Previous Year Question
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 a role in the sulphur cycle.
Irrigation over a period of time can contribute to the salinisation 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
Correct Answer: B
Detailed Solution
Statement 1 is incorrect: A high content of organic matter does not reduce the water-holding capacity of soil; it generally increases it.
Humus formed by the decomposition of organic matter can retain a considerable quantity of water relative to its weight.
Organic matter binds soil particles into stable soil aggregates. This improves soil structure, porosity and the infiltration of water.
In sandy soil, organic matter increases the capacity to retain water and nutrients, whereas in clayey soil, it can improve aeration and structure.
Therefore, the statement that a high content of organic matter ‘drastically reduces’ water-holding capacity is incorrect.
Statement 2 is incorrect: Soil is an important component of the sulphur cycle.
Sulphur enters the soil through the weathering of rocks, atmospheric deposition, fertilizers and the remains of plants and organisms.
Soil microorganisms mineralise organic sulphur and convert it into sulphate, which can be absorbed by plants through their roots.
After the death of plants and organisms, sulphur returns to the soil through organic remains.
Sulphate may be leached, and under oxygen-deficient conditions, its reduction may also produce hydrogen sulphide.
Therefore, soil plays an active role in the storage and transformation of sulphur, its uptake by plants and its leaching.
Statement 3 is correct: Irrigation water normally contains a certain quantity of dissolved salts.
After irrigation, water evaporates or is used by plants, but the salts contained in it may remain in the soil.
Prolonged irrigation, inadequate drainage and high evaporation can cause these salts to accumulate in the root zone and upper soil layers.
Where the groundwater table is shallow, saline water may move upwards through capillary action. After the water evaporates, salts accumulate near the soil surface.
This problem is particularly serious in arid and semi-arid regions, where rainfall is insufficient to flush salts out of the soil.
This process caused by irrigation is called secondary salinisation. It makes water absorption difficult for plants and can reduce crop productivity.
Appropriate drainage, controlled irrigation, good-quality water and the leaching of salts when necessary can reduce this problem.