Step 1: Definition of Mulching.
Mulching is an agricultural method involving covering the soil surface with organic or inorganic substances to preserve moisture, control temperature, and inhibit weed proliferation.
Step 2: Evaluation of Practices.
- Green Manuring: Involves embedding green plants into the soil to enhance fertility, rather than surface coverage.
- Conservation Tillage: Focuses on minimizing ploughing to protect soil, distinct from surface covering.
- Mulching: Correctly identifies the practice of covering the soil surface with materials such as straw, leaves, or plastic.
- Contour Farming: Involves cultivating along slopes and does not pertain to surface covering.
Step 3: Determination.
Therefore, the act of covering the soil surface is identified as mulching.
Final Answer: \[\boxed{\text{The practice is called Mulching.}}\]