Biological Activity

The vegetative cover and organism that occupy the parent materials from the start to later stages help in adding organic matter, moisture retention, nitrogen (nitrogen fixation by bacterias such as Rhizobium) etc. Dead plants provide humus. Some organic acids which form during humification aid in decomposing the minerals of the soil parent materials. Humus accumulates in cold climate as bacterial growth is low and thus layers of peat develop in sub- arctic and tundra climates.

The organisms affecting soil development range from microscopic bacteria to large mammals, including man. Besides providing much of the humus, vegetation influences the soil in several other ways. By intercepting direct rainfall and binding the soil with roots, plants check soil erosion. They counteract percolation by transpiration, reducing the effectiveness of the rainfall. They also help in maintaining the fertility of soil by brining bases (calcium, Magnesium) from the lower parts of the soil into stems and leaves, and then releasing them into the upper soil horizons. A change in vegetation may cause a change in soil.

The influence of animals on soils is both mechanical and chemical. For example, earthworms rework the soil by burrowing and also change its texture and chemical composition by passing it through their digestive systems. Equally, soil characteristics closely determine the type of animal present in the soil.