The PRS™-probes (15 cm x 3 cm x 0.5 cm) were designed to be used in the upper soil layers, because this is the principal location of soil nutrient dynamics. Furthermore, all plants take up the majority of their nutrients from this zone, especially during the early establishment phase, so it is important to measure the bioavailable nutrient supply rate in this area. The PRS™-probes can be effectively used at lower depths; however, digging a soil pit to the desired depth is required prior to insertion. Although the functionality of the PRS™-probes is unaffected when buried deep, it negates the ease and convenience of burying/retrieving the PRS™-probes on the soil surface.

Dimensions:

The ion-exchange resin membrane used in the cation and anion PRS™-probes are strong acid and base homogeneous membranes, respectively. Each membrane type has fixed ionic groups (i.e., permanent charges) within the polystyrene matrix. These membrane types are considered ‘general purpose’, non-specific ion-exchangers, and their selectivity is solely based on the principles governing electrostatic attraction. Published selectivity sequences of common cations and anions allow for a generalization of the preference of one ion over another; however these sequences are determined in a set free ion solution with no re-supply. Conversely in the soil, the soil solution is in a dynamic equilibrium with the ion-exchange complex (i.e., solid phase and organic matter), which provides a continuous re-supply of ions. Because the ion-exchange complex is much larger than the PRS™-probe ion sink, the source is a larger factor in determining what becomes adsorbed than the ion-exchange membrane’s affinity to adsorb specific ions. In fact, the adsorbed ion proportions after a one-hour PRS™-probe burial, accurately reflects those on the exchange complex (Greer and Schoenau, 1996). Listed below are the PRS™-probe maximum ion adsorption capacities (µg 10 cm^-2) for selected nutrient ions.

Note: Cl^- and Na⁺ adsorption capacities determined empirically in the lab. Remaining ion adsorption capacities determined by calculating meq charge on the membrane surfaces. Keep in mind that these maximums were determined for single nutrient ions only and, therefore, will decrease depending on the specific ratio (i.e., availability) of that ion in solution relative to the other ions.

Cl^- and Na⁺ ions are not routinely analyzed for in samples.  Advance notice should be given to the R&D Coordinators before PRS™-probes are shipped out to customer as changes must be made to the actual membrane and analysis methods, in order for these ions to be measured.  Please see A rapid method for assessing sodicity hazard using a cation exchange membrane (Greer, K.J. and Schoenau, J.J. 1996) for more information on this method.