Soil scientists sort land degradation into three broad buckets, physical, chemical, and biological, depending on what part of the soil is being damaged. This question asks which of the four listed processes belongs in the physical bucket.
- Fertility imbalance: This is about the ratio and supply of plant nutrients going wrong, a chemical issue, not a physical one.
- Organic matter decline: This is the loss of carbon and living matter from soil, treated as biological degradation.
- Erosion and depletion: This means topsoil is physically carried away by wind or water, and soil depth and structure are physically lost. This is the textbook example of physical soil degradation.
- Acidification: A fall in soil pH is a chemical change in the soil, not physical damage to its structure.
Only erosion and depletion involves the actual physical removal and rearrangement of soil material, so it is the correct match for "physical process of soil degradation."
Let's summarize:
- Physical degradation: loss or rearrangement of soil mass, such as erosion, compaction, crusting.
- Chemical degradation: fertility imbalance, acidification, salinization.
- Biological degradation: organic matter and soil life decline.
So the correct answer is option 3, erosion and depletion.