Agriculture Reference
In-Depth Information
At any level in this hierarchy there are also organisms that feed on (or para-
sitize) living plant roots. Examples of such organisms that attack grapevines are
discussed in “Biological Control in Vineyard Soils,” this chapter. Symbiotic,
or mutually beneficial, relationships also occur. These include the fungal-root
association commonly known as mycorrhizas, described in “Mycorrhizas and
Nutrient Uptake,” chapter 3, and the legume-
Rhizobium
symbiosis described
later in box 5.2.
Overall, a viable population of decomposers and reducers is essential for
many soil processes such as energy flow (from dead to living matter and back
again), nutrient cycling, disease control, and the maintenance of a stable soil
structure. Growers practicing organic or biodynamic viticulture believe that hav-
ing a thriving soil biota is the cornerstone of a healthy soil. However, we should
remember that healthy functioning of soil depends on the felicitous interaction of
many physical and chemical processes with the soil biota.
Functions of the Decomposers
Decomposer organisms vary greatly in their size, numbers, and diet. Table 5.2
provides a summary of the essential features of the main groups: Archaea, bacteria,
fungi, actinomycetes, and algae.
Among the decomposers, we recognize two broad nutritional groupings—
the heterotrophs and autotrophs, described in table 5.3. Irrespective of these dif-
ferences, however, there are some generalizations we may make about modes of
nutrition and metabolism within the decomposer group.
First, microorganisms in soil are short of food most of the time, so when resi-
dues are added, the population quickly expands to consume the fresh substrate.
Furthermore, if an unusual organic compound is added, microorganisms that can
attack and consume that compound begin to proliferate because of their competi-
tive advantage over other organisms that cannot.
1
When such a substrate is first
added, there is a lag of several days or even weeks before it starts to disappear.
However, once “enriched” with the requisite microorganisms, the soil quickly
responds to further additions of that substrate, which is then rapidly decomposed.
In vineyards, this behavior is important for the decomposition of some herbicides,
fungicides, and pesticides that can have an adverse effect on the environment
because of their chemical stability or potential to leach into runoff water. Regular
and prolonged use of some fungicides and herbicides in vineyards can be deleteri-
ous to soil organisms, as discussed in “Organic, Biodynamic, and Conventional
Viticulture,” chapter 6.
1
Because microorganisms such as bacteria have enormous “genetic plasticity,” strains capable of
consuming unusual substrates can evolve rapidly.
