Frequently Asked Questions

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

How are the PRS™-probes used?

Our service at Western Ag Innovations works such that we send the PRS™-probes for use, then they are sent back to our lab for analysis once the PRS™-probes are removed from the soil and thoroughly cleaned with deionized water. We charge based on the number of PRS™-probe analyses rather than on each individual PRS™-probe. This is because the cost of a single analysis can include the use of up to 4 PRS™-probe pairs (4 anion + 4 cation). This works much like a composite soil sample, where the pairs of PRS™-probes are distributed within a single experimental unit, removed from the soil, cleaned with deionized water and combined into one sample bag. Our lab will elute the sample bag containing the PRS™-probe pairs with one solution of HCl and analyze the resulting eluant for nutrient ions. When it comes to ordering the PRS probes, it is more important for us to know how many samples are needed (i.e. how many data points per nutrient ion) rather than how many PRS™-probes are needed in total.

Below is a general summary of the various research applications using PRS™-probes, specifically the different types of PRS™-probe burials: in situ; in-lab; growth chamber; saturated paste; under crop residues or amendments; by soil horizon; underwater; and, duration of burials (i.e., short-term and long-term). Obviously, space does not permit reviewing all of the PRS™-probe research applications; therefore, if you have any questions regarding the use of PRS™-probes within your specific research methodology, please do not hesitate to contact one of our R&D Coordinators.

Types of Burials:

• In situ: The greatest advantage of the PRS™-probe compared to traditional soil tests is its ability to be buried directly in the field, thereby integrating the nutrient supply rate measurement of all the important edaphic factors affecting nutrient availability to plants. Unlike destructive soil cores or ion-exchange resin beads in mesh bag formulations, the PRS™-probe is inserted and removed with minimal disturbance. Depending on the research objectives, the PRS™-probe can be inserted either vertically or horizontally at the desired depth in the soil.

  In situ placement of PRS™-probes vertically in a high mountain environment
  Photo provided by: Federico Osorio who used probes in his study on High Elevation Alpine Ecosystem productivity and Biogeochemistry classification.

• In Lab: A homogenized soil sample is moistened to field capacity with deionized water prior to inserting a PRS™-probe. In lab PRS™-probe burials typically are short-term burials, but also may involve long-term burials using intact soil cores sampled from the field (Qian and Schoenau, 1995). Researchers often will perform saturation experiments a priori to determine a safe maximum burial length in situ, based on the nutrient adsorption capacities of the PRS™-probe. In 2002, in lab PRS™-probe burials provided a basis for determining fertilizer recommendations for a variety of annual crops on over thousands of acres of farmland in Western Canada.

  Note: Although the PRS™-probe works well in soil slurries, it is not recommended, because such tests are not representative of natural conditions (i.e., soil matrix) under which plant roots grow.

  PRS™-probes inserted into a field soil sample to judge the nutrient supply rate over a 24-hour period.

• Growth Chamber: The most accurate means of validating the effectiveness of the PRS™-probe is to correlate the nutrient supply rates with plant nutrient uptake and growth within a growth chamber (i.e., controlled environmental conditions). Simply prepare pots of homogeneous soil, with some growing the plant of interest and others containing only PRS™-probes. Cumulative nutrient supply rates measured using the PRS™-probes then are compared to plant nutrient uptake and growth at the end of the growth period (Qian and Schoenau, 2000).

  Note: as with any growth chamber study, a companion field study should be carried out in order to validate the inferences made and test their applicability to field conditions.

  Soil cores in incubation chamber.
  Photo provided by: Jeff Schoenau

• Saturated Paste: Assessing potential soil sodicity hazards with the PRS™-probe can be done rapidly and accurately using a cation-exchange PRS™-probe (Greer and Schoenau, 1996). Current methodologies of determining exchangeable sodium percentage (ESP) directly, or indirectly through correlations with sodium adsorption ratio (SAR) using the Gapon convention, are costly, time consuming, and subject to multiple errors. The factors controlling the ion-exchange dynamics of the soil exchange complex (i.e., colloidal clay and humus) also control the cation composition on the PRS™-probe. Therefore, when allowed to equilibrate with the soil exchange complex via the soil solution during a one-hour burial, the cation-exchange PRS™-probe yields an accurate representation of the cation composition on the soil exchange complex.

  It is not surprising then that the PRS™-probe exhibits a one to one correspondence (r² = 0.91) with ESP measured using standard techniques. Considering that Na+ is the major ion of interest, the counter-ion used to saturate the cation-exchange PRS™-probe is either H⁺ or NH₄⁺. Such a simple and accurate procedure has tremendous potential for streamlining routine analysis of detailed salinity and sodicity hazards.

  Note: a saturated paste method is not appropriate for measuring supply rates of certain nutrients (i.e., N and P) due to the reducing conditions created, unless these are the conditions of interest.

