Introduction to Soil Chemistry

40 soil basics ii
NH4
Nitrosomonas spp.
NO 2 –
(2.3)
When observing the oxidation of ammonia to nitrite in soil, it is found to be
a slower reaction than is the oxidation of nitrite to nitrate. When the energy
available from each of these reactions is considered [see (2.3)], it is obvious
that this observation is directly related to the amount of energy available.
In this case nitrite is not expected to occur or build up to appreciable levels
in the environment. Nitrobacter species must use approximately 3.6 times as
much nitrogen in terms of nitrogen atoms to obtain the same amount of energy
as Nitrosomonas spp.Thus it can be expected to take up nitrite at a higher rate
to compete. This type of energy calculation is simply done by factoring in the
amount or energy required for bond breaking and the amount released
in bond making. Alternatively, the energy can be measured directly by
calorimetry.
2.8. ALL FACTORS TOGETHER
In a purely chemical approach to how reactions take place, all of these factors
come together; specifically, the rate is related to total energy used and released,
the energy of activation required, steric effects, and the types of bonds
being broken and formed. For this reason it is most common to measure
the energy and rate quantities directly for the conditions of the reaction.
Because of the complexity of soil, it is even more important to measure these
directly.
2.9. MICELLES
Nitrobacter spp.
+ –
NH4 + 1.5O2 NO2 + H2O + 2H +
Nitrosomonas spp.
Nitrobacter spp.
–
NO2 + 0.5O2 –
NO3 +
Difference
NO 3 –
275 kJ
76 kJ
3.62 ¥
In Chapter 1 it is observed that sand, silt and clay do not act independently of
each other. In a similar fashion clay particles alone do not act independently
of each other; rather, they form groups of particles called micelles. The model
for a micelle is a group of long-chain fatty acid salts in water. The hydrophobic
ends are associated with the ionic “salt” end exposed to water. An idealized
micelle with some associated water is shown in Figure 2.6. The structure
is often described as being a ball-like. This ideal is hard to visualize when
looking at the typical shape and size of a clay particle. However, it is possible
to envision individual clay crystals associated with each other through hydro-
(a)
(b)