2 - Pedogenic processes and pathways of horizon differentiation

Summary

Soils acquire and maintain their characteristics and composition while undergoing simultaneous alteration by an almost infinite number of biogeochemical reactions. The possible number of pedogenic events and combinations and interactions among them in soils is staggering. Although laboratory experiments can demonstrate that specific processes can produce specific soil features, the actual course of events within undisturbed soil will probably never be fully known because the cumulative impact of soil-forming processes spans such long periods of time relative to the lives of humans who observe those impacts.

Horizonation processes

The entire volume of material defined as soil is but one layer within a larger context of the lithosphere. Soil is a layer of the lithosphere where minerals formed at high temperatures in the absence of water during the cooling of the Earth's magma are being decomposed by water, and new minerals (secondary minerals) are being formed at lower temperatures. In soil, organic compounds formed in plants primarily from carbon taken from the air are mixed into the mineral material of the lithosphere. Soil can be conceptualized as an open system where material can be added, transformed, translocated and removed. Generalized processes responsible for the presence of identifiable horizons and other features within soil are outlined in Fig. 2.1.

These processes include:

1. Energy exchange as the soil surface is daily heated by the sun and cooled by radiation to space each night.

2. Water exchange as soil is periodically wetted by precipitation and dried as water evaporates and/or taken from the soil by plant roots and transpired through the plant leaves.

3. […]