Soil Taxonomy

My last blog focused on the soil forming factors, and I explained how soils vary from place to place based on those 5 factors (ClORPT). Soil scientists not only recognize these differences, but they set out to classify these soils into individual  soils called soil series. There is a whole system dedicated to classifying the uniqueness of each soil, and a whole 900 some page book dedicated to explain these characteristics. But the entire classification can get a little exhausted and hard to explain to a general audience for the purpose of this post I really just want to focus on the broadest tier of soil classification and that's soil orders. I will only touch on a few this post as it could get quite lengthy if i discussed each in detail. Below I have highlighted the 4 orders I will go over. I will touch on the others in a later post.

Soils are classified into soil orders on the major differences in soil forming factors (ClORPT) and the absence or presence of diagnostic horizons. So what is a soil horizon?

A soil horizon is a distinct layer of soil, whose properties develop from the combined actions of living organisms and percolating water. One or more horizons make up what is know as the soil profile. Which is the vertical sequence of distinct layers that is unique to each soil type.
Definition of Soil Horizons





Soil Profile

This figure shows what a soil profile might look like. With O, A, B, C and R signifying the distinct soil layers. So now that we know what a soil horizon is what is a diagnostic horizon?

A diagnostic horizon is defined as a well-defined soil layer whose structure and origin may be correlated to soil-forming processes and can be used to distinguish among soil units. Definition of Diagnostic Horizon
Diagnostic horizons may be at the top in the O or A layers or they may be in the subsurface or the B layers.
So now that we have a generalization of the factors that make up soil orders lets discuss the 12 soil orders.

   Soil Orders

 

1. Alfisols

 2. Andisols

  3. Aridisols

4. Entisols

 5. Gelisols

  6. Histosols

    7.Inceptisols

   8. Mollisols

9. Oxisols

    10. Spodosols

11. Ultisols

   12. Vertisols

 

 

http://passel.unl.edu/Image/mmamo3/TimKettler/alfisolsLG.gif

 

Alfisols are soils that do not have a mollic epipedon but have an argillic or natric horizon and are moderately leached.

 

An argillic horizon is normally a subsurface horizon with a significantly higher percentage of clay than the overlying soil material. This layer shows how clay has moved from the upper layers (O and A) into the subsurface layers (B). (Soil Taxonomy Twelfth Edition, 2014)

 

You will most commonly find alfisols under forest canopy and in temperate humid and sub humid regions of the world. Alfisols occupy about 10% of the global ice free land area. Alfisols have generally high fertility and are found to be very agriculturally productive.

 

 

 

 

 

http://croptechnology.unl.edu/Image/mmamo3/TimKettler/mollisolsLG.gif

 

 

 

 

Mollisols are soils of grassland ecosystems. They have a dark, thick surface horizon as shown in the image to the right. Mollisols have a diagnostic horizon of a mollic epipedon.

A mollic epipedon has a dark layer of organic matter that formed after many years of grassland vegetation.

 This long term accumulation of organic matter is what makes these soils so agriculturally productive. Mollisols are found in prairie regions such as the Great Plains. they occupy 7% of the ice free land area globally, but they are the most extensive soil order in the U.S (about 22%). Mollisols are one of the most important soils for agriculture due to the high amounts of organic matter, which increases fertility.

 

 

 

 





http://croptechnology.unl.edu/Image/mmamo3/TimKettler/entisolsLG.gif


 

 

 

 

 

 

 

 

 

 

 

Entisols are soils that have recently formed. These soil have developed in unconsolidated parent  material with usually no genetic horizons except an A horizon. These soils have very little profile development as you can see in the figure to the left. These are the soils that do not really fit into on the other 11 soil orders, so they are commonly characterized. Making them the most extensive soil order globally occupying about 18% of the ice-free land area.

 

http://croptechnology.unl.edu/Image/mmamo3/TimKettler/vertisolsLG.gif

Vertisols are clay-rich soils that shrink and swell with changes with moisture content. During the dry periods the soil "shrinks" or contracts. During the wet periods the soil "swells" or expands. This causes huge cracks in the soil profile which you can visibly see in the image to the right. These soils cause many problems in engineering, which many people in the Red River Valley are very familiar with. Vertisols only occupy about 2.4% of the global ice-free land area.







