Agriculture Reference
In-Depth Information
3.1.2.4
Sulphur
Sulphur is an essential major element for all living organisms and plays important roles
in a number of metabolic pathways. It is a component of the essential amino acids
cystine‚ cysteine and methionine and thus forms a structural part of tissue proteins and
enzymes important in photosynthesis and nitrogen fixation. In higher animals‚ it also
occurs in certain vitamins and in the hormone insulin (McDonald
et al
.‚ 1988).
In soils‚ sulphur occurs in inorganic form as sulphates‚ sulphides‚ elemental sulphur‚
sulphur-containing gases and a variety of other forms‚ depending on the climate‚ soil pH‚
the degree of soil aeration and biological activity. Concentrations depend on the parent
materials and are generally higher in soils formed from basaltic than other parent
materials. Total soil sulphur concentrations in surface soils are usually in the range
(Table I.2). Figure I.32d presents the frequency distribution of the con-
centrations of total S in the surface horizons of 572 Australian soils while Table I.15
presents typical concentrations in plant materials from a range of natural and semi-
natural environments.
In most soils‚ sulphur occurs largely in organic form and this may be subdivided
between that associated with the soil organic matter and that with decaying plant residues.
As with the other essential nutrient elements‚ a further small labile pool occurs in the
microbial biomass which may be either a source or a sink of sulphur. In fourteen
temperate-climate acid soils this pool ranged between 0.7 and 1.9
per cent.
of microbial
biomass carbon with an average of 1.2
per cent.
(Wu
et al
.‚ 1994); Banerjee
et al.
(1993)
found that this pool comprised slightly higher values‚
ca.
3
per cent.
of microbial
biomass carbon. Organic sulphur is mineralised to sulphate which is the form normally
taken up by plants. The sulphate ion is relatively mobile in soils and may be subject to
leaching losses. Sulphur dioxide a common atmospheric pollutant‚ is absorbed
through the leaf surfaces and while it may contribute to plant nutrition at low concen-
trations‚ it quickly becomes toxic with increasing concentration.
In arid and semi-arid areas‚ sulphate sulphur may accumulate in soils as gypsum
epsomite or as thenardite and other minerals.
Toward the drier end of the climatic spectrum‚ gypsum may form cemented horizons
at varying depths from the surface. More complex sulphur salts such as the jarosites
(
e.g
.‚
may occur in certain environments depending on the pH and
other factors.
In contrast‚ in wetland soils and those that are submerged frequently enough to
become effectively anaerobic‚ inorganic sulphur may occur largely in reduced form as
sulphides such as pyrite and in other reduced forms. Hydrogen sulphide is
one of the characteristic end products of the mineralisation of organic sulphur under
such conditions and is largely responsible for the distinctive odour associated with
such wetland environments as mangroves communities. This gas is also produced in
aerobic environments although it is quickly oxidised to elemental sulphur. Other
sulphur containing gases such as
and COS are also produced in soils through
microbial activity.
