What are litter, duff and humus?

Explanation

### Step-by-Step Solution:

1. **Definition of Litter**:
- Litter refers to the layer of undecomposed organic matter found on the surface of the soil. This includes fallen leaves, twigs, and other plant materials that have not yet broken down.

2. **Definition of Duff**:
- Duff is the term used for organic matter that is in the process of decomposition. It is the layer beneath the litter, consisting of partially decomposed organic materials. This means that while it is not fully broken down, it has begun to decay.

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