The A layer of the soil, also known as the O layer or the organic layer, is composed of decaying litter. It is the uppermost layer of the soil profile and is characterized by the presence of organic matter in various stages of decomposition. This layer plays a crucial role in the formation and fertility of the soil.

Decaying Litter:
- The primary component of the A layer is decaying litter, which includes fallen leaves, twigs, branches, and other organic materials from plants and animals.
- As these organic materials accumulate on the soil surface, they undergo decomposition by the action of microorganisms such as bacteria and fungi.
- The process of decomposition releases nutrients into the soil, making them available for plant uptake and contributing to the fertility of the soil.

Organic Matter:
- Organic matter in the A layer is rich in carbon compounds, which are derived from living organisms.
- This organic matter serves as a source of energy and nutrients for soil organisms, helping to support soil biodiversity.
- It also improves the soil structure, enhancing its ability to hold water and nutrients.
- The presence of organic matter in the A layer also contributes to the dark color of the soil, as it absorbs and retains heat from sunlight.

Nutrient Cycling:
- The decaying litter in the A layer undergoes a process called mineralization, where organic compounds are broken down into simpler forms, releasing nutrients such as nitrogen, phosphorus, and potassium.
- These nutrients are then available for uptake by plants, promoting their growth and productivity.
- The A layer acts as a reservoir of nutrients, storing and slowly releasing them to the underlying soil layers.

Conclusion:
In conclusion, the A layer of the soil is composed of decaying litter, which provides a source of organic matter and nutrients for the soil. It plays a vital role in nutrient cycling, soil fertility, and supporting soil organisms.