How should the subgrade be prepared to ensure bonding of the topsoil with the subsoil?

The preparation of the subgrade plays a crucial role in establishing a strong bond between the topsoil and subsoil, which is essential for promoting healthy plant growth and maintaining soil stability. Discing or scarifying the surface creates a rough texture that enhances adhesion between the two soil layers. This disturbance allows for better integration of the topsoil with the existing subsoil, improving water infiltration and nutrient exchange.

By using discing or scarifying techniques, small fractures and roughness are introduced into the subgrade, which facilitates mechanical interlocking. The process also helps to break up any compaction or crusted surfaces that might impede the movement of roots and water into the subsoil. Ultimately, this preparation helps create an optimal environment for root establishment and growth.

Other methods, such as flattening or rolling, typically focus on compacting or smoothing the surface rather than establishing a conducive bond between soil layers. Applying herbicides is unrelated to bonding and addresses weed control rather than soil preparation. Thus, discing or scarifying is the most effective method to ensure adequate bonding of the topsoil with the subsoil.