Introduction to Soil Chemistry

kjeldahl—organic nitrogen 121
Nitrogen in soil organic matter is mostly in the form of proteins and amino
acids. Although the specific analysis for these important and interesting
compounds can and is done, it is more often the case that the total inorganic
and organic nitrogen in soil is determined. This is because inorganic nitrogen
compounds are used by plants and are of environmental concern. Decomposition
of organic nitrogen containing compounds results in the release of
ammonia into the soil solution, where it immediately reacts to form ammonium.
Once in this form, it is readily oxidized by soil bacteria to nitrite and
finally into nitrate. Because of the ready conversion of organic nitrogen into
inorganic forms and the ready interconversion of inorganic nitrogen in soil, its
total concentration, both inorganic and organic, is important. Kjeldahl is the
basic and most commonly used method for determination of total nitrogen in
soil.
In a total soil nitrogen analysis a soil sample is first digested in a kjeldahl
flask to convert all organic nitrogen into inorganic ammonium. Two kjeldahl
flasks are shown on the left-hand side in Figure 6.7; the flask with the bulb at
the bottom is an older-type kjeldahl flask, while the large test tube is a newerdesign
digestion tube. Digestion is accomplished using concentrated sulfuric
acid and a catalyst. A salt such as potassium sulfate is added to increase the
boiling point of sulfuric acid such that decomposition of organic matter occurs
more readily. This mixture plus soil is heated until all organic matter has
been destroyed. In Figure 6.6 a heating block for heating the kjeldahl flask is
shown next to the distillation unit. After digestion the solution is cooled and
a concentrated basic solution, usually 50% NaOH, is added and the released
ammonia steam-distilled into a receiving flask containing a standard acid
that reacts with the ammonia. On completion of the steam distillation, the
unreacted acid is titrated and the amount of ammonia distilled is calculated
by difference.
The kjeldahl procedure has been used for many years to determine the
nitrogen in human tissues and in both animal and human foodstuffs. For these
materials, the procedure works well and is straightforward. For soil, such is not
the case. All soils naturally contain some ammonium, and when the steam
distillation is carried, out this distills along with the ammonium produced by
the decomposition of organic matter. This then gives a measurement of the
total ammonium in soil after digestion. It cannot distinguish between ammonium
derived from organic matter and from the soil itself.
If a soil kjeldahl organic nitrogen determination as described above has
been carried out, then this can be used along with simple ammonia steam
distillation, to measure the amount of nitrogen from each source, that is, inorganic
ammonium and organic matter. However, this still does not provide a
measurement of the total nitrogen in soil because it does not account for that
present as either nitrite or nitrate. See Figure 6.7 for a flow diagram for determining
all nitrogen in soil using a kjeldahl apparatus.