September 2014
116
soil fertility
S
oil microbiology is the basis of sustainable agriculture
and sustainable agriculture is the basis of the survival of
mankind.
Contrary to erroneous belief, soil is not a mere matrix to support
crop plants, nor an unlimited resource that we can exploit for our
short-termbenefit without replenishingwhat wehave taken from it.
The relationship between the soil’s physical, chemical and biologi-
cal properties is exceptionally complicated and, depending on ex-
isting soil conditions, these relationships can be either positive or
negative toplant and soil health.
A system where communities of organisms interact with other or-
ganisms, as well as the environment in which they live, is called
an ecosystem. Ecosystem functioning is greatly influenced by soil
organic matter availability, the activity and biodiversity of certain
organisms (viruses, bacteria, fungi, yeasts, nematodes, protozoa,
earthworms, etc.) aswell as the flowof nutrients.
Plants respond toandalter their environment through root exudates,
i.e. chemicals secreted into the soil by roots. Root exudate composi-
tion is determined by the physiology and genetics of the plant, as
well as by environmental factors. Different plant species will con-
sequently attract different soil microbial communities (SMCs) with
the ability to utilise specific chemicals or carbon sources secreted
byplant roots.
This microbial activity is most effective and efficient in the rhizos-
phere – the narrow regionof soil surrounding the plant root system.
Irrespective of the environments in which plants find themselves,
they all rely on SMCs for their crucial role in processes such as the
cyclingof nutrients in an ecosystem.
Soil inhabiting nematodes are of particular significance to the pro-
ducer, since they play a very important role in soil organism com-
munities that are interdependent for sourcesof carbonandenergy in
the soil (the soil foodweb). Plant parasitic nematodes are generally
seen as enemies to the producer, while free-living nematodes are
seen as benign.
Plant parasitic nematodes, in the act of penetrating and feeding on
the host root, create a weakened site that provides access to bac-
teria, protozoa and fungi. These organisms are then able to invade,
reproduce anddisperse throughout the root tissue, causingdisease.
As a result, they alter their surrounding environment, leading to
changes in soil microbial community composition, activity and di-
versity in the surrounding soil environment.
Some soil organisms play a critical role in soil fertility by serving
as biocontrol agents and biofertilisers. These include free-living
nematodes, symbiotic (interdependent) rhizobia (
Photo 1
) and plant
growth promoting rhizobacteria (PGPR) – groups that have been
widely studied and recognised for a long time. Nematodes, for in-
stance, may control outbreaks of algae, fungi, bacteria and other
microscopic fauna in soil andmaintain the balance of life forms that
constitute thedelicatebalanceof nature (
Photo 2
).
Many authors have duly emphasised the fact that, although free-
livingnematodesareconsideredbenignbymankind, theyconstitute
oneof thevital components in thepreservationof thebalanceof our
world’s lifeprocesses. Considering this, it canonlybeadvantageous
for crops to “optimise” theirmicrobial allies in the rhizosphere.
Thisknowledgecanbeappliedbyseveralmeans toenhance thesus-
tainability of agricultural systems by favouring beneficial SMCs to
increaseplant health, while enhancingdefences against pathogens.
With pressing global issues, such as food security, an ever-increas-
ing human population, arable land lost due to urbanisation, exhaus-
tion of fossil fuels and natural resources, aswell as climate change;
wemight be surprised to realise that the solution lies in something
wewalk on andoverlook everyday:Our soils.
Wemight relyon the illusion that our financial wealth andpowerwill
keep us alive, but we all know that you can’t stay alive by having a
stack of roastedR100 notes for dinner? There can be no lifewithout
agriculture, precious soil and the populations ofmicroscopic organ-
isms itmaintains.
Traditional agriculture has left soils unhealthy anddepletedof nutri-
ents, unable tonaturally sustain cropsunless chemical fertilisers are
applied. We have turned our soils into a veritable drug addict who
cannot get through thedaywithout a “chemical fix”.
Just as a drug addict cannot be rehabilitated overnight, we cannot
expect our soils to be rehabilitated in a matter of a few years after
decades of chemical fertiliser abuse.
Conventional intensive agriculture that resulted in declining crop
yields, the increaseduseof chemical fertilisers, and soil erosiondue
towindand raincausinga lossof 24billion tonsof soil due toerosion
in2011 alone, have ledproducers to search for alternative strategies
thatwouldnot leave arable landbare andunprotected after harvest.
These increased environmental concerns brought about the realisa-
tion that fertile soils are urgently needed to dramatically increase
yield on all available arable land. This resulted in farming strategies
aimed at increasing and sustaining soil organic matter, crop rota-
tion, integrated pest management and other ecologically-oriented
practices.
The application of current knowledge led to the implementation of
conservation agriculture (CA). CA integrates three main principles,
i.e. crop rotation (cropdiversification) in combinationwithminimum
soil disturbance (no-till, minimum-till) and permanent soil cover
(mulching), which significantly influence soil biological diversity and
activity.
With soil organicmatter being responsible for the energy supply in
anecosystem, CA iswinning the raceasameans to restoreour soils.
Influence of crop diversification
Cropdiversification (
Photo3
) encourageshigh soilmicrobial activity
and increased diversity. Such diversification promotes synergistic
associations between organisms such as rhizobia and mycorrhiza,
the latter being abeneficial relationshipbetween the roots of vascu-
lar plants and a fungus colonising thehost plant’s roots.
Soil / Fertility /Microbiology / Sustainability
JOHANHABIG, ANTOINETTE SWART
and
AHMEDHASSEN,
ARC-Plant ProtectionResearch Institute
ON FARM LEVEL
Conservationagriculture
