The most mature layer of soil is the A-horizon. It is the topmost layer of soil, typically dark in color due to the presence of organic matter.
The A-horizon is also the most fertile layer of soil, as it contains the nutrients that plants need to grow.
The A-horizon, also known as the topsoil, is the most mature layer of soil because it has undergone the longest period of time for the decomposition of organic matter and the formation of soil particles. This process, called soil genesis or pedogenesis, takes millions of years to complete. Over time, organic matter from plant and animal debris decomposes and becomes humus, which is a dark, sticky substance that provides nutrients and improves soil structure. The A-horizon also contains a variety of soil organisms, such as earthworms and bacteria,