INTENSIVE

YET

SUSTAINABLE

Irrigated rice fields are found to be fertile after continuous cropping for decades using chemical fertilizers, which debunks the notion that intensive rice farming is unsustainable.

Limited scope for expanding rice production area
necessitates intensive rice farming to meet growing
demand. However, some people contend that modern,
intensive farming is unsustainable. It is said to degrade
soil, eventually causing it to lose its ability to support
crops. In the case of rice, this is untrue.

Most rice, unlike other major food crops, is grown
on submerged soil, which supports fundamentally
different biological and chemical processes and
requires different management practices. Conclusions
on the sustainability of intensive rice farming must
therefore consider the unique features of rice production
compared with other major food crops.

Scientists at the International Rice Research Institute
(IRRI) established experiments in the 1960s that
subsequently served as “living laboratories” for
strategic research quantifying the sustainability of
intensive rice farming. One of these experiments, the
Long-Term Continuous Cropping Experiment, had by
the end of 2009 produced 137 crops of irrigated rice
with fallows of only 3 weeks between harvesting one
crop and planting the next.

A report published in the Soil Science Society of
America Journal by Roland Buresh, Mirasol Pampolino
and Eufrocino Laureles of IRRI and Hermenegildo
Gines of the Philippine Rice Research Institute used
findings from four long-running experiments to show
that, with proper fertilizer management, continuous
rice cultivation on submerged soils can sustain soil
organic matter and the capacity of soil to supply
nitrogen that is available to plants. In fact, in all four
experiments, total organic carbon and total nitrogen in
the topsoil — which serve as measures of soil organic
matter — were not only consistently maintained but
even increased slightly during 15 years.

“The floodwater overlying soil during rice cultivation
is a favorable environment for the growth of aquatic
biomass such as algae, which, upon death, settle onto
the soil and add carbon to soil,” explains Buresh. In
addition, organic matter in submerged soil decomposes
more slowly than in aerated soil. Thus, under intensive
rice cultivation, soil organic matter is maintained.
Furthermore, the unique properties of the flooded rice
system help sustain soil fertility because of biological
nitrogen fixation, in which soil and floodwater organisms
convert atmospheric nitrogen into a nutrient usable by
plants. The nitrogen produced through biological
fixation per hectare per crop is equivalent to all the
nitrogen in a 50-kilogram bag of urea fertilizer.
The findings reveal that farmers need not apply
crop residues to fi elds to maintain soil organic
matter as long as the nutrients that have been
removed are replaced by appropriate applications
of chemical fertilizers. Contrary to widely held belief,
the long-term application of manufactured fertilizer
does not damage soil health.
The unique features of flooded soils help sustain
intensive irrigated rice systems that now occupy 24
million hectares in tropical and subtropical Asia, on
which 1.5 billion rice farmers and consumers depend.
But more research is needed.

“Scientists must increasingly develop management
practices to ensure continued sustainability as
irrigation water becomes limited,” Buresh concludes.