Frequently Asked Questions

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

What are some important considerations when using PRS™-probes in situ?

Although the PRS™-probes greatly simplify the use of ion-exchange resin technology in situ, a few fundamental principles still need to be considered in order to ensure the accuracy and precision of the nutrient supply rate data. These important considerations include: proper contact between the PRS™-probe ion-exchange membrane and soil; soil temperature and moisture; the presence of other competing sinks; and, burial length.

Soil Contact

In order for the PRS™-probe to be an effective tool for assessing soil nutrient dynamics, it is critical to ensure that there is complete contact between the ion-exchange membrane and soil. Nutrient supply rates generated with the PRS™-probe are reported as the amount of nutrient adsorbed per surface area of ion-exchange membrane per time of burial in the soil. If there is incomplete contact between the ion-exchange membrane and the soil, then the soil surface area actually supplying nutrients is smaller than that assumed in the nutrient supply rate calculation, leading to an underestimation of the actual supply rate. For instance, if only half of the ion-exchange membrane were in contact with the soil, then the nutrient supply rate calculated would be half of the true value. Furthermore, consistently burying the PRS™-probes improperly will yield significant variability within your data. If a PRS™-probe is removed from the soil and a portion of the ion-exchange membrane appears too clean, there likely was poor contact between the PRS™-probe and the soil. If two or more pairs of PRS™-probes were installed in the treatment plot, then discard this PRS™-probe from the composite analysis. However, if it is apparent what portion of the ion-exchange membrane surface was in contact, then estimate the area and we can adjust for this in the nutrient supply rate calculation.

Depending on the soil type and conditions, PRS™-probes can be directly inserted into the soil; however, if the soil is heavy, dry, hard, and/or rocky, then preparing a hole prior to insertion is recommended to avoid breaking the PRS™-probe (Figure 1a). After inserting a PRS™-probe, use either a spade or soil knife and apply a 'back-cut' to ensure good contact between the PRS™-probe and soil (Figure 1b). Finally, close the back-cut holes by firmly pressing the soil with your heel, which prevents water accumulation and further ensures complete contact between the PRS™-probe and soil.

Figure 1. (a) Creating a soil slit prior to PRS™-probe insertion, and (b) using a back-cut to ensure complete contact between the PRS™-probe and soil.

Soil Temperature

When burying PRS™-probes in the field, it is important to account for the effects of soil temperature on ion diffusion and microbial activity (i.e., mineralization and immobilization). At lower soil temperatures, ion diffusion in soil is slower and, therefore, nutrient movement to the PRS™-probes will be reduced. This is reflected in the smaller nutrient supply rates measured in the same soil at 4°C compared to 20°C (Table 1).

Table 1. Mean (n=3) nutrient supply rates measured at varying soil temperatures during one-hour PRS™-probe burial. (Unpublished data; P< 0.05)

	Nutrient Supply Rate (μg/10cm²/hour)
Soil Temperature (°C)	NO3--N + NH4+-N	PO4-P	K	SO4-S
4	63 b	2.3 b	217 c	17 c
10	74 ab	2.7 ab	248 bc	21 b
20	86 a	3.0 a	275 ab	25 a
30	90 a	3.0 a	292 a	25 a

Although greater differences likely will occur following longer burials (i.e., 24 h vs. 4 weeks), even short-term burials indicate an effect of soil temperature on nutrient ion movement to the PRS™-probe (Table 1). Similarly, microbial activity (i.e., mineralization) will be reduced at lower soil temperatures, which will be reflected in the nutrient supply rates for those nutrients governed by such process (i.e., N and S but not P and K; Tables 1 and 2). Therefore, it is prudent to account for soil temperature differences among treatment plots when using PRS™-probes, which will helps to delineate between direct treatment effects and indirect temperature effects on the measured nutrient supply rates.

