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How a PST-1 Resin Capsule Works


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A UNIBEST PST-1 resin capsule adsorbs chemical elements (as ions) from the soil solution (the water in a moist soil) like a plant root.

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

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Shifting the View

According to traditional theory, plants simply extract nutrients from a soil which we try to emulate with laboratory extraction methods. However, a lab extraction does not account for ion release and transport within a soil. In the nutrient bioavailability theory, soil and plant roots are seen as a dynamic system.  Ion exchange resins respond to nutrient availability because they resemble the plant root process of ion adsorption (Skogley 1994). This means resin adsorption directly accounts for the effects that soil chemical and physical properties have on the availability of chemical elements. Einstein said it best, "The theory tells you what you can observe" (Casti 1989).
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Plant Nutrient Uptake

Root hair and nutrient uptake.Plants (and other soil organisms) take most of their nutrients from the soil as ions to obtain chemical elements used for metabolism. Ions move from the soil solution "outside" of plant cells to the "inside" by "passive" and "active" processes. As a root hair (figure 1) removes ions from the soil solution, concentrations near the hair become lower, causing ions to diffuse toward the root from the high concentration in the bulk soil solution. This process continually supplies the root with nutrients.
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Resin Capsules

Resin capsule showing ion adsorption.Each PST-1 resin capsule (figure 2) has a mixed-bed resin of thousands of resin beads charged with H+ and OH- ions, within a porous fabric membrane. The mixed-bed resins act as a strong sink for ions in the soil solution. In a soil the capsule adsorbs chemical elements through an ion exchange process that occurs very close to the capsule surface. The amount of each ion adsorbed by a resin capsule (Schaff and Skogley 1982, Dobermann et al. 1994) during a specific time depends on the:
  • Initial soil solution concentration of each ion,
  • Release from soil solids,
  • Diffusion of each ion through the soil, and
  • Capsule surface area in contact with the soil.

Initial adsorption takes place across the capsule membrane when it first contacts the soil. The resins adsorb both positive (+) and negative (-) ions from the soil solution in exchange for H

+ or OH- ions from the capsule. This step is relatively rapid and occurs because the resins have a greater affinity for all other ions than those originally present on the resin. In chemical terms, the resins act as a "strong acid" and "strong base," thus giving up their H + or OH- ions in favor of other ions in the soil solution (Skogley and Dobermann 1996).

Because exchange reactions occur independently for each soil solution ion, the resin will simultaneously adsorb all types of available ions. This includes plant nutrients as well as heavy metals like Mercury, Lead, Cadmium, Arsenic, Selenium. Only the bioavailable ionic forms of an element are taken up.

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Ion Adsorption
Step 1

Resin adsorption kinetics - phosphorusThe quantity of each ion adsorbed by the resin during the first phase of accumulation depends on the amount of each ion in the soil solution when the capsule is placed in the soil. In figure 3, this appears as the amount of adsorption during day one.  For plants, this occurs when seedlings are planted or as roots grow into new soil. This initial ion exchange reaction sets the stage for the following steps. The soil solution concentration of each ion at the resin-soil or root-soil interface decreases relative to ion concentrations in the bulk soil solution (figure 4) creating a "diffusion gradient."
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Step 2

Soil ion concentration vs distance from capsule.Diffusion gradients causes soil solution ions to move toward the resin capsule from ever-increasing distances in the soil. When these ions reach the resin surface they are also exchanged for H + or OH- ions. In this way, the resin capsule continues to function as a "sink," similar to a plant root taking up nutrient ions. Soil properties regulate the rate of ion diffusion through a soil. These effects are different for each soil and each ion. This is the slowest part of the process, or the "rate-limiting" step and affects the amount of each ion accumulated by the resin during a given time. In figure 3 this step is shown as a continuous increase in adsorption over the long term (Dobermann, et al. 1994).  For the capsule to provide accurate results, it must be in place long enough for the effects of diffusion to be expressed.  This usually requires two to four or more days, depending on diffusion conditions.
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Step 3

An accumulation of ions occurs inside the capsule in the resin at the capsule-soil interface creating a diffusion gradient within the resin capsule. These ions then diffuse toward the center of the capsule where the specific ion concentration starts out at zero. The rate of ion diffusion through the resin capsule is much faster than ion diffusion through the soil, so this step is not rate-limiting. Step 3 in plants is the mass transport of ions by sap flowing from the root to other locations.
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Step 4

A continuous process  results as electrical neutrality must be maintained in all parts of the system, as required by Mother Nature. The resin counterions, H + and OH-, diffuse outward as other ions diffuse inward. Because most soils have more cations than anions that can diffuse to a sink, more H + than OH- ends up in the soil surrounding a resin capsule. This causes a slight decrease in soil pH near the capsule, which can also increase the solubility of some elements. Again, this is like a plant root function.
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Conclusions

This innovative evolution in soil chemical analysis gives you a POWERFUL TOOL to evaluate bioavailability of chemicals. At the end of an accumulation period the amount of each ion in a resin capsule is a measure of its "bioavailability" for the soil conditions during that period, therefore you benefit because:
  • Resin capsules are an unbiased measure of the bioavailability of plant nutrients and other ions in a soil.
  • Now you can see the dynamics of all soil chemical ions, simultaneously,  the way a plant does--in all types of soil.
  • Management decisions can be made from data representing actual soil processes.
  • Site management strategies can be developed to deal with actual ion bioavailability.
  • Neutralize or isolate problem chemicals while knowing which ones are not going to be a problem.
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References

The following technical papers have more details about this topic.

Casti, John L. 1989. Paradigms Lost: Images of Man in the Mirror of Science. William Morrow and Co., Inc., New York. 565 pgs.

Dobermann, A., H. Langner, H. Mutscher, J.E. Yang, E.O. Skogley, M.A. Adviento and M.F. Pampolino. 1994. Nutrient Adsorption Kinetics of Ion Exchange Resin Capsules: A Study With Soils of International Origin. Commun. Soil Sci. Plant Anal. 25:1329-1353.

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. 1994. Reinventing Soil Testing for the future. p. 187-201. In J.L. Havlin et al. (ed) Soil Testing: Prospects For Improving Nutrient Recommendations. SSSA Spec. Publ. 40. ASA and SSSA, Madison, WI.

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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