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Tiny creatures known to scientists as 'rhizobia' could become one of the most important means of
boosting food production in the hungry regions of the third world, perhaps even averting the threat
of global famine frequently predicted by demographers and economists. These highly
industrious
micro-organisms not only hold out the promise of providing farmers in developing nations with a
much cheaper alternative to scarce and expensive chemical fertilizers, but unlike the nitrogenous
fertilizers, present no threat to the environment. What they have in common with chemical fertilizers
is their ability to provide nutrients for the soil. Rhizobia absorb nitrogen from the air,
converting it
and the organic matter in the soil into a valuable nutrient to improve the yield of various crops.
Plants themselves cannot use nitrogen straight from the atmosphere. Rhizobia and related bacteria
are the only living organisms which can perform the miracle of converting it into a form which
growing plants can use. For more than a decade, scientists have been working to find the most
efficient strain, or culture of rhizobia, so that it can be isolated and used to increase food yields,
in much the same way as chemical fertilizers do - but at a fraction of the cost, with the added
advantage of being environmentally sound.
The vital role which these microorganisms can play was first discovered in the rice fields of the
Philippines in 1962. Scientists at the International Rice Research Institute, near Manila, were mystified
when a trial plot, where no chemical fertilizer was added, kept giving
sustained yields of rice year
after year with no decline in soil fertility. Research showed that millions of the tiny 'free living'
bacteria were working with the rice plants, drawing atmospheric nitrogen down through the water,
via the stems to revitalize the soil. It was observed that the rhizobia bacteria only performed the job
of nitrogen fixation effectively when the paddy field was under water. This was because this
particular strain of rhizobia reached the rice by way of the blue-green algae in the water.
The search for different rhizobia strains which would work effectively with other food crops, and
without the need for water, soon spread to other countries and continents. Attention was concentrated
on leguminous plants, since it was known that some rhizobia lived in symbiotic
relationships with
such vegetables as peas and beans. Many African peasant farmers, who have been cultivating their
maize for generations in fields, also planted peas or beans. They are
unaware of the scientific reason
for this, but experience has taught them how to achieve better yields.
In the legume roots, the bacteria live in nodules -- each one a virtual 'fertilizer factory' no bigger
than a peanut. Many varieties of peas and beans support effective rhizobia, but the most efficient
strain yet identified lives in the roots of the cowpea, a twining plant of the bean family, widely used
for food in Eastern Asia.
Even in countries where chemical fertilizers are available, they might be applied more sparingly if rhizobia were used. This would avoid the pollution of water supplies resulting from overuse of
chemicals. It could also prevent damage to the ozone layer: some scientists fear that the intensive
use of nitrogenous fertilizers may have even more serious effects than
aerosols, or supersonic transport planes. |
