7. With
reference to agricultural soils, consider the following statements:
1. A high content of organic matter in soil drastically reduces its water holding capacity.
2. Soil does not play any role in the sulphur cycle.
3. Irrigation over a period of time can contribute to the salinization of some agricultural lands.
Which of the statements given above is/are correct?
(a) 1 and 2 only
(b) 3 only
(c) 1 and 3 only
(d) 1, 2 and 3
Explanation:-
Statement 1 is wrong because organic
matter increases water holding capacity.
Higher content of organic matter in soil will result into more humus and
increase in its water holding capacity.
“Each 1 percent increase in soil organic matter helps soil hold 20,000 gallons more water per acre.”
“Each 1 percent increase in soil organic matter helps soil hold 20,000 gallons more water per acre.”
Statement 2 is right because soil plays
a role in Sulphur cycle. Sulphur, like
nitrogen and carbon, is an
essential part of
all living matter
because sulphur containing
amino acids are
always present in almost all kinds of proteins. Plants can
absorb directly the sulphur containing amino acids, but these amino acids
fulfil only a small proportion or requirements for sulphur. To fulfil rest of
the requirements of plants, sulphur passes through a cycle of transformation
mediated by microorganisms. It accumulates in
the soil mainly
as a constituent
of organic compounds
and has to
be conve rted to
sulphates to become readily
available to the plants.
• Sulphur Cycle is the circulation of sulfur in various forms through nature. Sulphur
is key to protein structure and is released to the atmosphere by the burning of
fossil fuels. Sulphur occurs in all living matter as a component of certain
amino acids. It is abundant in the soil in proteins and, through a series of
microbial transformations, ends up as sulphates usable by plants.
Sulphur-containing proteins are degraded into their constituent amino acids by the action of a variety of soil organisms. The sulphur of the amino acids is converted to hydrogen sulphide (H2S) by another series of soil microbes. In the presence of oxygen, H2S is converted to sulfur and then to sulphate by sulfur bacteria. Eventually the sulfate becomes H2S.
Hydrogen sulphide rapidly oxidizes to gases that dissolve in water to form sulphurous and sulphuric acids. These compounds contribute in large part to the “acid rain” that can kill sensitive aquatic organisms and damage marble monuments and stone buildings.
Sulphur-containing proteins are degraded into their constituent amino acids by the action of a variety of soil organisms. The sulphur of the amino acids is converted to hydrogen sulphide (H2S) by another series of soil microbes. In the presence of oxygen, H2S is converted to sulfur and then to sulphate by sulfur bacteria. Eventually the sulfate becomes H2S.
Hydrogen sulphide rapidly oxidizes to gases that dissolve in water to form sulphurous and sulphuric acids. These compounds contribute in large part to the “acid rain” that can kill sensitive aquatic organisms and damage marble monuments and stone buildings.
Thus cycle can be divided as:
· Sulphur Cycle in Soils
Sulphur enters the trophic cycle in terrestrial plants via root adsorption in the form of inorganic sulphates (e.g., calcium sulphate, sodium sulphate) or by direct assimilation of amino acids released in the decomposition of dead or excreted organic matter. Bacterial and fungal (Aspergillus and Neurospora) mineralization of the organic sulphhydryl in amino acids followed by oxidation results in sulphate; this adds to the sulphate pool for root adsorption.
· Sulphur Cycle in Atmosphere
Sulphur in the atmosphere comes from several different sources: decomposition and/or combustion of organic matter, combustion of fossil fuels, and ocean surfaces and volcanic eruptions. The most prevalent form of sulphur entering the atmosphere is sulphur dioxide (SO2). It, along with other atmospheric forms such as elemental sulphur and hydrogen sulphide, is oxidized to sulphur trioxide (SO3), which combines with water to form sulphuric acid (H2SO4), leading to acid rain.
Atmospheric sulphur, largely in the form of sulphuric acid, is removed by two general processes: rainout, which includes all processes within clouds that result in removal; and washout, which is the removal by precipitation below the clouds. Depending on the amount of the various sulphur compounds available to form the sulphuric acid, the degree of acidity can be strong enough to ap-proximate that of battery acid. Atmospheric inputs of sulphuric acid provide the dominant source of both hydrogen ions (H+) for cation replacement.
· Sulphur in Sediments
The sedimentary aspect of the cycle involves the precipitation of sulphur in the presence of such cations as iron (Fe) and calcium (Ca) as highly insoluble ferrous sulphide (FeS) and ferric sulphide (Fe2S3, pyrite) or relatively insoluble calcium sulphate (CaSO4). The oxidation of sulphides in marine sediments is a key process, though poorly understood.
Irrigation
cause organic matter to leach and makes land saline. Over-irrigation can lead to salinity in soils,
because of over-use of ground-water and/or rise of water-table. Salinity is also caused due to excessive irrigation in dry conditions which
promotes capillary action.