Page 520 - Biosystems Engineering

492 Cha pte r Se v e ntee n

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moment of application. The equilibrium between NH and NH
4 3
depends highly on the pH and the ionic strength of the manure. This
means that the pH of the manure or the manure/soil mixture has a
major effect on ammonia emission. Ammonia emission lowers the
pH, whereas CO emission, by decay of organic matter, increases the
2
pH. Because the solubility of CO in water is 200 times lower than
2
that of NH , the CO will be emitted at a higher rate directly after
3 2
application of the manure. This increases the pH of the manure, which
will then promote ammonia emission and results in a very high
ammonia emission in the first hour after application. After a period
17
of 24 to 48 h, the pH in the soil surface will decrease again by the
buffering action of the soil and manure. Once the manure is in contact
+
with the soil, the NH will be captured on cation exchange places of
4
the soil.
The rate of ammonia emission is thus largest directly after
application, due to the high initial concentration of ammonia and
the increase in pH by the emission of CO . After this initial emission,
2
the rate decreases rapidly because the ammonia concentration
decreases by emission, infiltration, and nitrification. Typically, emis-
sion already reaches 50 percent of its cumulative maximum within 12 h
after application. However, sometimes emission can go on for longer
periods when the meteorological conditions limit evaporation, and
there is a very low speed of infiltration. It can, therefore, be concluded
that the contact surface and time between manure and air should be
minimized. To reach this goal, several manure application strategies
(band application, incorporation, injection) have been introduced.
When weather conditions minimize ammonia emission, there can
still be important emissions originating from manure in the form of
surface runoff. As long as the manure is lying on top of the soil, runoff
of excess precipitation can transport the nutrients away from the field
into surface waters. In the context of the manure problem, this is
directly attributed to the nutrient concentrations in the surface waters
and should therefore also be avoided. This can be brought about by
putting the manure into the soil as soon as possible so that the nutri-
ents will not be brought into solution and transported away by the
water flow.
Apart from the negative impact of these emissions on the envi-
ronment, there is another reason why we want to minimize them. It
would be very difficult to do precise application of manure when
nutrient losses by gaseous emission of ammonia and surface runoff
reduce the nutrient content available to the crop.
Manure application techniques that bring the manure directly
into the soil reduce both ammonia emission and surface runoff. The
deeper the manure is placed in the soil, the higher the emission reduc-
tion, but energy consumption and crop damage will also be higher.
Therefore, there exists an optimal injection depth. This injection depth