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Soils of the World Cheat Sheet by

A brief tour through the most common types of soils, how they're formed, and where they can be found.

About soils

Soils can be described as 'the intera­ction between the lithos­phere, hydros­phere, and atmosp­here'. They can be classified in a number of ways- i.e. based on colour, moisture, texture, structure etc.
Soil texture- The relative mixing of primary particles. Most commonly, we assess the levels of sand, silt & clay.
Soil struct­ure- The organi­sation of primary particles into secondary units (called peds). We can describe peds as either prismatic, platy, blocky, or crumb.

Soil fauna

Micro­biota- Micros­copic organisms, e.g. bacteria and nematodes.
Mesof­auna- Soil animals of an interm­ediate size, e.g. spring­tails.
Macro­fauna- Soil animals larger than 2mm, e.g. earthw­orms. Lead to larger pore spaces in the soil.

Factors in soil formation

Temp­era­ture- Rates of chemical reaction, and microbial activity increase with increasing temper­ature. This increases the rate of weathering and decomp­osi­tion.
Clim­ate- Temper­ature and rainfall influence weathering and throug­hflow through soils, leading to the formation of eluvial and illuvial horizons. In polar regions, regolith is weathered by freeze­-thaw.
Orga­nis­ms- Micro, macro, and mesofauna mix organic and mineral material thoughout soils through the process of biotur­bation. Humans also modify soils to meet our demands (usually agricu­ltu­ral).
Topo­gra­phy- Altitude and slope aspect can influence soil temper­atures, which influences rate and extent of weathe­ring.
Parent materi­als- Different parent materials (bedrock types) are more easily weathered than others.
Time- The most important factor in soil format­ion. Eventu­ally, bedrock will be weathered and incorp­orated with organic matter if given enough time. Soils in the tropics are much older than those in temperate regions.

Global soils

Key soil types to note are:
Oxis­ols- highly weathered, deep soils. Red in colour, typical of tropical forests.
Ulti­sols- highly weathered, similar to oxisols. Reddis­h/y­ellow in colour, typical of warm, humid climates.
Arid­iso­ls- very little organic content. Typical of deserts.
Ince­pti­sols- little in the way of horizons, e.g. brown earth.
Geli­sols- containing permaf­rost.
 

A typical soil horizon profile

Common types of soil

Brown Earth- Forms in temperate deciduous woodland. Well-i­nco­rpo­rated organic and mineral material due to biotur­bation by soil fauna; this results in an isotropic soil which is good for agricu­lture. Neutral/ mildly acidic pH.
Podzols (spodo­sol­s)- Form in coniferous woodland due to the presence of acidic needles. This needle litter decomposes and releases organic acids which form complexes with aluminium and iron in the E horizon. These leach down into the B horizon, giving an orange­-brown colour. (See image below).
Gley Soils- Form in anoxic conditions in areas of saturated ground. This saturation allows chemical species to be reduced. If drained, gleys can be effective agricu­ltural soils. Grey in colour.
Peat- Formed by the slow decomp­osition of organic matter under wet or cold condit­ions. Blanket peat is fed by precip­ita­tion, and raised bogs (which form in topogr­aphic hollows) is fed by ground­water running in from the sides.

A podzol soil profile. Note the distinct horizons:

                           
 

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