Dear Gregory,

a sequential extraction with BaCl ist the recommended method in the ICP Forests Soil manual (see annex).

Another method is a percolation with NH4Cl. For this method, there is only a German protocol available (see annex)

Chemical sequential extraction for metal partitioning in environmental solid samples

General applications of sequential extraction schemes can be summarized as follows:

(i) Characterisation of pollution sources

(ii) Evaluation of metal mobility and bioavailability

(iii) Identification of binding sites of metals for assessing metal accumulation, pollution and transport mechanisms

Thus, sequential extraction results proved useful to distinguish between anthropogenic and geochemical sources of most

metal species in sediments.

See A. V. Filgueiras, I. Lavilla and C. Bendicho*, J. Environ. Monit., 2002,4, 823–857

For forest floor soil, unbuffered 1 M NH4Cl would be my choice. We use a single extraction with a ratio of 20 mL of extract solution per gram of dry soil. The extract is "friendly" in an ICP.

Because forest floor litter is partly decomposed plant material, I would use the same method as for the digestion of plant materials, i.e. acid digestion, provided litter is thoroughly washed with water to remove soil particles and oven-dried at 60 oC for 3 to 7 days. Then use the digested material to determine cations using atomic absorption spectroscopy.

Hi,

What about to use a bit nontraditional but very efficient way: nutrient supply rates by PRS membranes. Just check out: http://www.westernag.ca/innovations

Tony

Http://karnet.up.wroc.pl/~weber/ekstrak2.htm

I recommend CEC Determination by the BaCl2 Compulsive Exchange Method

(Gillman and Sumpter, 1986)

I favor an approach like Chris Johnson.

Saturation of the OM is critical as you suggest by your concern about floating and recovery.

Hello Pere,

I enjoyed your detailed answer, but one point in it should be better detailed. In many cases, especially when calcium carbonate or gypsum are presented in the soil, prewashing step (by distilled water) would result in alteration of the composition of exchangeable cations - Ca will dissolve during this step and will out-compete exchangeable cations in which Gregory is interested... I would stick to your other suggestion - have two separate extractions -one for exchangeable + soluble, the other for soluble alone.

Cheers,

We have experienced the same issues when processing highly organic samples such as peats and forest litter through a sequential leach process. It really depends on what the methodology is that you are using. If the samples processed are in centrifuge tube for separation, when you decant the sample, you can use the lid of the tube as a strainer to retain some of the sample material. You can also re-centrifuge the decanted solution further in an attempt to recover any fine material that transferred over and by careful rotation of the centrifuge tube as you are pouring you can recover additional floating material as it sticks to the walls of the tube. Add your next reagent first to this tube, vortex, then transfer this solution back to the original sample residue you a going to continue leaching. Unfortunately it is a question of finding ways to minimize that loss with having to resort to filtering. It is way too time consuming to consider filtering (depending on the number of samples and as we normally analyse for many elements we don't want to risk the loss of element by adsorption onto the filter paper or leaching to trace elements from the filter paper into the solution. I have also found that much of the material that is floating (depending on how the sample was prepared for the leach) may be coarser bulk material that actually will only contribute minimally to the overall concentration of elements to be leached in the subsequent step. When we have total data to compare to a say 5 step sequential leach, the data is usually within 5 % of the total data for the sample. For interest sake you could collect, dry and weight this material and it may amount to a very insignificant amount of the total sample weight.

We would normally use a pH7 ammonium acetate leach for adsorbed/exchangeable to avoid leaching the carbonate phase, You could also do a water leach but it is really difficult with this type of sample. If you are looking for what elements are bound tot he soluble organic phase then I would suggest a sodium pyrophosphate leach.

Dear Dr Van der Heijden,

I suggest that you should try the hexaamminecobalt(III) trichloride method.

A standard method exists (ISO 23470:2007), but you can adapt this method (amount of sample, concentration and volume, S/L ratio, pH.. ), depending on the situation . For your kind of sample, a single extraction (30 minutes) followed by filtration (0.1µm) should be sufficient to extract all the exchangeable cations (including organic matter). The CEC can be cross-determined by colorimetric analysis and major cations (Na, K Ca, Mg, NH4...) analysis of the extract solution. If major cations analysis overtakes colorimetric value, this means that your sample contains solubles salts. You can correct this discrepancy, either by proceeding to leaching assays (pure water) on the sample, or by first washing it prior to CEC measurements. I would recommend the first option.

You should not encounter the reverse situation (i.e. colorimetry > major cations), unless you are working on preserved samples containing reductive species (e.g. reduced clays). This is the object of my next paper ..

One must note that this method may be aggressive for biological systems, because it used as well for DNA crystallographic studies. Hexaamminecobalt(III) replaces barely all major counter-ions, including Mg in DNA. But this worth a try!

Best regards