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Labratory Protocol for PST1 Resin Capsules

WECSA Home Consulting Service Reference Soil Monitoring

1. The Soil Sample
2. Sample Preperation
3. Handling the Resin Capsule
4. Ion Accumulation
5. End of Extraction
5a. Containers
6. Desorption
7. Chemical Analysis
8. Expressing Values
9. Evaluation of Results
10. Accuracy
11. Trouble Shooting
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UNIBEST resin capsules1/ are designed for use:
  1. in the laboratory on samples collected from the field and delivered to the lab in field-fresh condition, or
  2. in situ for detection of elemental bioavailability as influenced by existing conditions.

When used in the lab, this procedure is known as THE PHYTOAVAILABILITY SOIL TEST (PST). The following is a description of how to use these capsules for laboratory soil testing. For in situ applications, consult the appropriate protocol description from UNIBEST, Inc. Analysis of capsules after in situ use follows the same procedures described below, starting with Capsule desorption.

The PST is a modern approach to soil testing that is based on universal accumulation of nutrients and other elements by a capsule containing mixed-bed ion-exchange resins. The process is simple and convenient, but it must be performed correctly and carefully to ensure accurate results.

The Soil Sample:

As with any soil test methodology, the soil sample being tested must properly represent the area from which it was collected. Use recommended procedures for collecting the sample. Handle each sample with care to prevent contamination and to ensure that changes in nutrient status have not taken place prior to testing. As soon as the sample is collected, transport or ship it to the laboratory.

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Sample Preparation:

Each bulk sample should contain about 200 to 500g of field-fresh soil, protected in a plastic-lined sample bag or other appropriate container. Samples should be kept cool and immediately delivered to the laboratory. If the period between sampling and sample preparation extends to several hours, maintain temperature near 0°C, but not freezing, to minimize mineralization and chemical activity. Sample freezing alters the extractability of many elements [6, 8, 11]. Freezing would, however, be preferable to allowing samples to stand for several days when even greater changes could take place.

Make the entire sample into a (near) saturated paste, a condition that allows uniform ion diffusion. This is simplified by placing the sample in a mixing bowl with a tap for dispensing distilled-deionized water suspended directly above it. Remove coarse materials (rocks, roots, other debris) by hand and add water while stirring the sample. Using a dough mixer (fixed or portable) will speed up the mixing process. Saturated soil paste is described as the condition when the soil contains enough water so the "soil paste glistens as it reflects light, flows slightly when the container is tipped, slides freely and cleanly off a spatula, and consolidates easily by tapping or jarring the container after a trench is formed in the paste with the side of the spatula" [7]. Many soils require several hours to completely imbibe initially added water, with more water needed to obtain saturated paste status. Studies have shown, however, that the exact water content is not critical to obtaining repeatable resin capsule values [10]. After completion of water addition, thoroughly mix the sample to ensure uniformity. Transfer at least 50 to 60g of the paste to a plastic or glass container. A disposable 100ml specimen cup with screw cap works well, allowing disposal of the container and spent sample after extraction and eliminating the need for cleaning labware. The remainder of the main sample may be used for replication of extraction, or determination of other soil properties of interest (e.g., pH, EC, OM), using accepted saturated paste methods or appropriate adaptations.

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Handling the Resin Capsule:

Resin capsules are provided in plastic bags or screw-cap containers (UNIBEST, Inc., P.O. Box 5095, Bozeman, MT 59717) and must be protected from contamination during handling. Wear chemically clean (non-powdered) gloves during capsule handling. Many powders used to lubricate gloves contain zinc or other potential contaminants. Remove a single capsule from its container and place it into the soil sample, using a plastic tweezer. The capsule should be surrounded by a minimum of 10 to 15mm soil paste, as ions will diffuse several mm within a day or two. Intimate contact between paste and capsule can be assured by placing the capsule in the center of the paste and gently tapping the container on the lab bench to consolidate the mixture. Cap the container and set it aside under the appropriate conditions.

