Agriculture Reference
In-Depth Information
L
EGUMES
-
E
CONOMICALLY AND
E
NVIRONMENTALLY
I
MPORTANT
C
ROP
S
PECIES
With over 18,000 species, legumes represent the third largest family
(
Leguminosae
or
Fabaceae
) of angiosperms. They are also one of the largest
groups of food and feed crops grown globally, second only to the cereals.
Indeed, legumes have a global production value of over $200 billion per
annum. They are cultivated on 12-15% of available arable land and are
responsible for more than 25% of the world's primary crop production with
247 million tons of grain legumes produced annually (European Association
for Grain Legume Research 2007). They are often used as crop rotation
species as they can dramatically improve the organic content and volume of
soil. Key food and feed legumes crops include soybean, bean, pea, clover,
chickpea, alfalfa and mungbean. Species such as
Pongamia pinnata
are also
being pursued as sustainable biofuel sources due to their high seed oil content
(Scott
et al
. 2008).
Legumes also represent some of the most important members of natural
ecosystems as they play a critical role in the nitrogen cycle. This is achieved
via a symbiotic relationship formed with specific soil bacteria of the
Rhizobiaceae
family. This interaction results in the formation of specialized
root structures called nodules, which the bacteria colonize (Ferguson
et al
.
2010). Within the nodule, the bacteria convert or ‗fix' atmospheric di-nitrogen
(N
2
), a form of nitrogen that plants cannot use, into compounds such as
ammonia that the plan can use.
Modern agriculture has been highly reliant on industrial nitrogen
fertilisers to achieve maximum crop productivity. However, a great deal of
fossil fuel is required for the production and delivery of nitrogen fertiliser.
Indeed, industrial nitrogen fixation alone accounts for about 50% of fossil fuel
usage in agriculture. This can be exceedingly expensive. In recent years the
price of chemical nitrogen fertilisers has increased dramatically due to rising
fossil fuel costs. Moreover, carbon dioxide (CO
2
) which is released during
fossil fuel combustion contributes to the greenhouse effect, as does the
decomposition of nitrogen fertiliser, which releases nitrous oxides (NO
x
), itself
about 292 times more active as a greenhouse gas than carbon dioxide (Crutzen
et al
. 2007). In addition, applying chemical fertilisers is a largely inefficient
process as 30 - 50% is lost to leaching. This can result in significant
environmental problems, such as the eutrophication of waterways (Vance
2001).
