Agriculture Reference
In-Depth Information
Figure 5.14
Healthy vines grown under irrigation with reclaimed sewage water in
the McLaren Vale region, South Australia. (Photo courtesy of Dr. Belinda Rawnsley,
South Australian Research and Development Institute, Adelaide, South Australia.)
Summary Points
1. The C cycle of growth, death, and decay is essential for replenishing
a soil's nutrient store. Organic residues fall as litter and animal
excreta on the soil surface, and dead cells and exudates are deposited
around plant roots in a zone called the rhizosphere. A host of soil
microorganisms colonize the organic residues, deriving energy and
nutrients for growth.
2. During the decomposition process, CO
2
is produced and nutrients
such as N are mineralized (as NH
4
+
), depending on the C-to-N ratio
of the residues. The organic residues are converted into more and more
recalcitrant humic compounds. The combination of undecomposed
residues, dead organisms, and humic compounds is soil organic matter
(SOM).
3. Among the living soil organisms we recognize “reducers” and
“decomposers.” The reducers are larger organisms such as earthworms,
wood lice, mites, springtails, insects and their larvae, millipedes, and
centipedes that, although partially digesting organic matter, serve
the important function of breaking it down into small pieces that the
