| MadSci Network: Environment/Ecology |
Dear Michael,
Sorry it took so long to respond to your question. I have talked with a number of soil scientists and
soil testing technicians. Unfortunately there is no easy, accurate way of determining **TOTAL**nitrogen
in soil. Notice I emphasized TOTAL.
The standard method for measuring total nitrogen in soil, used by many testing labs and researchers,
is call the Kjeldahl ("Kel-dall") method, which, using concentrated sulfuric acid, some sort of oxidizer
(mercury, copper or selenium), and high heat, converts all forms of nitrogen in the soil (inorganic and
organic) to ammonium (NH4+). The ammonium can then be used for a quantitative recovery. I would
not consider this method applicable to a high school lab as it requires the use of some very dangerous
chemicals and very specialized apparatus.
There are ways, however, to measure the plant usable forms of nitrogen in the soil. This would give a
generalized idea as to the fertility of the soil. The two forms of nitrogen available to plants are nitrate
(NO3-) and ammonium (NH4+). Roots can absorb these forms, however, the nitrate form is generally
preferred. Hence the fact that fertilizer contains nitrate forms of nitrogen. There are microorganisms that
can change ammonium to nitrate through a process called nitrification. You can actually see evidence of
this. Pull up clover or alfalfa plants and look closely at the roots. You may see small nodules on the
roots. These are sites where microorganisms can convert ammonium to nitrate forms of nitrogen more
readily usable by the plant. That is why many farmers plant crops like clover and simply plow them under
at the end of the season. This crop adds nitrogen naturally to the soil to be used by other plants.
Getting back to original question. As I said, there are ways to measure ammonium and nitrate
(inorganic forms of nitrogen) levels in the soil using either electrodes or colorimetric methods. The
electrode method uses ion electrodes that are specific to ammonium or nitrate ions. The problem with
this method is the cost of the electrode and the meter are fairly high. The second method I mention
is the colorimetric version that changes ammonium and nitrate ions to a specific color and then, using a
spectrometer, you are able to quantitate the results. Again, the limiting factor for a high school lab may
be the spectrometer and also some of the chemicals, which are fairly dangerous.
If you would like to look at the various procedures that I mentioned, try finding the following book:
"Methods of Soil Analysis", Part 2-Chemical and Microbiological Properties, 2nd Ed. , American Society
of Agronomy, Soil Science Society of America, Madison Wisconsin, USA, 1982.
There are other ways that you may be able to use to determine the fertility of soils. You might be able
to convince a university laboratory or private testing company that normally runs soil tests to run a few
samples for you. Or you might be able to find a 'home' use test kit that would give you a very crude idea
of soil fertility. These might be available at local garden or home improvement stores.
I am afraid that is about all that I could tell you. If you have more specific questions, please feel free
to contact me at tedjeo@tc.umn.edu
I hope this helps you.
Best regards,
Ted