i. Soil horizon: a layer of soil parallel to its surface with chemical, physical, and biological properties different from the layers above and beneath.
Soil profile: a structure of different soil horizons. A good picture can be found in the corresponding article (Soil horizon) on Wikipedia:
ii. Disintegration: similar by chemical, physical and biological characteristics pieces of soil come apart. For instance, a rock destroyed by wind and water is disintegrated.
Decomposition: the transformation of chemical, physical and biological characteristics of portions of soil by bacteria and microorganisms. For example, a dead bug decayed by bacteria, other bugs and microorganisms will eventually become a part of soil. The bug took his part in decomposition.