Frequently Asked Questions

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

How long can the PRS™-probes be buried?

Long Term Over Winter Burial in an Alpine Ecosystem

Photo provided by: Federico Osorio who used PRS™-probes in his study on High Elevation Alpine Ecosystem productivity and biogeochemistry classification.

Short Term Burial in an Agriculture Ecosystem

Photo provided by: Dr. Dan Sullivan who used PRS™-probes in his study on soil amendment and cover crops in organic broccoli production. PVC root exclusion cylinders were used between broccoli rows.

The burial length of the PRS™-probes should be discussed with an experienced R&D Coordinator because it will depend on ecosystem, soil type, experimental treatments, and researcher’s objectives. Generally, the longer the PRS™-probes are left in soil, the greater likelihood of measuring every incident of nutrient availability (i.e., nutrient pulses following rainfall events), thereby increasing data accuracy and the inferences made. Extended burials are convenient when the study sites are a distance away; however, the PRS™-probes cannot be buried indefinitely. Once the ion-exchange membrane has reached a saturation point, it no longer follows Donnan exchange principles; instead acting as a dynamic ion-exchanger and the resulting data is suspect. Depending on the edaphic conditions during the burial period (i.e., over winter burial, xeric environments, etc.), the PRS™-probes can be safely buried for long periods of time. For other soil types and conditions, repeated measures over time in the same soil slot can be added together to achieve a cumulative nutrient supply rate over the growing season, while providing greater resolution in terms of temporal variations in nutrient availability.

The recommended maximum PRS™-probe burial length, safely avoiding saturation, obviously depends on the soil type. Under a heavily fertilized agriculture soil, a one-week maximum is recommended, but with a 'typical' agriculture soil two weeks is appropriate. In disturbed forest soils or following fertilizer amendments, a maximum burial time of four-weeks is recommended. For 'undisturbed' forest soils, experiencing tight nutrient (i.e., N) cycling, burials of six to eight weeks are acceptable. Notwithstanding the soil type, preliminary saturation experiments could be carried out in the lab to determine burial limits with your specific soils.

Note: regardless of the time period selected, PRS™-probes should be buried for equivalent time periods among treatments being compared. In addition, because ion adsorption is not linear, instead generally following first order kinetics over time, nutrient supply rates cannot be divided into time units smaller than the entire duration of soil burial. For example, nutrient supply rates measured over a two-week burial cannot be divided by 14 and reported in terms of per day.

Cumulative Nutrient Supply Rates

Researchers often will determine a cumulative measure of nutrient supply by removing buried PRS™-probes after a set number of days or weeks, and re-inserting fresh PRS™-probes in the same soil slot (See figure 1). This allows a measurement of temporal changes in nutrient supply due to changing environmental conditions. Adding supply rates from repeated burials can be used to assess changes in nutrient supply in situ over time. During a 1-24 h burial period, the nutrient supplying power of the soil is mainly derived from the rapidly available labile nutrient pool. However, if a new PRS™-probe is inserted into the same soil slot immediately following the removal of the initial PRS™-probe, there is a reduced labile nutrient pool remaining. The subsequent nutrient supply rate of the soil, therefore, is controlled by the buffer capacity of the soil (via: solid phase desorption), along with the replenishment of the labile pool by the more slowly supplying pools (i.e., mineralization of soil organic matter or organic amendments and dissolution).

Discrete Burials

When a researcher is interested in non-consecutive burials, nutrient supply rate measurements are made in different soil slots. PRS™ supply rates should NOT be added together, as the size of the initial labile pool included in the nutrient supply rate measure may vary (see figure 2 below).