Biological method of soil conservation refers to the use of living organisms or their by-products to prevent soil erosion and maintain soil fertility. Among the options given, contour farming is the biological method of soil conservation.

Contour farming is a technique that involves plowing and planting crops perpendicular to the slope of the land. By following the contour lines of the land, this method creates ridges and furrows that help to slow down the flow of water and prevent soil erosion. Here is a detailed explanation of contour farming as a biological method of soil conservation:

1. Explanation of contour farming:
Contour farming involves planting crops across the slope of the land rather than up and down the slope. This technique helps to slow down the movement of water downhill, reducing its erosive force and preventing soil loss. It works by creating ridges and furrows along the contour lines of the land, which act as barriers to trap water and prevent it from flowing downhill in a concentrated manner.

2. Benefits of contour farming:
- Reduces soil erosion: Contour farming helps to reduce soil erosion by preventing the rapid flow of water downhill. The ridges and furrows act as mini-dams, slowing down the water and allowing it to infiltrate into the soil, reducing the chances of runoff and erosion.
- Conserves soil moisture: By trapping water along the contour lines, contour farming helps to conserve soil moisture. The ridges prevent water from quickly draining off the land, allowing it to be absorbed by the soil and retained for plant use.
- Improves soil fertility: The reduced erosion and conservation of soil moisture through contour farming contribute to improved soil fertility. The soil is less likely to be depleted of nutrients and organic matter, leading to healthier and more productive crops.

3. Implementation of contour farming:
Contour farming can be implemented by identifying the contour lines on the land and plowing and planting crops along those lines. This can be done manually or with the help of modern agricultural machinery. It is important to maintain the ridges and furrows regularly to ensure their effectiveness in preventing soil erosion.

Overall, contour farming is a biological method of soil conservation that utilizes the natural contours of the land to prevent soil erosion and maintain soil fertility. By implementing this technique, farmers can protect their soil resources and sustainably manage their agricultural lands.