Frequently Asked Questions

Topics: General / Technical / Logistical / Ordering / Past Research

How do nutrient supply rates compare to conventional nutrient extractions?

No calibrated correlation can be made between PRS™-probe nutrient supply rate data and soil nutrient concentrations determined using conventional extractions, nor is it warranted. The nutrient supply rates measured with the PRS™-probes are unlike data generated from a conventional soil extraction for nutrient concentrations (i.e., ppm). Chemical extractions are static measures or indices of nutrient pools (i.e., total 'available', and organic) at a point in time, while the PRS™-probes provide a dynamic measure in in situ of ion flux over time to a sink with a constant (i.e., quantifiable) surface.

Note: the nutrient supply rates cannot be multiplied by two to provide an estimate of nutrient availability on a volume basis (i.e., kg/ha for 15 cm depth).

The PRS™-probe heralds a new area in functionally viewing the dynamic chemistry at an adsorbing surface. In simulating the mechanism of nutrient uptake used by plant roots, the PRS™-probe improves the accuracy of monitoring the soil nutrient supply. Although, short-term PRS™-probe supply rates can be well correlated to traditional chemical extractions (Table 1), one should not expect that the conventional chemical extraction indices to have anything more than a general relationship to the dynamic flux measured by the PRS™-probe during long-term burials. The reasons for this are simple, only the PRS™-probe accounts for temporal variability in nutrient availability due to: soil moisture and temperature; mineralization and immobilization; ion activity; buffer power; and ion diffusion.

The PRS™-probes measure nutrient supplies according to actual field conditions with minimal soil disturbance; therefore, providing a more accurate representation of actual nutrient supply to the plant. Furthermore, many chemical extractions provide an index that is meaningfully related to plant nutrient (i.e., P) uptake only under certain soil pH ranges. Functionally this means that an alkaline soil may require one type of chemical extraction, while the acidic soil type requires a different extraction to predict nutrient availability. The PRS™-probe measurement integrates all of the principal edaphic factors affecting nutrient uptake by plants and measure nutrient bioavailability regardless of soil type. This measured flux of soil nutrients or toxins over time is more biologically meaningful than chemically-extracted levels. It should not be surprising then that PRS™-probe supply rates are better correlated to plant uptake than traditional chemical extraction values. Please see Does the PRS™-probe simulate biological availability as verified by correlations with plant uptake? for more information.

The PRS™-probe acts as an ion sink to adsorb any ionic species that are supplied from the soil over time, with minimal soil disturbance. This patented technology is effective in tracking the dynamic availability of soil nutrient and toxins to plants. Relationships among PRS™-probe supply rates, conventional soil extractions, and plant uptake are shown below.

Comparisons among PRS™-probe supply rates, conventional extractions, and plant uptake: