With reference to agricultural soils, consider the following statements:
- A high content of organic matter in soil drastically reduces its water-holding capacity.
- Soil does not play any role in the sulphur cycle.
- Irrigation over a period of time can contribute to the salinization of some agricultural lands.
Which of the statements given above is/are correct?
- 1 and 2 only
- 3 only
- 1 and 3 only
- 1, 2 and 3
Explanation
Statement 1 is incorrect
- A high content of organic matter (humus) in the soil increases its water-holding capacity. Organic matter causes soil particles to bind and form stable soil aggregates, which improves soil structure.
- With better soil structure, water infiltration through the soil increases and improves the soil’s ability to absorb and hold water as well as reduces the potential for surface crusting of the soil.
Image Source: NRDC
Statement 2 is incorrect
- The sulphur cycle is mostly sedimentary except for its two compounds, hydrogen sulphide (H2S) and sulphur dioxide (SO2). Sulphur is locked in organic (coal, oil, peat) and inorganic (pyrite rock, sulphur rock) soil deposits in the form of sulphates, sulphides, and organic sulphur. It is released by weathering of rocks, erosional runoff and decomposition of organic matter and is carried to terrestrial and aquatic ecosystems in the salt solution
- Steps in the Sulphur Cycle:
- Sulphur enters the atmosphere as sulphur dioxide from sources like volcanic eruptions, combustion of fossil fuels (coal, diesel, etc.), from the surface of the ocean (Dimethyl Sulfide (DMS)) and gases released by decomposition.
- Atmospheric hydrogen sulphide also gets oxidised into sulphur dioxide
- Atmospheric sulphur dioxide is carried back to the earth as weak sulphuric acid (acid rain).
- Whatever the source, sulphur in the form of sulphates is taken up by plants and incorporated through a series of metabolic processes into sulphur-bearing amino acids, which are incorporated in the proteins of autotroph tissues.
- It then passes through the grazing food chain.
- Sulphur bound in living organisms is carried back to the soil, to the bottom of ponds and lakes and seas through excretion and decomposition of dead organic material.
Statement 3 is incorrect
- Excessive or improper irrigation can cause salinization of agricultural land. When irrigation water contains high levels of dissolved salts and drainage is inadequate, these salts gradually accumulate in the soil, resulting in soil salinization.
