Agriculture Reference
In-Depth Information
Unlike that of phosphorus but like nitrogen‚ the biogeochemical cycle of sulphur
has a significant gaseous component. The importance of this component has recently
been emphasised through the effects on the atmosphere of fossil fuel consumption and
the emissions from certain industrial processes. These have lead to increased atmospheric
concentrations of sulphur (and nitrogen) and increased deposition of acid forms of these
elements in both particulate form and in solution‚ particularly in areas associated with
industrialisation. In such areas‚ a major consequence has been the reduction of rainfall
pH to low levels‚ the 'acid rain' phenomenon‚ although this is also due to inputs of
nitrogen-containing acids.
When sulphur-rich wetland environments are drained during the processes of land
reclamation‚ the sulphur present in reduced form may be oxidised to sulphuric acid
leading to a drastic reduction in pH and the solubilisation and mobilisation of metals
(Dent and Pons‚ 1995). The effects of sulphur oxidation may be severe. The acid-sulphate
soils that result from such acidification are particularly difficult to manage and environ-
mental damage‚ including fish kills‚ may be incurred in down stream aquatic ecosystems.
A similar situation frequently follows exposure at the surface of sulphide-containing
overburden and processing waste materials associated with the mining of many minerals
of commercial importance (see‚ for example‚ Ripley
et al
.‚ 1996).
While much of the sulphur needed by agricultural crops is provided through the
mineralisation of organic matter‚ low sulphur concentrations commonly limit the growth
of crops and pastures. Sulphur adequate for the needs of many crops is supplied through
the application of such phosphatic fertilisers as superphosphate. In situations of intensive
production‚ however‚ regular sulphur additions may be required to maintain satisfactory
levels of productivity. The application of inorganic sulphur has an acidifying effect on
soils; elemental sulphur may sometimes be applied to alkaline soils to reduce their pH.
3.1.2.5
Calcium and magnesium
Calcium and magnesium are bivalent elements belonging to the second group of
the periodic table‚ the alkaline earth elements; a group whose hydroxides form strong
bases. They are amongst the few most concentrated elements in both the earth's crust
(Table I.1) and in soils (Table I.2).
Calcium is one of the soil macro-elements and median concentrations are
ca.
1.5 %
in non-calcareous soils. In contrast‚ airborne calcium inputs may lead to the formation
of discrete horizons of calcium and magnesium carbonates or gypsum in
the calcareous soils of semi-arid and arid areas (Birkeland‚ 1984). Surface efflorescences
or crusts dominated by calcium may form in depressions‚ in areas where subsurface
lateral flows emerge to the surface or through the capillary rise of saline waters.
Shallow calcareous soils also form from such calcium-rich parent materials as limestone
and chalk.
Both elements have essential roles in the nutrition of living organisms and‚ through
a strong association with soil pH status‚ calcium concentrations (in particular) strongly
affect the availability of other elements to plants. Both elements may strongly influence
soil structural stability.
