Introduction to Soil Chemistry

56 soil basics iii
2–
S2O3 S 0
H 2 S
Oxidation
Reduction
2–
SO3 R–SO 3 H
R–SH
2–
SO4 BIOLOGICAL AND ORGANIC CHEMICALS OF SOIL
3.4. BIOCHEMICAL
R–S–R
R–OSO 3 H
Figure 3.8. The sulfur cycle where S 0 is elemental sulfur, H2S is hydrogen sulfide, S2O3 2- is thiosulfate,
SO3 2- is sulfite, SO4 2- is sulfate, ROSO3H represents a sulfate ester, RSO3H a sulfonic acid,
RSR a thioether, and RSH a thiol (adapted from Coyne MS. Soil Microbiology: An Experimental
Approach. Boston: Delmar Publishers, 1999).
There are two sources of biochemicals in soil; one is the cellular constituents
released when cells are destroyed during the extracting process. It should be
kept in mind that any handling of a soil sample will cause the destruction of
some of the cells it contains. The simple acts of sieving, air drying, and weighting
soil will cause some lyses of cells and release of their contents. Extraction
will typically cause complete destruction of all cells in soil, with the release of
all their constituent parts. Some parts such as enzymes may continue to function
after release from the cell and continue to change the makeup of soil components
for some time.
The second source of biochemicals is molecules excreted from cells such as
extracellular enzymes and unused organic matter. A typical example is cellulase,
which is excreted by fungi such as Penicillium in order to break down
wood and woody material into sugars that can be used by the organisms.
Other common extracellular enzymes found in soil are ureases and amalyases.
Often enzymes are associated with clay particles, and in such associations their