are used to determine the limestone recommendation.
It should be emphasized that each step in the analytical process is performed according to standardized laboratory procedures to insure accurate and reproducible results. In addition many laboratories today are certified or else participate in a proficiency testing program to further insure high quality results.