85 / 116
83
October 2018
The
who’s who
of soil health
‘S
oil health’ is a very widely used term in agriculture these
days. Both producers and service providers have be
come increasingly soil health conscious. Soil health is
not only an important component of your soil’s current
productivity, but is also critical in the conservation of our food pro
ducing land (Di Cello
et al
., 1997).
Behind the term soil health there is a very complex interaction be
tween living and non-living elements in the soil environment. When
these elements are in balance, there
are synergistic interactions and re
lationships that promote better soil
and plant health.
Imbalances in the components in
volved in soil health result in a dis
ease promoting environment where
plants are stressed, disease levels
are high and soil productivity is low
(Garbeva
et al
., 2004).
When we look at the microbial com
ponent of soil health, we are deal
ing with microbial communities or
microbiomes. Overly simplified, we
can refer to them as ‘good’ com
munities (promoting soil and plant
health) and ‘bad’ communities (sup
pressing beneficial microbes and
causing disease).
Interestingly, the bad communi
ties (disease complexes) have
a very specific function in the
soil environment, i.e. to recy
cle plants with weak genetics as
food for the soil microbiome. It is
important to distinguish between the two microbiomes found
in soil since each of them has a unique contribution to soil
and plant health (
Table 1
).
Bulk soil microbiome
The bulk soil microbiome consists of communities of mostly free-
living microbes, i.e. microbes not directly associated with plants.
These microbes are all part of the soil food web. The soil food web,
as presented in
Figure 1
on page 84
,
is a complex interaction of soil
organisms (microbes, nematodes, insects, etc.) that each has a
specific role in making nutrients available to the soil inhabitants
and as a result has an impact on soil health.
Good soil health equals biodiversity. The extent of biodiversity has
shown to be critical to the maintenance of soil health and quality.
A large variety of microbial species results in a balanced and
stable soil food web that is disease suppressive and plant
growth promoting.
The opposite is an unbalanced
food web where plant pathogens
and parasitic nematode num
bers are high due to the lack of
natural enemies (bacterial and
fungal feeders as well as preda
tory nematodes) (Garbeva
et al
.,
2004; Wang
et al
., 2017a).
Factors to consider when the
goal is to improve the microbial
diversity in the soil are:
Different soil environments
select for different species
and strains of a specific mi
crobe. For example, mycor
rhizae that live in the desert,
will not grow in soil with
higher clay content.
Different soil management
practices have different ef
fects on the specific groups
of the soil food web.
Secretions from different
plants stimulate growth of
different microbial groups.
Different organic soil amendments stimulate growth of different
microbial groups.
The rhizosphere microbiome
Plant roots release a wide range of compounds into the surround
ing soil, including ethylene, sugars, amino acids, organic acids, vi
tamins, polysaccharides and enzymes. These root exudates create
unique environments for the microbes living in close association
with the roots, i.e. the rhizosphere.
on farm level
Management practices
Soil environment / Living / Non-living
Dr Ronél Hendriks,
chief scientific officer, ABM Africa
BULK SOIL MICROBES
RHIZOSPHERE MICROBES
Free living in soil (no direct interaction with plant)
Interaction/symbiosis with plant
Soil provides carbon and nutrients
Plant provides carbon and nutrients
Important role in soil = soil food web
Important role in plant nutrition = root development, enzyme and
metabolite production
Important role in soil health = pathogen/disease suppression
Important role in plant health = antagonism + induced resistance
Table 1: Comparison of the two microbiomes found in the soil.
“
Unfortunately, most
agricultural soils
have high numbers of
the pathogens and
as a result plants are
under constant
attack when
environmental
or nutritional
stresses surface.
“