Introduction to Soil Chemistry

ammonia 119
Figure 6.6. Setup for performing kjeldahl analysis of soil. On the left are two different types of
kjeldahl flask; to the right of the flask is a heating block used to heat the text-tube-shaped flask
during digestion and on the right, a steam distillation unit.
The equilibrium lies to the right unless the solution is rendered basic, at which
point the equilibrium shifts to the left and ammonia gas is released. This is the
basis for a common method for the determination of ammonium in soil. 1 Soil
is suspended in water and placed in a kjeldahl flask. The suspension is rendered
basic by the addition of a strong (5–50%) sodium hydroxide solution,
and the flask is immediately attached to a steam distillation setup. Steam distillation
of the suspension carries the released ammonia to an Erlenmeyer
flask, catching the distillate in a standardized acid solution that is subsequently
backtitrated, by acid–base titration.The amount of ammonia in soil can be calculated
from the endpoint of the titration. This procedure is similar to a standard
kjeldahl determination and can be carried out using the same equipment,
although no digestion is needed.
A steam distillation apparatus for determining the ammonia released by
the digestion of organic matter is shown on the right-hand side of Figure 6.6.
This same apparatus can be used to determine ammonia in soil as described
above. A flow diagram for the determination of ammonia in soil using a
kjeldahl apparatus is given in Figure 6.7 [6].
1 A cation containing solution is added before ammonium determination to release exchangeable
ammonium.