Agriculture Reference
In-Depth Information
tion is limited. Some elements, such as sulfur, can be reduced to their elemental
state by soil microorganisms; however, they both can also be easily oxidized
and the oxidized forms reduced and so are rarely found in their elemental
form in soil.
4.5.
DISSOLVED GASES
Molecular gases, the two most important of which are oxygen and carbon
dioxide, dissolve in soil water. Dissolved oxygen results in the soil solution
remaining oxidative or aerobic and thus tends to keep components in their
highest oxidation state. Microorganisms present will be aerobic and dissolved,
and suspended organic matter will undergo oxidative reactions.
Dissolving carbon dioxide produces carbonic acid, which ionizes to bicar-
bonate and carbonate ions, the reactions for which are shown in reactions
(4.1a)-(4.1c). This reaction sequence is extremely important because bicar-
bonate is a counterion to many cations, is active in buffering the soil solution,
and is involved either directly or indirectly in many soil chemical reactions.
Bicarbonates are generally more soluble than carbonates, which are generally
insoluble. Adding acid to carbonates or bicarbonate results in the release of
carbon dioxide and the formation of the salt of the acid cation. The acid is thus
neutralized.
At the other extreme, high levels of carbonate or high pH results in the for-
mation of insoluble carbonates, frequently calcium and magnesium carbon-
ates, thus removing them from the soil solution. These reactions thus limit the
lower and upper pHs normally found in soil [reactions (4.1d) and (4.1e)]. In
addition to carbon dioxide, methane is another common molecular gas in the
soil solution. It is largely insoluble in water but will be present because it is
constantly being produced by methanogenic bacteria in anaerobic zones in
soil. It may seem unlikely that methane would be found in an oxidizing envi-
ronment; however, as noted above, some pores in soil do not drain and at some
distance from their mouths microaerobic and/or anaerobic zones will be found
(Figure 4.5). Anaerobic zones are conducive to the formation of methane,
hydrogen, hydrogen disulfide, carbon monoxide, or other reduced species in
the soil solution.
Any low-molecular-weight organic compound that is normally a gas at stan-
dard temperature and pressure (STP) may also be found in the soil solution.
These compounds will be produced as a result of the decomposition of organic
matter in soil. Many of these will be readily taken up and used by micro-
organisms and thus their lifespans in the soil solution are short.
4.6.
COMPOUNDS IN SOLUTION
The soil solution will contain numerous inorganic and organic compounds
derived from the solid components making up the soil. Common compounds