Table 2. Mean (n=3) nutrient supply rates and microbial activity of a loamy sand soil (O.M. = 1.8 %) incubated for one week at varying soil temperatures. (Unpublished data; P < 0.05)

Soil Temperature	N Supply Rate	P Supply Rate	Cumulative Respiration
(°C)	(μg/10cm²/week)		(μg CO2-C/kg oven dry soil)
5	62 b	2.2 a	37 c
23	90 b	2.1 a	189 b
32	300 a	2.4 a	337 a

Soil Moisture

Soil moisture content has a large effect on nutrient availability to plants, specifically through its influence on: physical ion movement; biologically based nutrient cycles (i.e., uptake, mineralization, immobilization, etc.); and, chemical reaction of ions in the soil. At low soil moisture contents, there are smaller nutrient supply fluxes to plant roots and the PRS™-probe, because of restricted ion diffusion due to increased tortuousity of ion movement, whereas a traditional soil extraction will render the same nutrient concentration (i.e., mg/g) whether the soil is sampled moist or dry. In locations where soils are periodically saturated during the growing season, denitrification and leaching losses may cause reduced N supply rates. In extremely dry or wet soil, reduced microbial activity further affects nutrient supply to plant roots and the PRS™-probe. Such edaphic controls on nutrient availability are accounted for with long-term burials of PRS™-probes in situ, unlike traditional soil extractions that do not take such temporal variability into account if not taken consistently throughout the growing season. The effect of varying soil moisture on PRS™-probe nutrient supply rates is shown in Table 3.

Table 3. Influence of soil moisture content on PRS™-probe nutrient supply rates.

Soil Moisture Content	Nutrient Supply Rate (μg/10cm²/hour)
(% Field Capacity)	NO3--N + NH4+-N	PO4-P	K	SO4-S
Saturated	282	4.5	218	50
100	200	2.7	181	39
70	196	1.4	155	37
45	113	0.9	93	26
15	24	0.3	48	12

For short-term in situ burials (e.g., 1 to 24 h) soils should be moistened to field capacity using de-ionized water prior to insertion of the PRS™-probes (Figure 2). For long-term burials no wetting is required; however, soil moisture should be monitored among treatment plots and reported as interpretative data along with nutrient supply rates.

Figure 2. For short-term in-situ burials, de-ionized water is used to moisten soil prior to burying the PRS™-probes.

Depending on the site, there may be prolonged drought conditions throughout the growing season. Under such xeric conditions, the PRS™-probes will not measure appreciable nutrient fluxes. This may be perceived as a limitation of the PRS™-probes; however, if the soil is dry enough to form desiccation cracks, what is the expected plant nutrient uptake under such conditions? This variation in nutrient availability throughout a growing season measured using the PRS™-probes, which a traditional soil test is incapable of accounting for, is invaluable when attempting to explain observed plant growth.

Competing Sinks

When burying the PRS™-probes for extended periods, the effects of interspecific competition from plant roots need to be considered. When buried among plant roots, the PRS™-probes will provide a net nutrient supply rate (i.e., measuring the difference between total soil nutrient supply and plant uptake); therefore, yielding a measure of nutrient surpluses rather than net mineralization over the burial period. Depending on the research objective this may be desirable; however, if wanting to measure the effects of net mineralization on nutrient supply, then employing a root exclusion cylinder is required (Figure 3). Care should be taken to remove plants growing within the cylinder over the burial period. Following a heavy rain, ponding of water may occur within the PVC pipe leading to denitrification and lower N supply rates. To prevent this from occurring, make sure to level the PVC pipe with the ground and/or simply drill drainage holes in the sides of the exposed PVC pipe. Although studies have shown that the use of PVC pipes have no significant effect on edaphic properties or nutrient cycling processes, if their use is impractical, then cut a deep (i.e., 30 cm) slit around the PRS™-probes with a radius of at least 20 cm. This will work to effectively prevent root competition in a similar manner; however, this procedure will need to be repeated at least once every two weeks to a month.

Note: Large microscale variations in supply rates may be observed when PRS™-probes are buried within root competition due to differences in numbers and proximity of roots at different locations.

