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Resin Extraction Represents Soil Ion Bioavailability

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Using UNIBEST resin capsules to measure soil ion availability offers a distinct advantage over other methods because resins give results like using plant roots directly in the soil.

By G. E. Warrington and E. O. Skogley 1996.

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The true measure of soil ion bioavailability is that quantity of ions which reach a plant root surface where they can be adsorbed by the root. As ions are taken up by plants they become less concentrated in the soil near the root surface. This creates a concentration gradient so that more ions move by diffusion, through the soil toward a root surface. Many soil factors influence the rate ions move toward replenishing the depletion at a root surface. Some of these are; (1) physical characteristics related to clay mineralogy, soil bulk density, temperature, and water content and (2) chemical properties like pH and release of ions from the solid (non-available) forms. Therefore, soil test results from chemical extraction of soil samples often fail to correctly predict ion availability to biological organisms because there is no accounting for these soil chemical and physical effects (Skogley and Dobermann 1996).
Resin adsorption of K at 96 hours Resin adsorption of Ca at 96 hours Resin adsorption of Mg at 96 hours
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Moisture Effects

The above graphs illustrate how changing soil temperature and moisture affect resin adsorption of three ion species, potassium (K +), calcium (Ca2+), and magnesium (Mg2+). For Ca2+ and Mg2+ increasing soil water alone has more effect on adsorption than just increasing temperature (Schaff and Skogley, 1982).


Ion diffusion rates follow changes in soil temperature and water.
  • There is a highly significant interaction between temperature and water on adsorption of all three ions.
Soil concentration of K vs distance from resin Soil concentration of Ca vs distance from resin Soil concentration of Mg vs distance from resin
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Diffusion Distance

These graphs show average soil ion concentrations for each of four 1-mm thick soil layers away from the resin when exposed to a resin sink for 6 and 96 hours. Differences between the two times for the 25-mm layer representing the bulk soil are not significant, but are due to experimental error in measuring extractable ions.


The amount of an ion taken up by a resin sink is limited only by soil properties and not the resins.
  • The ion concentration is always lowest in layer 1, next to the resin sink, and decreases over time.


Ion diffusion replaces a depleted supply of ions near the resin sink.

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Physical and Chemical Effects

In 1985, Skogley and Schaff described effects of several soil factors on the depletion of three ion species, K+, Ca2+, and Mg2+ in 20 soils. They kept resins in contact with moist soils for 96 hours at two temperatures (5 or 30°C). After removing the resin, each soil was sampled in 1-mm thick increments extending to 4 mm away from the resin/soil contact. A fifth layer at 25-mm was used to represent the bulk soil.


Under cooler conditions that could occur in the field, extractable K+ did not provide a good predictive index of the amount available for plant use.
  • The amount of ammonium acetate extractable K+ was correlated with diffusion only for warm temperatures.


For each ion species, the set of soil factors affecting diffusion at 5°C was different from the set at 30°C. For example:

  • At 5°C potassium diffusion correlation was positive for soluble K+ from a saturated paste, amount of K+ in the clay, and was negative for the illite content of clay.
  • At 30°C potassium diffusion correlation was positive for the ratio of K+ to Ca2+  + Mg2+ in a saturated paste, extractable soil K+, and extractable soil Mg2+.


Other soil physical and chemical conditions were also significantly correlated with diffusion of one or more ion species.
  • For example, electrical conductivity, cation exchange capacity, clay content, clay mineralogy.


Soil tests based on chemical extraction can be made more accurate by including additional measurements of these soil chemical and physical properties.
  • Resins directly account for the effects of these soil chemical and physical properties.
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Resin extraction represents ion bioavailability as a function of soil chemical properties and physical conditions.


Resins can be used to directly measure the bioavailability of plant nutrients and other ions in a soil.

The user benefits because:

  • Now you can see soil chemical dynamics the way a plant does--in all soil types.
  • You will base management decisions on data representing actual soil processes.
  • You develop site management strategies that deal with actual ion bioavailability or toxicity.
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The following technical papers have more details about this topic.

Schaff, B.E. and E.O. Skogley. 1982. Diffusion of Potassium, Calcium, and Magnesium in Bozeman Silt Loam as Influenced by Temperature and Moisture. Soil Science Society of America Journal, Vol. 46:521-524.

Skogley, E.O. and B.E. Schaff. 1985. Ion Diffusion in Soils As Related to Physical and Chemical Properties. Soil Science Society of America Journal, Vol. 49:847-850.

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

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Gordon Warrington
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