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Ion Accumulation Conditions and Duration:

The rate of ion accumulation by resin is temperature sensitive [14] and time-dependent. A standard, uniform temperature should be maintained. We recommend a temperature approximately 20°C in temperate regions. Quantities of ions accumulated by the capsule continue to increase with time, so the duration of accumulation depends on study objectives. Typical accumulation curves are presented in Fig 1. In most cases there is a rapid initial accumulation of ions (dependent mostly on initial soil solution concentrations) lasting a day or so, followed by a quasi-steady-state accumulation that reflects continued nutrient release from solid phases and diffusion to the capsule. Two or more capsules, each removed at different times, are required to develop "nutrient accumulation curves" [1] as illustrated in Fig 1. If values for only one time interval are desired, one capsule will provide data for all target ions.

Fig 1. Resin capsule ion accumulation curves. Fig 1: Kinetics of Na, K, Mg, and Ca adsorption on the resin capsule during 4 week anaerobic incubation in a Typic Tropaquept at IRRI, Philippines. Symbols show observed values and the solid lines represent the power function fitted to the data [1].

Analyzable quantities of most macronutrient ions are accumulated in about 24 to 48h. Several days or weeks may be required to accumulate analytical quantities of some elements from some soils. Methods for enhancing detection levels for target elements may be required in some instances.

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Termination of Extraction:

At the desired time, uncap the container and remove the resin capsule, using gloves or a clean plastic tweezer, and place it on a small plastic screen. Rinse off soil particles adhering to the surface of the capsule by directing a stream of deionized water onto the capsule while rotating it. A gentle kneading action may enhance this process. It is important to use deionized water to prevent the desorption of accumulated ions during washing. Another method is to push a coarse stainless steel hypodermic needle into the center of the capsule and force water into the capsule, dislodging surface soil particles and flushing them away. After rinsing, place the capsule into a clean container. If the desorption process is not done immediately, the capsule can be placed into a 5ml snap-cap plastic cup (or other appropriate container) and stored. Capsules should be kept cool in the refrigerator (near 0°C) if the storage period will be more than a day or two.

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Several methods of desorbing (stripping) accumulated ions from the capsule can be used. Resins have different affinities for specific ions, so depending on the target ions, the rigor of the stripping process can vary. If only weakly adsorbed monovalent ions are of interest, they can be recovered more readily than can higher valence ions for which the resins have greater affinity.

For most general applications a simple, recommended method is as follows: place the capsule in a container with about 50ml capacity, add 20ml of

2 N HCl, cap the container, and shake on a reciprocating shaker for 20 minutes. Pour the solution into a bottle (minimum 60ml capacity) and repeat this process two more times (total of three times). All extracts should be combined into the same bottle. This provides 60ml of analyte for the analysis of target ions.

Several alternative methods will remove adsorbed ions, but it is important to understand that ion accumulation and removal from resins involves diffusion within resin beads. Because diffusion is time-dependent, instantaneous recovery does not occur. Thus, a reasonable period must be allowed for desorption. Also, different amounts of ions will be recovered if the extraction time is varied. Regardless of the approach, always use a uniform, standard method that will retrieve uniform proportions of adsorbed target ions from each capsule used in a particular study.

Spent resin capsules should be discarded, following approved disposal procedures. Capsules will not function according to their designed specifications if they are re-used. Capsules are initially saturated with H

+ and OH- and cannot be regenerated into this condition without first separating the cation and anion portions of resin. Regenerating the resins into other ionic forms destroys the capsule's function as an appropriate sink for many target ions in the medium. In addition, stripping of adsorbed ions (unless rigorous, time-consuming procedures are used) is not 100% efficient. Thus, errors of unknown magnitudes will be introduced for certain target ions during each re-use.
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Chemical Analysis:

The capsule stripping solution is the analyte for any element of interest. Requirements for analysis by any chosen method are:

  1. that the concentration of target ion be high enough to analyze, and
  2. that the analyte be modified appropriately if the 2 N HCl solution is not directly analyzable

The 2 N HCl analyte can be directly used for atomic adsorption, ICP, or other procedures. In cases where Cl or Br are of interest, 2 N H2SO4 may be used as the stripping agent. If too little target ion is present after a specific duration of adsorption, longer adsorption periods may increase concentrations to analyzable levels. Various methods of concentrating the analyte may also suffice. In some instances the concentration of specific target ions may be too strong, requiring dilution prior to analysis. Neutralization of the acid may be required for certain colorimetric procedures.

