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
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
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.
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.
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.
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.
