The orders of the world.

Welcome to part four of my series on soil and soil sciences.  In this article I will briefly cover all the different soil orders and what factors lead to their creation throughout the world.  The variations in geography, climate, biological organisms, topography and parent material all give rise to the various soil orders and their characteristics.

Global Soil Map-Natural Resources Conservation Service



These are immature soils that form on recent deposited sediments.  They will typically show no horizons due to their short existence.  These soils can form in very dry or wet conditions due to the stresses that those climates exert on soil development.  They can be used for limited agriculture such as grazing or hay land for cattle.  The exception to this is river deposited alluvium Entisols which support intensive agriculture due to the richness of the soil.  These soils will make up around 16% of the ice free land surface.     



Vertisols are soils that posses a high clay content that will show significant expansion and contraction when excessively wetted and dried.  These soils will form under grassland and deep rooting tree species and will typically be in areas with erratic moisture.  They are younger soils that have had very little weathering occur on the soil profile.  These soils make up around 2% or the ice free land surface.



A slightly older soil than the Entisols these soils will show some horizonation but the will still be very shallow soils when compared to most others.  This soil type is mostly formed under forest vegetation with a few suborders that may have form under grasslands.  They will typically form on glacial till deposits and will occupy upland positions.  These soils occupy around 17% of the world's ice free land surface.  



A desert soil that will typically be very low in organic matter but high in base content.  These soils are commonly affected by a process call salinization.  Salinization is when dissolved salts are brought to the surface when evaporation and net uptake of water by plants exceeds precipitation.  This salt is deposited near the surface by these actions.  Typical plants in these soils would include cactuses, mesquite and yucca plants.  Use of these soils is limited due to the low amounts of organic matter, low microbial counts, low amounts of nutrients and lack of water.  These soils occupy around 12% of the ice free land surface. 



A soil that is usually found in prairie regions of the world.  It is characterized by thick dark O and A horizons with soft crumbly consistency.  It is rich in base minerals and can be prone to the process of calcification.  Calcification is when water uptake by plants and evaporation exceeds precipitation.  This can lead to the formation of a caliche layer which is a hard compacted layer that plant roots have difficulty penetrating.  The climate that these soils form under temperature ranges from -40 to +100 and from seven inches of rain to 20 inches of rain.  They can be found on undulating plains to mountainous areas.  These soils occupy around 7% of the ice free land in the world. 



This soil will have a light gray or white surface horizon and a reddish or black subsoil.  These soils will be high in iron and aluminum which can be hindering to plant growth in excessive amounts.  The spodosol will form through the process of Podzolization which occurs in cool moist climates where coniferous vegetation dominates.  The organic matter deposited by the conifers creates acidic conditions which accelerates the removal of the iron and aluminum compounds from the A horizon.  These are leached down into the B horizon giving the spodosol its differentiated horizons.  Spodosols are largely confined to humid area extending from high latitudes to the tropics but are extensive in the cool humid mid to high latitudes.  They can be used from forestry, pasture, hay and for some cultivated crops that can tolerate acidity.  They make up about 4% of the ice free land surface.        



These soils form in semiarid to moist areas.   They are a result of weathering that leaches clay and other minerals out of the surface into the subsoil.  They formed under primarily under forest or mixed vegetation and are productive for most crops.    They make up around 10% of the ice free land surface.  



Intensely leached, strong clay translocation, low bas content, forms in humid, warm climates.  Formed by the process of Laterization, this process is common in tropical and subtropical regions and it occurs due to the large amounts of rainfall in the region.  This rainfall leaches the minerals and the nutrients out of the soil unless it is taken up by plants.  Only iron and aluminum are able to resist the leaching process and they are what gives the soil it reddish coloration.  This rainfall also causes high acidity in the soil.  These soils make up around 8% of the ice free land surface.



Highly weathered soil that is prevalent in the tropical regions.  They have a nearly featureless profile from the high amounts of leaching.  They will typically be red, yellow or gray in color and rich in iron and kaolinate.  These soils have low natural fertility as well as low capacity to hold soil amendments such as lime and fertilizer.  They have limited use has intensive agriculture soils due their age and weathering.  They will occur on stable upland summits or on landscape terraces.  They make up around 8% of the world's ice free land surface. 



High organic matter content is the signature feature to this soil.  It forms in areas that have poor drainage from high water table, high clay content in soil and/or high amounts of precipitation.  They will form boggy or mucky soil that is found in swamps or peat bogs across the world.  These soils will occupy around 1% of the world’s ice free land area 



Andisols are a soil derived from a volcanic parent material.  The upper layers of the soil are dark colored and is not highly weathered due to its relatively young age.  The soil also posses a low bulk density.  They are common in cooler area with moderate to high precipitation and due to their structure have a very high water and nutrient holding capacity.  All these factors combined lead to some of the most highly productive soils in the world.  They occupy about 1% of the planets ice free land surface. 



Soil characterized by the presence of permafrost or soil temperatures or 0 degrees or less within 2 meters of the surface.  The soil process that forms this soil is Gleization which occurs in regions with high rainfall or low lying area associated with poor drainage.  This high amount of moisture slows decomposition and allows organic matter to accumulate.  They are common at higher latitudes or elevations and will typically make up 9% of the worlds ice free land surface.


 All of these soil orders form the base on which all life on this planet is built.  Knowing more about them and about the soil in your area is critical to the better management of the soil.  For more information about soil and the soil science check out other articles I have here on Infobarrel.  They can be found by searching for Soils 101, Soils 102 or Soils 103 in the Infobarrel search bar.





Smith , Thomas M., and Robert Leo Smith . Elements of  Ecology. 6. San Francisco: Pearson Education Inc., 2006. Print.