Contour ploughing is the cultivating method that can best conserve soil on hills.

Contour ploughing involves ploughing along the contours of the slope, rather than in straight lines up and down the hill. This method helps to slow down the flow of water down the slope and prevent soil erosion. Here's a detailed explanation of why contour ploughing is effective in conserving soil on hills:

1. Definition and purpose of contour ploughing:
- Contour ploughing is a method of ploughing parallel to the contour lines of the slope.
- Its purpose is to reduce soil erosion by slowing down the flow of water and preventing it from carrying away the topsoil.

2. Slowing down water flow:
- Ploughing along the contour lines creates ridges and furrows that act as barriers to water movement.
- These barriers slow down the flow of water, giving it more time to infiltrate into the soil and reducing the chances of erosion.

3. Preventing soil erosion:
- As water flows downhill, it tends to gain speed and force, which can cause soil to be washed away.
- Contour ploughing prevents this by breaking up the slope into smaller sections, reducing the length of the slope that water can flow down.
- The ridges and furrows created by contour ploughing also help to trap water and prevent it from running off the slope.

4. Retaining moisture:
- Contour ploughing helps to retain moisture in the soil.
- The ridges created by ploughing act as barriers that prevent water from flowing downhill and allow it to collect in the furrows.
- This helps to increase water infiltration and reduce runoff, ensuring that the soil retains more moisture for plant growth.

5. Reducing soil compaction:
- Ploughing along the contour lines also helps to reduce soil compaction.
- In traditional ploughing methods, ploughing up and down the slope can lead to the formation of deep furrows, which can cause soil compaction and increase erosion.
- Contour ploughing avoids this issue by maintaining a more even distribution of soil and reducing the formation of deep furrows.

In conclusion, contour ploughing is an effective method for conserving soil on hills as it slows down water flow, prevents soil erosion, retains moisture, and reduces soil compaction. By implementing this method, farmers can ensure the long-term sustainability of their land and protect the valuable topsoil from erosion.