They call them
“suppressive” soils because they suppress disease in crops. Scientists are
racing to find out why. But they know 2 things: 1. Soil microbes are responsible for them. 2. Soil carbon
increases are the key. And “There are soils right across the country where the
incidence or severity of disease is suppressed, even in the presence of the
pathogen that causes it, a host plant and a favourable environment,” says Associate Professor Pauline Mele,
LaTrobe University and principal research scientist, Department of Primary
Industries Victoria (DPI).
“These
disease-suppressive soils have been found to develop under management practices
that supply higher levels of carbon inputs for more than five consecutive
years. The carbon from plant roots and crop residues is biologically available
and provides an important food source for soil biota, ” says CSIRO’s Dr Gupta
Vadakattu in GRDC’s GroundCover 96 Soil Biology Supplement.
Disease suppression
is the result of increased species density among microbial communities in soils
associated with increased carbon levels. We know that, when soil carbon levels
are rising, biodiversity increases and this has the effect of increasing
resilience (or disease resistance). “We know the effect is due to the presence
of a diverse range of ‘good’ micro-organisms,” says Professor Mele.
Three facts Dr
Mele mentioned provide further evidence that soil carbon is a key influence:
1. Balance in the
microbial community is critical:
“upsetting the balance or sterilising the soil can cause the disease to strike
with a vengeance”.
2. It is not soil
type specific; it could therefore be a soil health agent – such as carbon –
that is at work: “ we believe every soil has the potential to be suppressive”
3. It is a feature
of soil heavily influenced by a farmer’s management practices: “it’s just a
matter of working out what management techniques will encourage it.”
The fact that a
microbial community is a natural system and such systems exhibit ‘emergent
properties’ as they become more complex. It is not one variable at work.
However, reductionist science tends to look for the single factor. However the
Professor says, “At this stage, though, we’re still trying to identify exactly
what organisms, or combination of organisms, are doing the work.”
“HIGH rainfall zone (HRZ)
grain growers stand to increase yields and save significant amounts of money on
chemicals, if the secrets of suppressive soils can be unlocked,” reports The
Land. Growers lose an estimated $250 million each year from root lesion
nematodes alone. “Soil biology is tipped to be the ‘next big thing’ in terms of
productivity gains and a five-year research program is currently being funded
by the Grains Research and Development Corporation (GRDC) to address some of
the knowledge gaps.” Having poured scorn on soil biology as “snake oil” and
‘witches brew’ for so long, the GRDC’s epiphany is welcome.
“The soil
biological resource under our feet is seen as something of the ‘last frontier’
for the grains industry… We know it’s about competition for resources. If we
create a habitat that favours one type of soil microbe, say through repeated
use of the same management practice such as addition of fertiliser or sowing the
same plant types, the community may end up with fewer types of biota present;
thereby reducing the resilience of the system,” says Professor Mele.
The writing is on
the wall for chemical companies. “Using biological suppression to reduce crop
losses, without chemicals or with minimum chemical input, could improve the
profitability of growers worldwide,” says the Professor.
More information
about the Soil Biology Initiative II is available at www.grdc.com.au/soilbiology.
Research partners include the Victorian Department of Primary Industries (DPI
Vic), Queensland Department of Agriculture, Fisheries and Forestries (DAFF),
Department of Agriculture and Food WA (DAFWA), and CSIRO.
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