When it comes to Soil Solution Sampling – your questions answered

July 4, 2013

If your research requires the measurement or monitoring of soil solution please read on as we have put together some frequently asked questions on this specialist area of environmental research:

When can I use a simple ceramic sampler instead of the more expensive glass or plastic samplers?

Our ceramic sampler has many qualities for example some are fitted with pure aluminiumoxide ceramic (99.8%) which has little or no influence on phosphate sampling characteristics.  The non-pure aluminiumoxide used in the other samplers can reduce the phosphate content in a sample. If you are in doubt about the applicability of a sampler for your soil solution sampling project you need to do a comparative test: Collect soil samples in coring tubes (or with an auger and stack the soil in an inert column). Put distilled water (imitating rain falling on the soil) on top of the sample. Collect the water dripping out from the bottom of the column. This is the quality of water that moves to deeper layers / groundwater after heavy rains. In other words soil moisture with all natural levels of chemical parameters. Compare these levels with the levels you obtain from the sampler(s) you have in mind for your research.

What are the effects of soil temperature and mountainous areas (altitude) on the efficiency of the samplers?

Altitude will simply reduce the maximum applicable suction. A local barometer will inform you of this. In practice this means that the maximum theoretical suction at sea level at the moment of weather with a low air pressure is about 950 hPa minus the gas pressure of water at that temperature. So this means that in La Paz, Bolivia at 3600m altitude with a soil temperature of 20°C the effective suction is considerably lower than in Alaska at an altitude of 0 meters and a soil temperature near freezing point. This may mean that soil moisture content must be considerably higher to be able to collect a sample.

Can I use Macro-Rhizons at deeper depths?

Although there are theoretical and practical limits the answer is ‘yes’. Often Macro-Rhizon soil solution samplers are meant to sample the shallow root zone. In certain studies deeper applications may be desired. To maintain sufficient suction at the tip of the Macro rhizon when the sampler is being used at an important depth (e.g. between 3 and 7 m) extending the sampler can be done with PE tubing in combination with some PVC/PE lined extension tubing  to make a long extension tubing of the required length.

Can I clean Rhizon samplers to reuse?

This will depend on two main questions:

1. Is cleaning an efficient use of time compared to the cost of buying new ones? : This depends on the salary costs and lab costs but as these items are very competitively priced new samplers are usually more cost effective.

2. Will cleaning be successful? : This will depend on what parameters you are analysing and if you are willing to accept the reduced hydraulic capacity, increased risk of cross-contamination and need to take and analyse blank samples.  If you decide to try to clean your samplers the advice for decontaminating NEW large plastic plate samplers could be followed to try to clean a used sampler: wash with 0.1 molar hydrochloric acid; then rinse with deionized water. You could copy this procedure for Rhizons. If you have used a sampler to sample for organics one could flush the sampler with a detergent and after that flush with plenty of deionized water.

Can Rhizon samplers be used to sample the soil solution for microorganisms in pots (bacteria and fungi) from plant roots?

The water filtered by a Rhizon becomes sterile as the pores are < 0.22 microns which is the critical size for bacteria. So this is a dilemma!  All other (ceramic, plastic, glass) soil moisture samplers would, to a certain extent, cause this problem because even with a more common pore size of 1 micron most bacteria will be filtered out. Soil moisture samplers never have pore sizes larger than 1 micron, otherwise the bubble point pressure of the filter material becomes lower than 1 bar. If it is lower than 1 bar you will not be able to apply a good vacuum to extract water from the soil. If a good vacuum is put on a water saturated 5 micron sampler it will start leaking air through its pores instead of withholding air and letting water pass (which is necessary in all cases where the pores are not completely filled with water). So  a sampler with pores between 0.25 and 1 micron would not solve the problem either since a lot of bacteria will be larger than even 1 micron. So you would be unable to get a sampler as air would be collected not water except if it is submerged in a water saturated sample or you are able to use a sampler that does not pass through even a part of the bacteria but there are two ways to get a good impression of the bacteriological composition of the soil moisture in a pot:

1. Take a vertical core sample of the contents of the pot. Then saturate the sample by adding distilled water on top of the sample until water comes leaking out at the bottom.  This water resembles water as it is flowing through the ground by natural forces (like heavy rainfall, flooding, irrigation) and is not filtered.  As a trick (with concessions to the accuracy) you might very SLOWLY water the pot (with distilled water) and collect water that comes flowing out at the bottom.

2. The other way would be to put the pot (or a sample taken from it) in a centrifuge and swing out the moisture by mechanical forces.

For more information visit: http://en.eijkelkamp.com/products/soil/in-situ-soil-physical-research/soil-moisture-samplers/rhizon-soil-moisture-samplers.htm

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