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Expression of Values:

Resin capsules function as a sink, accumulating ions that contact the surface of the capsule. Some ions are present in the soil solution when the capsule is introduced, and other ions will diffuse to the capsule surface. Diffusion occurs because the continual removal of ions from the solution at the resin surface creates a concentration gradient. Multi-element diffusion is simultaneous and independent [13], and the longer the accumulation period, the greater the distance from which the ions may diffuse. Each soil has its own diffusion characteristics, hence, the volume of soil contributing ions to the capsule during a specific interval is soil-dependent. Unless additional measurements are made, this volume (or soil mass) will not be known. What is known after capsule analysis is the quantity of each target ion that was bioavailable at a point in the sample during the adsorption period. This is like analyzing plants to determine quantities accumulated; the quantity of soil contributing to plant uptake of elements also generally is not known.

Converting values from analysis of each ion in the analyte to mass (milligrams or micrograms) or chemical equivalence (millimole or micromole) allows units of expression as mass (or chemical equivalence) per unit time. If desired, values may be expressed per unit of contact surface area of the capsule (each capsule has a functional surface area of 11.4cm

2. Some research, has used "resin adsorption quantity" (RAQ) in units of µmoles cm-2 at time t (e.g., see Fig 1). If measurements are all taken over a constant time, the t component can be ignored and values can be compared directly. A general formula for calculating RAQ (mmol cm -2) is given by:

RAQ = c V/M A Equation 1

where c is the concentration of nutrient in the HCl extract obtained from the capsule (mg l-1), V is the volume of the extract (ml), M is the molar mass of the nutrient, and A is the capsule surface (11.4cm2).
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Evaluation of Resin Capsule Results:

Depending on the objectives of the study, capsule results may stand alone, or they may require supporting data. If testing is done in a uniform manner and for a standard time, values from capsule to capsule are directly diagnostic. Larger or smaller values for any ion can be interpreted to indicate greater or lesser bioavailability of that ion. The magnitude of difference between values can serve as an index of actual differences in the medium being tested.

It is important to realize that the ratio of ions accumulated by the resin capsule do not translate into identical ratios in the sample being tested. Results are influenced by soil dynamics. In addition, resins have different affinities for each ion, so they will accumulate more of a high-affinity ion from a solution having a given concentration than from the same concentration of a low-affinity ion. Determining relative affinities (or quantitative separation factors) of target ions in the specific medium being studied may help the interpretation of results [9].

A data base for evaluating PST values may be developed by conducting side-by-side comparison of PST vs. current soil test values, or, more appropriately, by correlating PST values directly with plant analysis, crop performance, or other applicable approach.

To better understand PST values, it is suggested that "nutrient release curves" (Fig 1) be developed for individual soils (or groups of similar soils). Separate PST extractions over increasing lengths of time are needed to develop these curves. To do this, place three separate portions of a sample, prepared as described above, in separate cups. A capsule is placed into the paste in each cup. Allow one capsule to remain in the paste for 2 days, the second one for 4 days, and the third capsule for 6 or 7 days. Results from these three times of extraction will reveal the nature of nutrient release from the soil to a "sink" over time for that particular soil. Release curves for each element of interest can be determined from the same three capsules, indicating how effectively each soil can deliver each nutrient (or other ion of interest) for plant uptake.

Once you know the nature of "nutrient release curves" for specific soils or groups of soils, subsequent analysis would require only one extraction time, with one capsule, to provide the data on which to base management recommendations for soils with each type of release curve.

As long as all results are obtained in an appropriate, uniform manner, and over equal times, PST results provide data showing relative phytoavailability of each nutrient (or other elements of interest) for a specific soil, differences among various soils, or changes from year-to-year on a given soil. Calculation of nutrient ratios for each sample can provide additional insight to assist in interpretations or development of management recommendations.

