Mr.Zethof,above 5.5 pH in soil, water soluble Al ions may not be present.So in highly calcareous soils with pH 7.8,delectable amounts of water soluble Al may not be present.In calcareous soils(pH 7.8) water soluble Fe also may be also small.However ,small amounts of iron may be mobilized by microbial activity and organic matter(organic matter decomposition,root exudates and secretion of organic acids ,protons by roots) in rhizosphere and also under reduced conditions(Fe3 can be converted into Fe2 form which is soluble).In calcareous soils iron deficiency and iron chlorosis  in plants is a known problem.

Thank you for your response. The ICP-OES didn't distinguish between Al3+ ions or other Al species (like Al(OH)4). So I agree I wouldn't expect to find Al3+ at this pH. But most literature on toxicity of Aluminium in soil (solutions) are mostly on Al3+ species (so with a maximum pH of 6.5/7) and not Al(OH)4. Do you have an idea if Al(OH)4 could be harmfull for plants and what normal levels should be in soil solutions? I can imagine that they are transformed in the "acidic zone" around the roots to Al3+.

 Dear Zethof,Interesting discussion on different species of Al.We paid attention so far on Al3,AlOH 2 plus and Al(OH)2 plus species but the Al(OH)4 with minus one charge behaves much differently.In aqueous phase,the activity of Al3 plus and AlOH 2plus  end by 7 pH and that of Al(OH)2 plus  by pH 8.6. But Al(OH)4 minus concentration increased from pH 4.4 to over 10.Of course, the local acidity/organic acids in rhizosphere may solubilize the  Al(OH)4 species. This would be an interesting study.The following references may be consulted for more information on chemistry of Al species at different pH.

Chemistry of aluminum in natural waters Forms and stabilities of aluminum hydroxide complexes in dilute solution By J.D.Hem and Roberson,C.E.

pubs.usgs.gov/wsp/1827a/report.pdf

Ecological Soil Screening Level for Aluminum Interm Final  US EPA

https://rais.ornl.gov/documents/eco-ssl_aluminum.pdf

Dear Jeroen,

We published years ago a paper on Al toxicity indices. You can find the link below. It refers mainly to acid soils, but maybe it can be of some usefulness for you.

Thank you very much, Dr Rao, for the link to the excellent publication of Hem and Roberson. I had a print copy of this publication in the past, but I lost it; I was regretful for that for years. I am happy with the pdf.

Article Application of aluminium toxicity indices to soils under var...

Dr.(Ms.)Maria Luisa,it is by chance  that in my search on internet I could locate that excellent publication .I am glad that you could get  your favorite paper .Your attached paper is also quite good and worth reading.

Thank you both for these very useful papers. I didn't encounter them at my search, but maybe I was looking in the wrong direction.

I dont understand well the significance of ICP-OES but I think that iron concentration (total iron or soluble iron which is uptake by plant) can be determined by two different protocols. for the soluble iron, you must add an EDDTA-Fe chelate to extract it. I hope that I contribute in this discussion.

first: iron contents must not be dosen on extract 1:10 thé extraction of soluble iron must be done as mentionned by PAwels and al. with adding a chelate agent...

second: iron chelation in calcareous soil with high pH is a big problem which causes chloroses in some fruit plants ..SO you can find high levels of iron in soil solution and contrary plants show iron deficiency...