These are only four of the twelve orders, but these are the orders that you will most likely see in the ND/ MN area. Below are a few helpful links if you want more information on the orders and there geographic areas. The distribution maps so how extensively they are mapped in the U.S. Be on the look out for more soil order descriptions in later posts.



• http://www.cals.uidaho.edu/soilorders/index.htm
• http://courses.nres.uiuc.edu/nres201/Lectures/Lec%207%20and%208%20Soil%20Classification%20Fa.%2008p2.pdf
• http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/survey/class/?cid=nrcs142p2_053580
• http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/survey/class/?cid=nrcs142p2_053589

 

Soil Forming Factors

 So I have focused a lot of my time discussing many management strategies that help promote soil health, but I realized today that in order to make sound management decisions you first need to know what your working with. What I mean by this is that not all soils are the same, soils are often generalized but they are very much different from each other. So what makes soils different? This is where the famous ClORPT saying comes into play . If you have ever watched a soil talk or researched the basics of soil probably one of the first things you learned about was ClORPT. If you have ever watched a soil talk or researched the basics of soil, probably one of the first things you learned about was the 5 soil forming factors or what we at NDSU say ClORPT.

ClORPT strands for...

Cl- Climate

O- Organisms

R-Relief

P- Parent Material

T- Time

I like to start with the P component or the Parent Material, because soil has to be made and it has to come from another material through weathering processes. Just like us soils have parents, called parent materials. These are the underlying materials that the soil was formed from after thousands to millions of years of weathering. Some might form from sandstone giving the soils sandy characteristics or from limestone giving the soils excess calcium carbonate. All soils take on characteristics from their parents but other factors help to influence these "traits" and make them individually unique from their parents and siblings.

The Cl or the Climate component is a real driving factor in molding these traits and developing them into stable soils.  Below is a figure showing major climatic regions in the world.

 

Each of these regions have unique characteristic's help drive soil characteristics. For example in areas that are humid and see significant amounts of precipitation we might see extremely leached soils. In areas with an arid climate or semi-arid climate we might see soils that have very little profile development showing only one or two horizons. You'd also see little organic matter or the "dark stuff" due to little vegetation on the soil, because in these areas there is not enough precipitation to support active plant growth.  These two contrasting examples should really bring home how climate influences soil formation.

Leached Soil

Arid-Semiarid Soil

 These two figures here are two soils that represent the two examples I provided above. The one on the left shows an extremely leached soil due to plentiful amounts of precipitation. The white area is the defining factor that we look for when determining if the soil is heavily leached. We call this the E horizon. The figure on the right shows an example of a soil that you would most likely see in an arid or semi-arid area. This is an example of a Aridisol. An Aridisol is a soil order ( this is a way of classifying soils, which I may discuss in a later post) that represents little profile development and little organic matter that dominate deserts. Much like we discussed above.


The O component or Organisms is another factor that may be harder to conceptualize. We know that soils are in fact living, they have millions of little organisms living in them that help make many processes function. It's said that in one tablespoon of soil there is more organisms than there are people on Earth. Now if you look at a soil profile down to 60 inches, imagine how many living organisms there are! So with that big of an influence it's hard not to say their making an impact. These organisms help to bind soil aggregates and really start to make soil structures. They help with many processes that involve plants, that help to provide nutrients back into the soil. All of these processes really start to determine soils and their functions. Below is a link to a video to on soils biology, it's a lengthy video (25 minutes) but if your interested in learning more about this aspect of soil health its worth a view.

http://www.bing.com/videos/search?q=soil+biology&qs=n&form=QBVR&pq=soil+biology&sc=8-8&sp=-1&sk=#view=detail&mid=67CE654F7E75399564B167CE654F7E75399564B1

 The R component or Relief component is another factor that defines how a soil forms. The best way to understand this component is to use a hill scenario. Say we have a hill and we take a soil cores from the top of the hill, the side/middle of the hill and one at the bottom of the hill. What would we expect to see? Well we know that after a rainfall event all the water at the top of the hill will most likely end up at the bottom of the hill due to gravitational movement and the path of least resistance so how does that effects soil formation. Well after many years of this type of event we would start to see erosion processes take place on the top and side of the hill, meaning that we would see a shorter or thinner A horizon and less organic matter than the bottom core. Similar to what we discussed in the climate section we would see more leaching going on in the bottom core due to more active water entering the soil system.

