Monday, August 22, 2011

How to Map Soils

Looking at this landscape a little knowledge of soils can explain a lot of things.  Learn how to piece together landscape stories with the information on soil distributions and mapping in this blog entry.
Unless you are a little unusual, you probably rarely consider the soil we walk across.  But its OK to be a little unusual, the benefits can be great.  If when we are hiking, or even in our own backyard, we were to consider the soil we stand on and how different types of soil are distributed around us, the education we would receive would be great.  Far to often even seasoned research scientists will forget about the soil they walk over, even if it makes a huge difference to what they are studying.  It would literally pay for these scientists to have a little soil education.  By using the clues outlined in this article you can begin to piece together where specific types of soils are located, and begin to develop a story to how the land, plants, and animals came into their present state.  This also helps in understanding gardening or landscaping projects.  Every landscape has a story to tell, and very often that story starts with the rocks and dirt.  Here are the clues to look for:

Parent Material
What is the general type of rock that you find in an area?  A good rock book will help you figure this out.  Certain rocks will produce certain soils (click here for a previous blog on this subject).  Fortunately, there are only a handful of different types of rocks most soils are made out of.  Where I live in the Arizona Sonoran Desert the most common rock parent materials are granite, gneiss, basalt, and schist.  Where I grew-up and still visit often in Iowa limestone and sandstone are the most common rocks.  In Iowa however, rocks are usually not present and the parent material is glacial deposits (glacial till), alluvial (water deposited along rivers or streams), or loess (wind deposited).  The lack of rocks also is a good indicator of a particular soil type.  If there are no rocks try to use landform or landscape position to identify soil.
An example of landforms in the Sonoran Desert.  The foreground of this picture is a flat basin with only 1 to 2% slope.  In the middle of the picture is a long narrow hill called a banali.  In the distance are mountains.  Each of these landforms will have their own soil types.   

What types of landforms are near you or are you on?  Hills, mountains, basins, plateaus, plains, depressions, small rises will all produce different soil types.

Landscape Position
Where in the landscape do you find yourself?  This is similar to landform but more specific to where you find yourself on a particular landform.  Do you find yourself on a hillside, a hilltop, a flat plain along a river, a mountain?  Are you close to a mountain or far away from a mountain?  All these different areas will have different soils.  Also consider elevation.  Are their multiple plains, or flat areas, each with different elevations?  Each individual plain will likely have different soils.

This granite rock and the lack of other rocks in the area indicates glacial till as the parent material.  This rock was carried by a glacier from Canada to Iowa during the ice age.  The landforms in this picture also indicate different soils.  The flat area in the mid to foreground are one type of soil and the hill in the background has a different type of soil.  Behind where this photo was taken is another flat area, lower in elevation and with finer textured soil along the Cedar River, indicates another soil type.  Difference in vegetation does not indicate different soil types in this picture due to different disturbance histories.  The grassy area is disturbed by burning every few years while the forest is not burned.  
What is the general slope of the landform you stand on?  Again, this overlaps with landscape positions and landform, but helps you narrow down a particular area to a particular soil type.  Is the slope steep, flat, moderate?  Approximately, what is the percent of slope?  Consider landform or landscape position again, hilltops will have one particular slope, hillsides another, and basins or plains another.  Each will have a different soil type.

Soil Color
What is the soil color?  Refer to this previous blog entry to find more about what that soil color means.  One particular soil is going to have one general soil color.

Soil Texture
What is the soil texture?  Refer to this previous blog entry to find out how to determine and what that soil texture means.  One particular soil type is going to have one general soil texture.

Plant Communities
Generally, plant communities correspond directly to soil types.  Meaning, two different soil types are going to have two different plant communities.  The differences in plant communities may not be obvious.  Also, if there is a history of disturbance using plant communities as an indicator for soil becomes worthless.  For example, using plant communities as an indicator in deserts works extremely well due to the fact that deserts usually lack fire or agricultural disturbance.  In the Midwest however, fire is a normal part of prairie management but not of forest management.  And depending on when agricultural or logging disturbances in the Midwest happened will also determine what stage of succession a forest is in.  So depending on when and what type of disturbance has happened historically, one particular soil type may have several different plant communities.

So piecing all these things together, you can determine that an area has one particular soil type if: 1. the area is on a particular landform, 2. the area is on one particular landscape position on that landform, 3. that area also has similar parent materials, 4. The area also has a general soil color, 5. the area also has a similar soil texture.  Lastly, plant communities may or may not indicate a soil, depending on disturbance history.

This may seem like a lot of information, but with some practice this becomes a lot easier.  Landscape stories can be quite fascinating and often, most of the information you need is right under your feet.

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