These aren't the horizons your looking for....

Please pardon the Star Wars pun there I just couldn't help it I guess, please feel free to blame the child in me if you like.  If perhaps you did not get the reference then feel free to check out one of the many clips in the internet that make fun of that Star Wars scene.


Welcome to soil horizons the breakdown (a little more soil humor there).  Today I will be leading you on a journey down through the soil profile as I outline and define the common soil horizons and the factors that cause their formation.  After a soil has completed the soil forming process and has built up sufficient depth that the parent material is no longer being weathered, soil horizons will begin to form.  Soil horizons are layers that develop in the soil due to the downward movement of organic matter and mineral material.  Over time this downward movement will lead to the creation of four distinct layers or horizons in most soil profiles.  The layers that form will vary in color, mineral composition, organic matter content thickness, chemical composition and many other ways.    



The topmost layer in the soil profile is the O-Horizon.  This layer is composed of mainly organic matter in three stages of decomposition.  On the top you will find the most recently deposited material such as grasses, leaves, animal remains, and woody materials.  In the middle you will find partially decomposed materials that were deposited in earlier years and finally on the bottom the organic matter will be completely decomposed into a black homogenous mixture called a humus layer.  The thickness of this horizon will vary with the season and geography.  It will be thickest in the fall after that season’s deposition of organic Soil Horizons-From the USGSmatter and thinnest in late summer after a season of decomposition has taken place.  This layer will also be thicker in temperate or northern latitudes because of the slow rate of decomposition that occurs from the winters that affect these areas. 

A well-developed O-Horizon can be crucial in plant growth and soil health and water retention in the soil.  In many long-term agricultural fields the O-Horizon has been depleted by heavy cultivation and the use of monocropping systems.  This could lead to reduced yields and possible pest and disease issues if not controlled or prevented.



The next layer in the soil profile is the A-Horizon which is essential a mixture the organic matter leaching down from above and the mineral matter that was weathered out of the parent material.  This soil horizon will typically be similar in color to the O-Horizon but usually a shade lighter due to the presence of lighter colored mineral matter.  For example in the Mollisol soils the O-Horizon will typically be black to dark brown while the A-Horizon can almost be indistinguishable depending the on the lighting or it can easily be seen as a distinctly lighter brown layer.  Since a large amount of water passes through this horizon this can sometimes lead to the formation of a E horizon which is a zone of greatest leaching.  This layer would be mainly composed of mineral and finer soil particles such as clay that washed down from the upper horizons.  This horizon is typically found in forest soils because of the higher amount of rain fall than what is found in grasslands or deserts.



The B-Horizon or the subsoil is the last horizon above the unconsolidated parent materials.  It will typically be light in color and composed mainly of fine soil particles such as clays and small silts and minerals such as salts.  This accumulation of salts and fine particles forms a dense soil that plants roots have trouble penetrating.  If this layer is close to the soil surface it could lead to stunted plant growth and reduced yields on crops or gardens.

 Soil Horizons-From the USGS


The C-Horizon is the final layer above the bedrock and is typically the parent material from which the soil formed.  This layer is comprised mainly of minerals, large rocks and any soil particles that may have been brought down by water.  This material has been exposed to little or no weathering activities by either climate or biological life and as such represents the unaltered state of the original parent material.   Quite often this a the layer you will find underground water in because it has been trapped by the bedrock below and the somewhat impermeable B-Horizon above it.

All of these horizons will form the soil profile or pedon and understanding these horizons and how they function can lead to better understanding of how the soil works and how to better manage it.  For more information about soils you can check out other articles I have written here on Infobarrel either by searching for Soils 101 or Soils 102 or by clicking on the links provided.  

I hope you have enjoyed learning more about soil and soil horizons today and I look forward to sharing more articles on soil and soil science in the future.





|Source= |Date=unknown |Author=USDA employee |Permission=This image is a work of a United States Department of Agriculture employee, taken or made during

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