PRS™-probes measure the bioavailable nutrients in the soil solution over their burial period. This means that all nutrient ions adjacent to the PRS™-probe already in the available form, along with nutrients that are converted to the available form, will be adsorbed onto the membrane surface. The amount of nutrient ions absorbed on the PRS™-probe at the end of a burial period represents the nutrient supply rate to a plant for the duration of the burial. The PRS™-probes attract and adsorb ions through electrostatic attraction. All bioavailable ionic species adsorbed by the PRS™-probes can be removed with an eluant solution (i.e., weak acid or salt). The analytical instrument (i.e., colorimetry, ICP, AA, FES, HPLC, etc.) used to measure the analyte concentration will depend on the ion of interest. Also, certain eluate matrices can be stored indefinitely, allowing sample archiving for further nutrient analyses if desired. The common ionic species measured by the PRS™-probes are listed below, indicating PRS™-probe membranes maximum ion capacity:

*Cl- and Na+ capacities determined empirically in the lab and are not routinely measured with the PRS™-probes. The remaining ion capacity is 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.

Nutrient supply rates represent the rate of nutrient ion adsorption by the PRS™-probe, expressed as a weight of nutrient adsorbed per surface area of ion-exchange membrane over time and are reported as such. Nutrient supply rates generated with the PRS™-probes are reported as the amount of nutrient adsorbed per amount of adsorbing surface area per time of burial in the soil. The units used by Western Ag Innovations are μg nutrient/10 cm² ion-exchange membrane surface area/time of burial (i.e., 24 h, 2 wks, 4 wks, etc.). The convention of reporting nutrient supply rates in terms of 10 cm² is to facilitate working with data for diffusion-limited nutrients (i.e., P) having smaller fluxes. However, it is also acceptable to report the number per cm² membrane surface area instead of per 10 cm². The units also may be reported as μg ion/PRS™-probe/time of probe burial providing that the total surface area (17.5 cm²) of the ion-exchange membrane is mentioned somewhere within the document.

Important Note: it is invalid to divide the burial duration into smaller time units since adsorption of ions to the PRS™-probe is not linear over time, particularly when there is a large initial labile nutrient pool.

Figure obtained from: Khasawneh, F. E. 1971. Soil Sci. Soc. Amer. Proc. 35: 426. Solution Ion Activity and Plant Growth.