ISIS Report 06/01/04
Organic Outperforms Conventional in Climate
Extremes
Long-term research has shown that organic cropping systems give
higher yields than conventional during periods of drought or torrential rains.
Lim Li Ching reports.
Sources
for this report are available in the ISIS members site.
Full details here
The Rodale Farming Systems Trial was started in 1981 at the Rodale
Institute in Pennsylvania, USA. It compares the benefits and risks of three
farming systems, two organic – manure-based (MNR) and legume-based (LEG) -
and one conventional (CNV), on a long-term basis.
The MNR system is a 5-year maize-soybean-wheat-clover/hay rotation, the
LEG a 3-year maize-soybean-wheat-green manure, and the CNV a 5-year
maize-soybean rotation. The MNR system includes livestock and uses manure as
fertilizer, while LEG incorporates leguminous crops into the soil. Both the
organic systems rely on mechanical cultivation and heterogeneous crop mixes for
their weed and pest control. The CNV system uses mineral fertilizer and
pesticides.
After a transition period of four years, crops grown under the organic
systems yielded as much as and sometimes better than conventional crops.
Average maize and soybean yields were relatively similar in all three systems
over the post-transition years (1985–1998).
Five moderate drought years, with total April-August rainfall less than
350 mm, occurred between 1984 and 1998. In four of them the organic maize
out-yielded the CNV by significant margins. For example, in 1998, organic maize
yielded 141% and 133% relative to CNV in the LEG and MNR treatments,
respectively.
In 1999, a severe crop season drought in the northeastern US was
followed by hurricane-driven torrential rains in September, offering a unique
opportunity to observe how the systems responded to climate-related stress. As
evidence of global warming gathers (see "Life of Gaia" series,
SiS 20), there
is greater likelihood of increased incidence and severity of droughts, flooding
and other extreme climatic events. Long-term crop yield stability and the
ability to withstand climatic stresses will be crucially important for
sustainable and secure food production.
The Rodale researchers examined crop and water dynamics by measuring
cover crop and crop biomass, weed biomass, grain yields, percolated water
volumes, soil water content and water infiltration rates.
Crop season rainfall for April, May, June, July and August 1999 were
55%, 66%, 17%, 29% and 40% of normal for those months, respectively. September
1999, with 268%, was a complete reversal of previous months as a result of
Hurricane Floyd, and was the wettest September on record in the Northeast
US.
Due to the severe drought, all crop systems suffered heavy depressions
in yield in 1999: to less than 20% of the long-term average in maize and 60% in
soybean.
There were substantial yield differences between systems. With one
exception (LEG maize), organic systems gave significantly better yields than
the conventional system.
Organic LEG and MNR maize yielded 38% and 137% respectively relative to
CNV. LEG soybean yields were significantly higher than MNR, and MNR in turn
yielded significantly higher than CNV. Organic LEG and MNR soybean yielded 196%
and 152% respectively relative to CNV.
The severely reduced yields in the LEG maize plots were largely due to
pressure from weeds and excessive cover crop biomass (nearly twice that
necessary for adequate nitrogen inputs) in that year. Skill in managing weeds
is therefore an important part of organic farming. Both these factors
contributed to over-consumption of the limited soil water, thereby more than
canceling out the benefits of the LEG soil’s good water-holding capacity
and infiltration rate.
The primary mechanism thought to be responsible for the higher yields in
the organic systems is indeed the improved water-holding capacity of the soils
during water deficits. Data collected over the past 10 years of the Rodale
research show that the MNR and LEG treatments improve the soils’
water-holding capacity, infiltration rate and water-capture efficiency. LEG
maize soils averaged a 13% higher water content than CNV soils at the same crop
stage, and 7% higher than CNV soils in soybean plots.
In combination, these factors have led to optimum drought adaptiveness
in the MNR maize plots and both the MNR and LEG organic soybean plots. In
contrast, the CNV system had poor soil water-holding capacity and infiltration,
limiting the ability of the crop to adapt to drought despite negligible water
use by weeds and no water use by a cover crop.
Earlier research showed that organic techniques significantly improve
soil quality, as measured by structure, total soil organic matter (a measure of
soil fertility) and biological activity. The improved soil structure created a
better root-zone environment for growing plants and allowed the soil to better
absorb and retain moisture. Apart from the benefit during low-rainfall periods,
it reduced the potential for erosion in severe storms. The higher organic
matter content also made organic soils less compact so that roots could
penetrate more deeply to find moisture.
Furthermore, the organic soils showed good late-season flood
performance, with good water capture and reduced runoff. Water capture is
important for groundwater recharge. Soils in the organic plots captured more
water and retained more of it in the crop root zone than in the CNV treatment
in 1999. Water capture averaged 30% higher in the organic plots than in the CNV
plots, and in September, following high rainfall, water capture in the organic
plots was approximately 100% higher than in CNV plots.
In May, at the onset of drought, the CNV plots, for the only time in
1999, had more percolated water than the other treatments, indicating that in
the LEG and MNR plots more water was retained in the soil for crop use during a
time when water was limiting. Over a 5-year period, the LEG and MNR systems
captured 16% and 25% more water than the CNV system, respectively. The MNR
plots captured significantly more water than the LEG system.
The Rodale Institute’s research provides yet more evidence that
organic agriculture can help ensure sustainable long-term food production (see
also The Case
for a GM-Free Sustainable World by the Independent Science Panel, ).
These results highlight the benefits to soil quality organic farming brings,
and its’ potential to avert crop failures. "Our trials show that improving
the quality of the soil through organic practices can mean the difference
between a harvest or hardship in times of drought", said Jeff Moyer, Farm
Manager at Rodale Institute.
Given the increasing incidences of climate extremes, and projections
that these are likely to occur more frequently, organic crop management
techniques will be important in providing soil and crop characteristics that
can better buffer environmental extremes.
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