About Resin Capsules and Their Use
Please provide any information you may have regarding colorimetric analyses of ammonia, nitrate, and phosphate in the 2 M HCl extracts.
This is the one question that most people have concerning analysis of a 2 M HCl solution. We "neutralize" by taking an aliquot for analysis (e.g., for nitrate and ammonium) then add an equal quantity of 2 M NaOH. This will work like most other analytes for most colorimetric determinations. The neutralized solution can also be used for AA or ICP methods, but we often use the HCl solution directly from the desorption for these procedures.
What is the maximum possible in situ residence time for resins without coming too near to saturation?
For most practical purposes, the PST-1 resins are an infinite sink for ions as compared to the amount of available ions that are usually in a sphere of soil near the capsule. If used in highly salty conditions, it is possible that resin capsules may approach ion saturation after several days or weeks in the medium. At this time we do not know of any situation where the resin in UNIBEST capsules has neared saturation.
Does the bead size of the resins ensure a saturated water-conductivity similar to that of the surrounding soil?
The resin beads are similar in size to a fine sand but, all resin beads shrink and swell in response to wetting and ion exchange. Although the resin beads are sand-size, they are highly porous, having varying proportions of macro- and micropores. In this way, their water relations are similar to those of soils having both macro- and micropores. It is not necessary to have the same amount of water in the capsule as in the soil because ions are moving along a diffusion gradient. Experience shows that there is enough water bridging the capsule - soil interface for ion diffusion to occur, even when soil water tensions are high.
How big a problem is swelling/shrinking of the resins, does this affect water flow or contact to the soil?
Resins do change volume depending on water content and types of ions present. If kept wet, the amount of volume change is minimal unless a pretty complete change in ion saturation takes place like what happens when stripped of adsorbed ions with HCl. Since resin beads in the capsule have both micro- and macro-pores they will act "similar" to soils in terms of water movement inside the capsule. However, we have not, measured the water relations involved. The WECSA access system is designed, with the spring loaded carrier assembly, to compensate for any in situ changes due to soil or capsule shrink and swell action.
Several different resin adsorption systems are discussed in various publications. Are there any differences between the techniques?
Yes, there are important differences between various resin adsorption systems affect the way results from each method can be interpreted.
Suggested reading for more information is;
Skogley, Earl O. and Achim Dobermann. 1996. Synthetic
Soil and Environmental Studies. Journal of Environmental Quality
ASA, CSSA, SSSA, 677 South Segoe Rd., Madison WI 53711 USA
How long should resin capsules be left in contact with the soil for adsorption to occur?
Resin capsules respond to the same environmental conditions that affect nutrient adsorption by plants. When a soil sample is made into a saturated paste in the laboratory and held at room temperature, we have obtained satisfactory results from capsules that were in situ for 1- to 2-days. On the other hand, in the field under cold dry conditions it will take more time for a capsule to adsorb the available elements from the soil.
Suggested reading for more information is;
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 46:521-524.
Warrington, G. E. and E. O. Skogley. 1996. Resin Extraction
Soil Ion Bioavailability.
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Can the resin capsules be reused?
Regenerating and reusing capsules is not recommended for two reasons. Foremost is that using a salt or acid to regenerate leaves the resin saturated with ions that do not allow the capsule to function effectively as a "sink" for soil ions. Second, each time the capsule is extracted, the process is not 100% efficient. Small, but unknown amounts of contamination would occur unless extreme processing was used. In practical terms, a resin capsule is like film in a camera, you take a picture (expose a capsule to the soil chemistry) then develop the film (desorb the capsule) to see the results.
When the soil is washed from a capsule, will the ions be washed out too?
All ions are adsorbed by the resin in a capsule by displacing the counter ions H resin. Therefore, adsorbed ions are held in the resin and will not be removed by a brief washing with deionized water. Desorption of these attached ions requires a strong acid like 2M HCl where the H the NO3-N and NH4+ on the resin. This and other characteristics of resins are discussed in the paper;
Skogley, Earl O. and Achim Dobermann. 1996. Synthetic Ion-Exchange Resins: Soil and Environmental Studies. Journal of Environmental Quality 25:13-24.
Also, see the paper in SCIENCES of SOILS 1997 -Abstract:
for Resin Capsules: Capsule Storage and Ion Recovery on the web at;
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Ion availability is different for each crop and is influenced by weather, can the resin capsule adsorption reflect this?
Actually, ion availability is a function of the soil system. When a resin capsule is used in situ with the soil access system, it will respond to the local environmental conditions. In situ sampling of the soil allows capsules to respond to the same soil variables of temperature, moisture, nutrient solubility, and transport that regulate how much of each element plants can take up.
Individual plant species and crops have unique requirements for nutrients and some plants are more efficient than others at taking up a particular nutrient or element. This is the basis for bioremediation, use a plant species that will accumulate an element that may be toxic to many other species and then harvest the plant to remove the element. The resin capsule adsorption would need to be correlated with the uptake by individual species to develop an interpretive data base.
Reference: Warrington, Gordon E. and Earl O. Skogley. 1994. A
Jump in Monitoring Soil Chemistry. In; Keammerer, Warren R. and Wendell
G. Hassell, Eds. Proceedings; High Altitude Revegetation Workshop No.
March 16-18, 1994, pgs 196-206. Colorado Water Resources Research
Available from: Cooperative Extension Resource Center, General Services
Center, Colorado State University, Fort Collins, Colorado, USA 80523.