This reference might help you:

https://publications.waset.org/10008000/methods-for-preparation-of-soil-samples-for-determination-of-trace-elements

Sounds like you are looking for the ISO 11466:1995 method

"Soil quality — Extraction of trace elements soluble in aqua regia"

Also, you should try to add more clarity to your questions. Do you need to do acid digestion? Are you analyzing total concentrations of metal in soil?

I think rather than acid digestion, DTPA extraction or microwave digestion method is the right method for heavy metal analysis in soil.

Pending on the metals you are looking for you will need to add hydrofluoric acid you need to be careful with this acid and you would be best using a microwave digestor

Thank you Zeyede Aregahegn Carol M.S.T Sukhn

Akshatha Banadka

There are several published methods for the analysis of heavy metals in soils. I like this method from the research group of Professor Chen at NTU Taiwan. Article Digestion Methods for Total Heavy Metals in Sediments and Soils

You can also download for free a pdf from Google scholar.

Thank you so much Victor B. Asio

The heavy soil element analysis method is the DTPA method. As for the plant nutrient microelement method, it is the acid digestion method.