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.
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 [1-4]. Average maize and soybean yields were relatively similar in all three systems over the post-transition years (19851998).
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 soils 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 Institutes 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.
Article first published 06/01/04
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