Chemistry Reference
In-Depth Information
Irrigation is required to turn marginal dry lands into productive agricultural
land. Soil moisture not only provides water for plant growth, but is a vital soil
stock whose quality (pH, salinity) and quantity (volumetric water content)
control many soil processes.
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Life depends on water, and soil is no different,
with the soil moisture content controlling the soil microbial activity and, in
turn, the soil biogeochemical cycling. However, if irrigation is managed poorly
it results in waterlogging and salinisation. In the San Joaquin Valley of
California, modelling studies have shown how irrigation practice over the last
40 years has led to increased soil salinisation.
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It is estimated that
approximately 0.45 Mkm
2
, representing 20% of the world's total irrigated
land, suffers from salinisation or waterlogging.
Addition of chemicals to soils (in the form of both fertilisers and pesticides
to enhance productivity) brings about soil change. According to Foley et al.
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there has been a ca. 700% increase in global fertiliser use during the past 40
years. In the case of reactive N, production via the Haber-Bosch process has
gone from 0 prior to 1910, to more than 100 Tg N in 2000, with 85% going into
fertiliser production.
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Fertiliser application has helped increase food
production, but addition tends to acidify soils and creation rates are faster
than denitrification, so reactive N is building up in the environment. It has
been proposed that reactive N levels have already exceeded planetary
boundaries,
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where the boundary represents a safe operating space for
humanity. Fertiliser application has a range of effects on soils; obviously it
increases nutrient content, and food production wouldn't be sustainable
without them. The long-term Broadbalk (Rothamsted) experiment that has
grown continuous wheat since 1843, shows a strong linear correlation between
the amount of fertiliser N applied annually (ranging from 0-200 kg N ha
21
)
and the quantity of organic C accumulated in the soil.
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Long-term fertiliser
applications have also been reported, in a number of cases, to cause increases
in water-stable aggregation, porosity, infiltration capacity and hydraulic
conductivity, and decreases in bulk density as compared with unfertilised crop
production. However, detrimental effects are observed when large doses of
ammonium (NH
4
+
) salts are added to soils, causing colloid dispersion in fine,
poorly aggregated soils, and - if persistent - can cause crusting and reduced
infiltration.
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Pesticides include a wide range of organic and inorganic materials, applied
to kill different organisms such as weed plants and disease-causing fungi and
nematodes. These materials have a direct effect on the target organisms, but
may also have indirect effects on the structure and function of wider biotic
communities in soils. Chemicals can cause alteration of metabolism of endemic
soil microorganisms and arthropods, or result in the eradication of some
components of the primary food chain. Although early research was upbeat
that pesticides had no major detrimental impact on soil biota, more recent
work indicates that pesticides, and particularly repeated application, can result
in a range of effects on the soil biota.
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