The "A horizon" is the top layer of soil, often referred to as the topsoil. It is rich in organic matter, nutrients, and microorganisms, making it crucial for plant growth. This layer typically appears darker than the layers below due to the accumulation of decomposed organic material. The A horizon is essential for agriculture and gardening, as it supports root development and water retention. It is usually found just beneath the O horizon, which consists of organic litter, and above the E horizon, characterized by leaching of minerals. Understanding the A horizon helps in soil management and conservation practices.