Agriculture Reference
In-Depth Information
Figure 2.12
An example of fungal mycelium and fruiting bodies. Photograph by the author. See color
insert.
live as parasites and many are pathogenic. Downy mildew of grapevines is one
such example. Of special benefit, however, are the
mycorrhizal fungi
, which live
symbiotically in plant roots, deriving C compounds from the host and in turn
supplying the host with mineral nutrients, especially P and Zn (section 4.7.3.2).
Soil fungal colonies are much less numerous than bacteria (1-4
10
5
organisms/g). But because of their filamentous growth, their biomass is generally
larger than the bacterial biomass. Fungi, excluding yeasts, are intolerant of anaer-
obic conditions. They grow better in acid soils (pH
5.5) and tolerate variations
in soil moisture better than bacteria. Fungi are all heterotrophic and are most
abundant in the litter layer and organically rich surface horizons of soil, where
their ability to decompose lignin gives them a competitive advantage.
Algae
. Algae can photosynthesize and therefore live at the soil surface. But
many will grow heterotrophically in the absence of light, if simple organic sub-
strates are provided. The two major subgroups are the Cyanobacteria, or blue-
green algae, and the green algae. The “blue-greens,” as exemplified by the genera
Nostoc
and
Anabaena
, are important because they can reduce atmospheric nitro-
gen (N
2
) and synthesize amino acids for growth. Therefore, they can contribute
a substantial amount of N to wet soils. Algae that occur in soils are much smaller
than the aquatic or marine species and may number from 100,000 to 3
10
6
or-
ganisms/g.
Protozoa
. Protozoa are the smallest of the soil animals, ranging from 5 to 40
m in length. They live and move in water films in the soil. Nearly all protozoa
prey on other small organisms such as bacteria, algae, fungi, and even nematodes.
Protozoan biomass is comparable to that of earthworms, so they are important in
controlling bacterial and fungal numbers in the soil, and hence they are impor-
tant in nutrient cycling.
