It is a known fact that there is not one "plant availability". Availability of metals and metaloids depends on many parameters of the soils (pH, EC, CEC etc.) as well as on plant physiology (rhizosphere processses). Until today a lot of methods have been developed (column elution methods, single step extraction, sequential extraction), however if one is interested in the availability of non essential trace elements for plant growth (e.g. lanthanides, uranium) what method is the best? Does anybody have experience in that?
The only real way to measure plant availability is to be specific and measure update by the target plant of interest in the soil type that it needs to grow in(or that you want to grow it in). All other methods are just approximations and to have most confidence in any extraction methods they need to be calibrated against targeted pot trials.
A good approximation of plant availability of trace elements in soils is to determine the concentration of the free ionic form in solution AND the buffer capacity of the soil for the free ion. This means performing for example isotopic dilution or soprtion/desorption experiments together with speciation determinations. Not so easy!! To account for the plant effect on the availability the best option is to grow the plant in the soil!
If the availability is limiting the uptake, DGT may be a good proxy but must be calibrated.
There are sequential extractions and buffered extraction methods that can approximate availability. My experience is that DTPA works fairly well in calcareous soils and 0,05M CaCl2 gives also a good approximation. But, as A.P. Newman saids, you must check the method in your environment.
See my resent paper "Larrea tridentata Uptake of Trace Metals in Mine Tailings within an Arid Region of Nevada, USA" as we looked at trace metal uptake in the creosote bush in an arid environment. Look at the references page and mine that for great ideas.
As you pointed out, availability of metals and metaloids depends on many parameters. However, a crucial base is the knowledge of the generally available concentration in soils, which can be found out by sequential elution methods. By this you can discriminate the very stable silicate bound portions or stable oxides which are not plant available and at least can find a rough discrimination of available and not available. This does not depend on the element you are investigating. Whether or not an uptake of easily and moderately available portions occurs (as determined by seq el), and the amount of which, depends on the many soil parameters you mention. So it remains a challenge and a great question - the more, as the different plant parts show different uptakes. It remains also a challenge in analysis (REE in soils; plant ashes; ICP-MS).
Thank you very much for the great answers and suggestions!
I heard about a rhizosphere based approach to assess plant available fractions of elements in soil. In this paper the authors found significant correlations between the citric acid extractable amounts of some toxic trace elements in soil and the corresponding concentrations in plant tissues. I checked this with oat, barley and maize and there was no correlation, while there were significant results with rapesees and sunflower. From this it seems that the use of organic acids is restricted to the determination of bioavailable fractions for dikots. Thus I plan an experiment with siderophores to check for correlations with monokots. Is anyone familiar with that?
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