FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 457

Environmental influences on N-fixation
N-fixation rates are highly dependent on environment conditions. For example, N-fixation
in soybean crops varies from 10 to per crop, and some legumes may fix no
N at all (Dommergues et al., 1985). Factors that adversely influence N fixation rates are:
(i) edaphic, particularly drought and water logging, and (ii) the presence of assimilable
mineral-N as or (iii) environmental effects, notably insufficient light,
(iv) climatic, especially very high temperatures, (v) biological, including the presence
of root parasites such as nematodes, and (vi) the energetic constraints mentioned
in the section above. In this last respect, N fixation is clearly dependent upon an efficient
photosynthesis.

3 2.2 STIMULATION OF GROWTH AND DEFENCE AGAINST COMPETITORS

AND PATHOGENS

3.2.2.1 Plant growth promoting substances

Rhizosphere organisms produce a diversity of substances that stimulate plant growth.
Bacteria and fungi produce vitamins of the B group, and plant hormones including
auxins, gibberellins, kinetins and indole-3-acetic acid (IAA) (Brown, 1972; Rempe,
1972; Rademacher, 1994). The stimulatory effects of earthworm activities on plant
growth have also been attributed to a 'hormone-like' effect (Tomati et al., 1988).
Other substances may indirectly influence plant growth through allelopathic effects
on competing plants and the suppression of pathogens.

3.2.2.2 Allelopathy

Allelopathy includes any directly or indirectly harmful effect by one plant upon another
organism through the production of chemicals released into the environment.
These chemicals are secondary compounds and may be produced through exudation
or the decomposition of plant litter.

Chemical compounds implicated in allelopathy
Rice (1974) has identified 14 different groups of allelochemical compounds based on
their composition and metabolic pathways. The most important groups are phenolic
acids and their derivatives: terpenoids, steroids, coumarins, flavonoids, alkaloids,
cyanohydrins and tannins (Table IV. 11). They are generally water soluble, except for
a few volatile compounds that diffuse from roots and leaves (Jordan et al., 1993).

Effects of allelopathic compounds
Secondary plant compounds are chemicals produced by plants and micro-organisms
that are not essential to the growth of the organisms involved although they may play
important roles in mediating interactions with other organisms. They may directly
inhibit seed germination or the growth of other nearby plants; they may affect water
uptake (Einhellig et al ., 1985) or nutrient supply through, for example, the inhibition
of nitrification or nitrogen fixation (Balke, 1985; Hoagland and Williams, 1985).