A principal factor controlling plant nutrient uptake is the relative activity (i.e., effective concentration) of nutrient ions in the soil solution. When assessing soil nutrient dynamics using the PRS™-probes, specific ion activities will influence the extent to which nutrient ions are adsorbed. Conversely, a conventional soil extraction only measures the total extractable ‘available’ nutrient pool, which depending on soil conditions may not be necessarily bioavailable. A PRS™-probe measurement, therefore, may be poorly correlated with a traditional soil extraction, but strongly correlated with plant nutrient uptake. This especially is true when the relative abundance of a single cation is skewed, which often is the case with Ca²⁺ in highly calcareous soils. High Ca²⁺ concentrations in the soil will reduce the activity of other ions of lower concentration, such as NH₄⁺ and K⁺; therefore, the plant takes up greater amounts of the more highly concentrated ion and induces a deficiency in the other. Similarly with the PRS™-probes, excess Ca²⁺ affects the adsorption of NH₄⁺ and K⁺ from the soil. Herein lies another example of why difficulties exist when attempting to compare the PRS™-probe data to conventional extraction techniques, measuring total extractable ion concentration instead of 'bioavailable' ions.

To illustrate the importance of measuring ion activity instead of total extractable levels, look at K⁺ bioavailability in a highly calcareous soil. The PRS™-probe K⁺ supply rates represent the flux of K⁺ from the soil to a plant root under conditions specific to the plant root environment, while chemical extractions are used as an index of the exchangeable K⁺ in the soil. Although there may be an abundance of NH4OAc-extractable K⁺, the excess Ca²⁺ in soil solution interferes with plant K⁺ uptake resulting in low K⁺ bioavailability to plant roots. It is not surprising then that fields with soils having abundant K⁺ (i.e., >1000 kg K/ha; NH₄OAc-extractable), although considered to be K-deficient using the PRS™-probes, have exhibited significant crop growth response to added fertilizer K. This principle is illustrated in the figure below comparing PRS™-probe K⁺ supply rates to NH₄OAc-extractable K (a) and extractable K percentage (b).