The PRS™-probe acts as an ion sink based on Donnan exchange principles, which is the principal mechanism plant roots use to attract and absorb nutrient ions from the soil (download PRS™-probe movie requires QuickTime). PRS™ membrane is composed of a network of polymers, which are polystyrene cross-linked with divinylbenzene, having ionic function groups. These ions are covalently linked to the polymeric network. Associated with each of these is an ion of opposite charge, the counter-ion. These counter-ions neutralize the charge of the resin’s functional groups and are exchangeable with other ions from the surrounding liquid phase. There are cations and anion exchange resins. A PRS™-probe pairs consists of one anion and one cation PRS™-probe.

The permanently charged functional groups on the PRS™-probe’s ion-exchange membrane are saturated with sodium (Na⁺) for the cation probe, and bicarbonate (HCO₃^-) for the anion probe. The anion-exchange PRS™-probes are shaken in 0.01 mol/L ethylenediaminetetraacetate (EDTA) until EDTA is exchanged at approximately 30% of the ion-exchange sites. The EDTA allows for adsorption of micronutrients, in particular polyvalent metal cations such as Al, Fe, Mn, Cu, and Zn. Once inserted in the soil, the PRS™-probes will adsorb similarly charged ions from the soil solution and labile nutrient pools. This results in further release of ions from the more slowly supplying ion pools in an attempt to maintain the solution ion concentration (i.e., soil buffer power). The PRS™-probe continues to measure nutrient release from these pools while in contact with the soil and/or until the buffering capacity of the soil is reached. Nutrient supply rates are a function of the size of nutrient pools and edaphic properties (i.e., physical, chemical, and biological) controlling nutrient availability; therefore, all soil environmental factors must be considered when interpreting PRS™-probe data.