• Under Crop Residue or Amendment: To measure nutrient release and leaching potential from crop residues, manure, or biosolids in the field, PRS™-probes can be placed horizontally on the soil surface below the crop residue. Similarly, in forestry-related applications, PRS™-probes can be inserted horizontally below the LFH layer (or between the organic layers) to measure nutrient fluxes over time in ‘undisturbed’ forest soils or following different management practices (Huang and Schoenau, 1996).

  Probes inserted for measuring exchangeable phosphate at surface of manured soils.
  Photo provided by: Jeff Schoenau

• By Soil Horizon: Nutrient supplies often are desired according to soil genetic horizon. Nutrient supply rates can be measured in these different horizons by digging a soil pit and inserting PRS™-probes horizontally into different layers (Huang and Schoenau, 1997). After inserting the PRS™-probes, replace the excavated soil according to horizon in order to maintain as close to original soil environmental conditions as possible.

  In situ placement of PRS™-probes horizontally in a forest soil profile.
  

  (Below) Comparison of N and P (mean + one standard deviation) adsorbed on resin membranes during a 2-wk burial in the field and laboratory. Resins were buried at 4 different forest soil depths (L, F, H, Ae).
  Figure obtained from Huang, W.Z. and Schoenau, J.J. 1996. Can. J. Soil Sci. 76: 373-385. Forms, amounts and distribution of carbon, nitrogen, phosphorus, and sulfur in a boreal aspen forest soil.

  
• Underwater: PRS™-probes can be used to measure nutrient dynamics in situ within oligotrophic environments. The PRS™-probes can be inserted into peat at any depth and fishing line attached to the handle for easy removal after the burial period. The PRS™-probes not only represent a tremendous potential for simplifying existing methodologies (i.e., frozen cores taken back to lab for analysis) of measuring nutrient dynamics in these environments, but also provide more accurate and precise data.

• Short-term Burials: A short-term (i.e., 24 h or less) PRS™-probe burial is used to measure a "snapshot" of the soil nutrient supply rates at a particular time and is a quick test for determining the balance of available nutrients in the soil.

  However, depending on the site, significant microscale variability may lead to large variation in nutrient supply rates among individual PRS™-probes. Therefore, it is recommended to combine several PRS™-probes or pairs of PRS™-probes scattered through each experimental unit for analyses, which will act similarly to compositing several soil cores. Considering the PRS™-probe is mechanistically similar to a plant root in its natural environment, short-term assessments of nutrient availability support the modeling of increased nutrient supply over the growing season given an increase surface area of ion sink (i.e., growing root system). The nutrient supply rates measured during short-term PRS™-probe burials often are well correlated to concentration of available nutrients obtained by conventional chemical extractants.

  

  Effects of long term fertilization on a North Vietnam soil’s nutrient supply rate at two soil depths over a 24 hour period (Figure adapted from Nguyen, et al., 2001)
  Nguyen, H., Schoenau, J.J., Van Rees, K., Nguyen, D., and Qian, P. 2001. Can. J. Soil Sci. 81: 481-488. Long-term nitrogen, phosphorus and potassium fertilization of cassava influences soil chemical properties in North Vietnam.

• Long-term Burials: Long-term (i.e., two to several weeks) PRS™-probe burials are used to measure nutrient supply rates throughout the growing season and account for the temporal factors affecting nutrient supply. These include ion diffusion from greater distances and the slow-release of nutrients from mineralization (i.e., soil organic matter or organic amendments) and dissolution. A long-term burial also has the advantage of accounting for differences in nutrient dynamics among soil types as affected by management practices (Hangs et al., 2004). Depending on soil type and ecosystem of interest the recommended maximum burial length of the PRS™-probes may vary.

  In a study by Nguyen, et al. (2001) the influence of nine years of annual applications of NPK fertilizers on nutrient availability was assessed using PRS™-probes. Soils were brought to field capacity with DI water and probes were left in soils for 24 hours to determine the nutrient availability.

  

  Single PRS™-probe burial - Total nitrogen (N) supply rate absorbed on PRS™-probes after 60 days as a function of N applications. Error bars are ± standard error of the mean.
  Vasquez, E., Sheley, R., and Svejcar, T. 2008. Invasive Plant Science and Management, 1:287-295. Nitrogen Enhances the Competitive Ability of Cheatgrass (Bromus tectorum) Relative to Native Grasses.

  

  Multiple PRS™-probe burials - Cumulative long-term supply of plant nutrients in soil amended with 1x compost, and synthetic fertilizer in an organic snap bean cropping system in New Brunswick, Canada.
  Owen, J., Leblanc, S., and Fillmore, S. A. E. 2008.Season-long supply of plant-available nutrients from compost and fertliser in a long term organic vs. conventional snap bean rotations experiment. 16th IFOAM Organic World Congress, Modena, Italy.