Figure 3. Root exclusion cylinder used for isolating PRS™-probes from plant root competition during long-term in-situ burials.

Installing PRS™-probes into Root Exclusion Cylinders.

Some researchers use a combination of PRS™-probe burials with/without root competition to gain a more complete understanding of total nutrient release, plant uptake, and nutrient surpluses. The difference between nutrient supply rates within a root exclusion cylinder and those measured outside the cylinder can be used as an index of plant nutrient uptake (Huang and Schoenau, 1997). However, the presence of roots may have secondary effects (i.e., rhizosphere effects) on nutrient availability, which the PRS™-probes are incapable of reproducing. For example, when N-fixing plants are involved, the additional N fixed by plants outside of the cylinder will greatly affect the relationship between N supply rates measured inside and outside the root exclusion cylinder and plant uptake.

The impact of plant root competition on soil nutrient supplies measured over a growing season is shown in the following table:

Table 4 & Figure 4: Impact of plant roots on cumulative NO3-N ion adsorption by PRS™-probes buried in different pasture systems biweekly (12 times) over a growing season at Lacombe, Alberta 1999. Data courtesy: Dr. Vern Baron, Agriculture & Agri-Food Canada Research Center, Lacombe, Alberta.

Table 4

Pasture Type	No Roots	Roots	N Uptake
	μ NO3- 10cm-2 week-1		kg ha-1
Old Perennial	1713	237	140
Alfalfa	2686	1328	330
Mixed Brome/Alfalfa	2446	896	320
Annual Rye Grass	2955	2007	200
Low Input	1349	262	140
Significant F	**	**	*
SE LSMeans	185	165	40

Figure 4

This data shows that PRS™-probe NO₃^--N supplies were much lower in the presence of roots. By burying PRS™-probes both inside and outside of the cylinders, a more complete picture of soil nutrient dynamics can be obtained.

Plant roots are not the only ion sinks requiring consideration. Similarly, microorganisms remove ions from the available soil nutrient pool, thereby competing with the PRS™-probes for nutrient ions, resulting in reduced nutrient supply rates. For example, if a soil is amended with material having a wide C:N, C:P, or C:S ratio, then soil microorganisms will be competing with the PRS™-probes for available nutrients. However, it is important to remember that this nutrient competition is similar to that experienced by the plant root. Burying the PRS™-probes in plots with/without the imposed treatment will provide a more complete picture of soil nutrient dynamics following any amendment.

Burial Length

It is essential that the duration of PRS™-probe burial be equivalent for all treatments being compared, because this will have an effect on the amount of nutrients adsorbed. Depending on your objectives, there are either short-term or long-term PRS™-probe burials. The longer a PRS™-probe is buried, the greater the opportunity to adsorb ions released from the soil and the higher the resultant nutrient supply rates measured. Burial length is important particularly when measuring nutrients whose availability is governed primarily through biochemical reactions (i.e., N and S) rather than the size of labile pool (i.e., P and K). It is also important that the PRS™-probes not be buried for so long that the ion-exchange membrane becomes saturated. Depending on the soil type, the recommended maximum burial length of the PRS™-probes will vary.

Note: 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. However, repeated measures over time in the same soil slot can be added together to achieve a cumulative nutrient supply rate (i.e., over the growing season), while providing greater resolution in terms of temporal variations in nutrient availability.

Considering that the PRS™-probes integrate all of the principal edaphic factors affecting nutrient uptake by plants, this flux measurement will change in response to changing soil conditions. Therefore, it is very important that these conditions be considered when interpreting nutrient supply rate data among sites and/or treatments. The sensitivity of PRS™-probes to conditions in situ makes them a powerful functional tool for measuring nutrient availability under conditions analogous to those in which plants grow. However, this sensitivity also demands that care must be taken in planning experiments with the PRS™-probes and when interpreting PRS™-probe nutrient supply rate data. If possible, it is recommended that at least soil moisture and temperature be measured for the duration of PRS™-probe burial and that plant root competition be accounted for.