Introduction to Soil Chemistry

78 soil basics iv
Ped
Microaerobic
zone
Anaerobic
zone
may include, in low concentrations, oxides, particularly those of silicon, aluminum,
iron, and titanium.These compounds move down the soil profile sometimes
contributing to formations such as the spodic horizon, which can contain
aluminum and iron oxides along with highly decomposed carbon.
A large variety of dissolved organic compounds are released by decomposition
of organic matter or from the activities of plants and animals in soil.These
compounds range from relatively simple molecules such as acetic acid, to relatively
complex materials such as enzymes and antibiotics and various cellular
components released on cell lyses (see Chapter 3). These and intermediate
products serve as sources of carbon and energy for organisms in soil and thus
do no persist. However, it is possible for almost any relatively simple watersoluble
organic compound to be found in the soil solution at any given time.
Other inorganic and organic compounds are brought into solution by the
decomposition of their parent materials. Rocks and minerals will be decomposed
by physical, biological, and chemical mechanisms. Enzymes released
into the soil solution by microorganisms will decompose insoluble organic
materials such as wood. Most of the organic material released with be taken
up by the organism releasing the enzyme, but some will find its way into the
soil solution.
4.7. INORGANIC IONS IN SOLUTION
Water layer aerobic
Figure 4.5. Aerobic, microaerobic, and anaerobic zones in a soil ped.
Ionic species are generally more soluble in water than are neutral molecules.
The polar hydrogens in water are associated with anions and the partially