Chemistry Reference
In-Depth Information
crops such as wheat grown in the cold dry winter than to crops such as maize
grown in the wet summer, whereas organic materials may provide a more
available source of nutrients in the wet season when decomposition is more
rapid.
91
For the rice and millet crops grown on residual nutrients, integrated
nutrient treatments yielded more than fertiliser applications alone, and slightly
less than with manure treatments alone. On average, treatments integrating
both organic and mineral fertilisers yielded 35% more for both crops in the
maize/millet rotation and 16% more for both crops in the wheat/rice rotation
compared
d
n
1
r
2
n
g
|
1
with
the
treatments
where
inorganic
fertiliser
alone
had
been
applied.
Despite the demonstrable benefits of integrated nutrient management from a
scientific viewpoint, there are, however, several limitations to its adoption in
practice. Two important limitations are the availability of suitably nutrient-
rich sources of organic materials and the availability of labour to both generate
the organic materials and to spread them. In the mid-hills of Nepal, tree leaves
and crop straw are fed to cattle to generate farmyard manure,
92
but partial
cost-benefit analysis favours fertiliser application because of the large labour
costs involved in the production and transportation of manure.
93
Similarly, in
western Kenya, the transfer of biomass from trees to land producing crops is
constrained
d
n
6
h
3
both
by
the
availability
of
materials
and
the
availability
of
labour.
94
4.1.2
Water
Agriculture accounts for 80-90% of freshwater use globally so there is
considerable focus on using this resource more efficiently.
95,96
Irrigated crops
occupy about 15-20% of the total cropped area but contribute 33-40% of the
production, so they are crucial to the world's food supply. Most irrigation is
applied on the surface (84.5% of the total), with smaller amounts via sprinklers
(13.5%) and localised systems (2%). Generally, surface irrigation systems have
been used to apply large quantities of water, but their on-farm application
efficiency is, on average, low, and over-irrigation is common.
95
Globally,
storage and conveyancing efficiencies are around 70%, implying a 30% loss of
water before delivery to the field.
97
On the farm, irrigation efficiency is
typically only about 37%, so that almost two-thirds of the water delivered is
lost as drainage or runoff, or both.
95,97
Improving the efficiency of this practice
will be critical for future improvement in water-use efficiency (Table 3).
Until recently, most irrigation was scheduled on the basis of fully meeting
crop water requirements, but with sprinkler and localised drip systems it has
been possible to demonstrate that deficit irrigation strategies can not only
sustain yields and profitability, but also reduce water use.
98,99
More
developmental work is required to turn these into widespread, commercial
systems.
In both irrigated and rain-fed production systems, substantial amounts of
water can be lost as evaporation directly from the soil surface, and many
