Why might you want to know the total content of particular element(s) in a soil?

Here are few reasons why total determinations of trace elements might be of interest:

• To help determine the background (natural) levels in the soil parent material.
• To assess the total metal content of the soil; the total element-oxide content serves as a check for the “accuracy” of such a determination.
• To get an idea of how much of a particular metal might become available in the long-term.
• To see if there has been accumulation over time.
• To assess if metals are being mobilized by forming complexes with organic matter.
• To help determine how much of an organic amendment, such as biosolids, can be safely added to a particular land base and soil type.
• Guidelines for soil contamination are often based on total metals.

A digestion method that includes hydrofluoric acid (HF) is the only way to break down the primary silicate structures of the more resistant quartz and feldspar soil minerals and thus release the total amount of trace elements into solution to be determined.

Note: HF is a very strong acid. Extreme caution should be used and special laboratory equipment is required (e.g., teflon containers are needed because regular glassware will be dissolved). Digestions with perchloric acid are also not recommended for safety reasons; this strong oxidizing acid can cause explosions.

Because HF is very dangerous to work with, it is sometimes preferable to use other oxidizing acids or acid mixtures to do a pseudo-total digestion. These other digestion methods will not completely dissolve the silicates, but the results can often be related to “total” content for the elements of interest.

Some options that are considered estimates of total-recoverable metals are:

• Digestion with concentrated nitric acid and hydrogen peroxide (H2O2)
• Aqua regia digestion, which uses concentrated nitric (HNO3) and hydrochloric (HCl) acids.

Elemental concentrations of the digest solutions can be determined by spectroscopic methods such as atomic absorption spectroscopy (AAS) and inductively coupled plasma optical emission spectroscopy (ICP-OES).

References and Resources

• Burau R.G. 1982. Lead. In A.L. Page, R.H. Miller, and D.R. Keeney (eds). Methods of soil analysis. Part 2, 2nd Ed. American Society of Agronomy and Soil Science Society of America, Madison, WI.
• Chen, M., and L. Q. Ma. 1998. Comparison of four USEPA digestion methods for trace metal analysis using certified and Florida soils. J Environ Qual 27:1294-1300.
• Edgell, K. 1988. USEPA Method Study 37—SW-846 Method 3050 Acid digestion of sediments, sludges and soils. EPA Contract no. 68-03-3254.
  Available at: US EPA Method Study 37 SW-846 Method 3050 Acid Digestion Of Sediments, Sludges, and Soils
• Edgell, K. 1996. US EPA Method 3050B: Acid digestion of sediments, sludges, and soils.
  Available at: http://www.epa.gov/waste/hazard/testmethods/sw846/pdfs/3050b.pdf
• Hendershot, W.H., H. Lalande, D. Reyes, and J.S. MacDonald. 2008. Trace element assessment. In Carter, M.R., and E.G. Gregorich (eds). Soil Sampling and Methods of Analysis. 2nd ed. Canadian Society of Soil Science, CRC Press and Taylor & Francis Group. Oxford, UK.
• Hossner, L.R. 1996. Dissolution for Total Elemental Analysis In Sparks, D.L. (ed). Methods of Soil Analysis: chemical methods. Part 3. Soil Sci. Soc. Am. Book Series No. 5. ASA-SSSA, Madison, WI.
• Kabata-Pendias, A., and H. Pendias. 1992. Trace Elements in Soils and Plants. CRC Press. Boca Raton, FL.
• Risser, J.A., and D.E. Baker. 1990. Testing Soils for Toxic Metals. In Westerman, R.L. (ed.) Soil Testing and Plant Analysis. 3rd edition. ASA-SSSA, Madison, WI.