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Accuracy of Resin Capsule Results:

Soil testing procedures based on chemical extractions are affected by numerous errors, including sample processing (drying, sieving, grinding) [2], subsampling for chemical analysis [12], chemical extraction (chemicals, soil to solution ratio, shaking conditions), and the actual determination of nutrient concentrations in the extract. Efforts to standardize methods of soil and plant analysis have been reported [3], but many inter-laboratory comparisons have demonstrated high variability. For example, a soil sample submitted to 23 laboratories in Brazil resulted in CVs ranging from 11 to 57% for organic matter and extractable cations among the laboratories [5]. Coefficients of variation for Olsen-P were 10 to 16% during long-term analyses of two laboratory samples [4].

Resin capsule results may have errors due to subsampling (quality of homogenization of the soil paste), resin nutrient extraction (indigenous variation among capsules, micro variability in the soil paste, capsule washing, and chemical extraction), and analysis. Duplicate or triplicate analyses of the same soil sample provide a measure of the precision.

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If problems are encountered, call UNIBEST, Inc. at; 1-406-587-4630, send a fax to 1-406-587-4870, or send an Email to: eskogley@montana.com

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For More Information from a published paper, click on:

Earl O. Skogley, Achim Dobermann, Gordon E. Warrington, Mirasol F. Pampolino and Ma. Arlene A. Adviento.  1996.  Laboratory and Field Methodologies for Use of Resin Capsules.
Sciences of Soils, Vol. 1 - 



[1] Dobermann A., Langne H., Mutscher H., Skogley E.O., Neue H.U., J.E. Yang, Adviento M.A.A. and Pampolino M.F. (1994): Nutrient adsorption kinetics of ion exchange resin capsules: A study with soils of international origin. Commun. Soil Sci. Plant Anal., 25, 1329-1353.

[2] Houba V.J.G., Chardon W.J. and Roelse K. (1993): Influence of grinding of soil on apparent chemical composition. Commun. Soil Sci. Plant Anal., 24, 1591-1602.

[3] Houba V.J.G., Novozamsky I. and van der Lee J.J. (1994): Standardization and validation of methods of soil and plant analysis as conditions for accreditation. Commun. Soil Sci. Plant Anal., 25, 827-841.

[4] Kalra Y.P. and Maynard D.G. (1991): Methods manual for forest soil and plant analysis. Forestry Canada, Edmonton.

[5] Quaggio J.A., Cantarella H. and van Raij B. (1994): Evolution of the analytical quality of soil testing laboratories integrated in a sample exchange program. Commun. Soil Sci. Plant Anal., 25, 1007-1014.

[6] Rayment G.E. (1993): Soil Analysis: A Review. Austral. J. Exp. Agr., 33, 1015-1028.

[7] Rhoades J.D. and Miyamoto S. (1990): Testing soils for salinity and sodicity. In: R.L. Westerman (ed.) Soil testing and plant analysis. Third ed., Soil Science Society of America, Madison, WI, p. 299-336.

[8] Ron Vaz M.D., Edwards A.C., Shand C.A. and Cresser M.S. (1994): Changes in the chemistry of soil solution and acetic-acid extractable P following different types of freeze/thaw episodes. Europ. J. Soil Sci., 45, 353-359.

[9] Skogley E.O. and Dobermann A. (1996): Synthetic ion-exchange resins: Soil and environmental studies. J. Environ. Qual., 25, 13-24.

[10] Skogley E.O., Georgitis S.J., Yang J.E. and Schaff B.E. (1990): The Phytoavailability Soil Test-PST. Commun. Soil Sci. Plant Anal., 21, 1229-1243.

[11] Walworth J.L. (1992): Soil drying and rewetting, or freezing and thawing affects soil solution composition. Soil Sci. Soc. Am. J., 56, 433-437.

[12] Wang D., Snyder M.C and F.H. Bormann (1993): Potential errors in measuring nitrogen content of soils low in nitrogen. Soil Sci. Soc. Am. J., 57, 1533-1536.

[13] Yang J.E. and Skogley E.O. (1992): Diffusion kinetics of multinutrient accumulation by mixed-bed ion-exchange resin. Soil Sci. Soc. Am. J., 56, 408-414.

[14] Yang J.E., Skogley E.O. and Schaff B.E. (1991): Nutrient flux to mixed-bed ion-exchange resin: temperature effects. Soil Sci. Soc. Am. J., 55, 762-767.

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