One of the most important factors is time! Just like Rome wasn't built in a day, soil takes thousands to millions of years to form. In order for all the of the processes discussed above to effectively influence the soil it needs a plentiful amount of time for these processes to take shape.

Tillage and No-Till Operations

http://www.nrcs.usda.gov/wps/portal/nrcs/detail/ia/home/?cid=nrcs142p2_011847

    You will hear a lot of common themes when learning about soil health, last week I talked about cover crops which is one of those themes. This week I would like to dedicate my post to another common theme of soil health and that's no-till or minimum  tillage. You will hear a lot about these systems, but it may be very unfamiliar so I want discuss the basics of no-till and how it differs from conventional tillage.

What exactly is tillage and why do we do it? Well tillage is simply the preparation of land for growing crops using a plow, chisel, disk etc. Tilling has been used for centuries to ensure that we have a proper seed bed when planting to allow the crop to grow with out being impeded. Tillage helps the roots grow by breaking up soil structure and allowing soil water and nutrients to become available to the roots. With all the benefits of tillage in recent years there has been an increase in the popularity of no-till or minimum tillage systems.

Disking Operation

On major reason is because intensive tillage has been shown to cause major soil erosion. This is somewhat easy  to understand. When we break up the soil and bring it to the surface we are allowing the soil to become more susceptible to elements such as wind and water. Which are major erosion causing forces. Tillage also takes a lot of time and with the large amounts of time it takes, that means it takes a lot of money in fuel, equipment and maintenance. We have also seen that extensive plowing causes a phenomena called plow pans. This is the compression of soil causing a layer in the soil that impedes water and nutrient movements. This causes increased surface runoff and decrease in nutrient infiltration, which is a major loss in money. So other options such as no-till and strip-till are becoming increasingly popular due to the soil health properties and less expenditures. So with that being said what is no-till and why is it used?

No till is a way of growing crops or pasture from year to year without disturbing the soil. This simply means that after harvest, one will plant directly into the past years residue instead of preparing a seed bed through tillage. No-till increases organic matter in the soil which is often destroyed during conventional tillage. With the increase in organic matter that therefore increases the soil tilth which therefore increases the amount of water that infiltrates in the soil.  Soil tilth is the state of aggregation of a soil especially in relation to its suitability for crop growth as defined by Merriam-Webster Dictionary.http://www.merriam-webster.com/dictionary/tilth By not disturbing the soil the soil microbes are allowed to grow and flourish, which are often distorted by tillage. These microbes are essential in the soil as they provide beneficial nutrients to the crops. The easiest to understand benefit is the reduction  of soil erosion. By keeping the soil covered it is protected from the elements such as wind and water. This is one of the main reasons it is implemented over conventional tillage. As discussed tillage promotes soil erosion and loss of essential organic matter. Organic matter is extremely important in soils and there is no true monetary value that you can put on this precious soil component. No-till in it's essence is promoting sustainability. We want to ensure that we have healthy land to farm many years into the future. By preserving our organic matter and promoting a diverse soil atmosphere we are therefore participating in sustainability.

This is really just a sentence of the book that could be written on the advantages of no-till. I suggest that you check out more blog and other website for more information. For more information here are a few links that may help you understanding more about the systems.

• http://extension.psu.edu/agronomy-guide/cm/sec1/sec11g
• http://www.nrcs.usda.gov/wps/portal/nrcs/detail/ia/home/?cid=nrcs142p2_011847
• http://cropwatch.unl.edu/tillage
• http://notill.okstate.edu/publications/notillcroppingsystemsoklahoma/chapter06.pdf

Also coming soon to the blog will be my first journey into no-till gardening!