Tutorial:Soil Processes

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[edit] Soil forming processes

Soil forming processes are important archaeologically as they determine the nature and fertility of soils associated with sites. In periods of stability, soils develop in previously deposited sediments, and if deposits are not sufficiently sealed from the surface by deep, rapid burial they can be an important part of the post-burial taphonomic process.

The nature of soil development is affected by five soil forming factors (Jenny, 1941):

• Climate – Temperature, rainfall, humidity and evapotranspiration. These influence the net movement of particulates and dissolved solids in the soil water. Temperature and rainfall also influence the rate of weathering processes.
• Relief – In many ways relief is linked with climate as rainfall generally increases with altitude. Slope influences the drainage of water and susceptibility to erosion; differences in drainage and soil depth along a slope leads to the formation of a distinctive series of soil types called a catena.
• Organisms - Soil development is reliant on biological activity. Plants and animals provide the organic component of soil; this is mixed with the mineral component by the soil fauna. Micro-organisms play a crucial role in the decay of organic matter and in the formation of soil structure. Humans also play a key role in many soil forming processes, as a vector of erosion and deposition, through cultivation and land management and in creating whole soil profiles (anthrosols).
• Parent material – This is the material in which the soil develops and may be the consolidated geology of a natural or an anthropogenic deposit. The parent material affects the mineral status, drainage and susceptibility to erosion.
• Time – The passage of time allows soil features and horizons to develop. When the soil is largely in equilibrium with its environment it is considered to be mature. The time this takes to happen varies according to the environment and the parent material. Generally soils develop more quickly on free-draining sandy soils, than on slowly draining clay soils.

The balance of these factors influences the soil forming processes that can operate. These include:

• Weathering
• Biological processes
• Leaching
• Gleying/hydromorphism
• Podzolisation
• Clay eluviation
• Calcification and salinisation