GY 111 Lecture Notes D Haywick 2008 09 1 GY 111 Lecture Note Series Soils Lecture Goals A What is soil B Typical soil profile s C Soil Types Reference Press et al 2004 Chapter 7 Grotzinger et al 2007 Chapter 16 p 373 384 A What is Soil As we discussed last time rocks exposed at the surface of the Earth are subject to physical and chemical weathering Given sufficient time and a suitable climate hot and wet is best for chemical weathering cold and wet is best for physical weathering even the most resistant rock will be reduced to a shadow of its former self The product of weathering will ultimately be sediment the major ingredient of sedimentary rock however we reserve the term sediment for material that is moved from its place of origin i e the place where weathering is occurring to some depositional sink i e a place where sediment is collected In situ weathering of rock may leave a layer of weathered residue atop the rock If the residue is more or less broken bits of rock which was primarily the result of physical weathering we generally refer to it as regolith If the rock has been physically and chemically weathered and if there is an appreciable amount of organic material it is better to refer to the layer as soil Soil is the most important Earth resource that we have with the possible exception of water and as one smart ass student said air Without soil we would not have potatoes lettuce peas corn bread beef chickens trees etc Life without potatoes would be pretty tough It is for this reason that soil conservation is taken so seriously In the past we had many serious experiences with soils erosion the Great Dust Bowl of the early 20th Century is but one example Today agricultural practices are better and soil erosion by wind is not as much of a problem however water runoff as a cause of soil erosion is still a major concern B Soil Profile s Soils are complex but they all more or less share one important characteristic They are layers Soil scientists usually agricultural geographers recognize 3 or 4 layers properly termed horizons within most soils The following cartoon summarizes a typical soil profile GY 111 Lecture Notes D Haywick 2008 09 2 The horizons are labeled from top to bottom A including O B and C The A Horizon is also called the Zone of Leeching and is the part of the soil that is most intensely chemically weathered The term leaeching implies that all of the soluble minerals have been removed through dissolution or hydrolysis In the A Horizon all of the K SiO44 Ca2 Mg2 etc has been washed downward deeper into the soil The majority of the remaining material is Fe2 and Al2 This explains the generally red orange colour of the A Horizon in soils like those in southern Alabama where chemical weathering is pronounced The A Horizon also contains the majority of the soils organic material Usually the organics are contained in the top portion of the A Horizon in what is called the top soil because it is the best soil for agricultural reasons the most nutrient rich interval Scientists prefer to call the top soil humus or the O Horizon for organic The best soils have the thickest O Horizon Where does all of the K SiO44 Ca2 Mg2 from the A Horizon go to A lot of it ends up in the B Horizon which is also called the Zone of Accumulation because this is the interval where a lot of chemical precipitation can occur The B Horizon differs from place to place because the climate differs from place to place Along the Gulf Coast which is generally hot and wet the minerals that a precipitated in the B Horizon are largely restricted to iron oxides hydroxides and clay minerals Our B Horizon looks pretty much the same as the A Horizon although the grain size may differ between the two In drier climates other minerals may precipitate in the B Horizon e g calcite in soils this would be called calcrete quartz in soils this would be called silcrete halite in soils this would be called saltpan We will discuss climate as a factor in soil development more extensively in just a few minutes but first check out the photo to the right It s a rather nice soil profile A B C GY 111 Lecture Notes D Haywick 2008 09 3 atop a basalt outcrop in New Zealand The A horizon covered in slumping grass and the B horizon light yellow are well developed but the C Horizon is exceptional What is the C Horizon It s defined as the Zone of Physically Weathered Bed Rock what a mouthful because it is the transition interval from soil to unaltered rock It is a zone where physical weathering dominates By the way the term bedrock has nothing to do with the Flintstones Geologists call unaltered rock exposed at the surface of the Earth bedrock Sedimentologists like your humble instructor may also call bedrock basement rock if they are discussing the bottom successions of layered rocks The term basement means the bottom portion of the succession Now getting back to the photo on the bottom of the previous page as well as the image on this page to the right Both images show that the C Horizon is variably coloured Black is unaltered basalt but the yellowish orange is limonite FeOOH which is forming through hydrolysis of the ferromagnesium minerals in the basalt This was one of the reactions that I told you about in the lecture on weathering There is no need to go to New Zealand to see chemical weathering in action Just look around you Mobile is weathering central if you get my drift C Soil Types One of our geographers in the Department of Earth Sciences Dr Roy Ryder is an expert at soils and were he talking to you today instead of me He would doubtless tell you that there are dozens and dozens of different types of soils He is correct Soil scientists classify soils in a very minute way Geologists tend to be a little less systematic We distinguish several soil types based primarily on the thickness of the soil horizons and the nature of precipitation in the B Horizon Please note it is climate that is the dominant control on the development of these soils but other factors are also important Fe3 Al3 Fe3 Al3 SiO44 Soil 1 Pedalfers This type of soil see profile to the right forms in areas with warm and wet climates like the Gulf Coast The A Horizon is pretty thick there is a well developed O Horizon and the B Horizon contains iron and aluminum oxides hydroxides The soil is typically red orange in colour and is pretty fertile The name pedalfer recognizes the dominance of Fe3 and Al3 in the A and B Horizons Soil 2 Laterites This type of