Concept:Mechanical tillage uses heavy machines, and its main drawback is the damage they cause to soil structure.
Explanation:Mechanical tillage requires expensive equipment, so the initial cost is high, not low.
This makes Option A incorrect.
Option B mentions a need for special training, which is a limitation but not the main agricultural disadvantage.
The key issue is that heavy tractors and implements press down on the soil, leading to soil compaction.
Compaction reduces soil aeration, lowers water infiltration, and restricts root growth.
Option D about foreign exchange for spare parts is not directly related to the tillage practice itself.
Therefore, the most significant disadvantage is the use of heavy equipment that causes soil compaction.
Answer:C. use of heavy equipment leading to soil compaction