In which of the soil layer does most biological activity occur?

Most biological activity in soil occurs in the A-horizon. The A-horizon, also known as the topsoil, is the uppermost layer of soil that is composed of a mixture of mineral particles, organic matter, and living organisms. This layer is particularly rich in organic matter, which provides a food source for microorganisms such as bacteria and fungi that play a vital role in breaking down organic materials and releasing nutrients that plants can use for growth. Additionally, the A-horizon is where plant roots grow and interact with soil organisms, which can influence the health and productivity of both plants and soil. Therefore, the A-horizon is considered the most biologically active layer of soil.