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BIOSPHERE2 ARIZONA DESERT SOIL PROFILE
Five soil forming factors that interacted to form this particular soil are:
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Parent material:
This soil was formed from alluvial (water deposited) materials
that were deposited more than 2 million years ago. Scientists
can tell that they are alluvial deposits because they contain
many large round rocks and boulders that are very well sorted
(rocks are deposited in layers according to their size, so
that rocks of the same size are found in the same layer).
Climate: The climate in this part of the United States is arid, which means it gets very little rainfall (about 30cm per year), and it is sunny most of the time.
Topography: This particular soil was located on the side of a steep (8%) slope. The slope is north facing, so that it is cooler and less dry than the same soil facing south. The type of landscape is called an "alluvial fan" which means it is sloping and fan-shaped, having been deposited by a stream from above. Since the time it was deposited, there has been a great deal of erosion from the top part of the slope where materials were removed and deposited in other parts of the landscape.
Vegetation: The common type of vegetation in this region is called "desert scrub", which include cactus, grasses, shrubs, and trees that are adapted to these desert climate conditions. The most common trees are "mesquite" (which is used to give flavor to charcoal), and "palo verde" (which has green bark for photosynthesis). These trees are commonly known as "nurse plants" because they provide shade, nitrogen, and organic matter to the soil, which allows it to be moister, cooler, and more fertile than the soil around it.
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Because of these favorable conditions, more
plants are found growing under these trees than out under
the hot sun. In a simlilar way, the north facing slope, on
which this soil profile is found, has many more plant species
than the same soil on the south facing slope which is much
hotter and drier.
Time: The parent materials of this soil were deposited over 2 million years ago, and have undergone a lot of weathering since that time. Some features that tell us this is a very old soil are:
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The bright red color of the A and upper B
horizon (this indicates that there are many of coatings,
such as iron, around the soil particles
The high clay content in the whole profile (this indicates that clay particles have formed or moved into the soil from above, over time
The calcium carbonate deposits in the lower B horizon (white color). This is a result of calcium carbonate salts moving down into the soil and accumulating over time. These carbonates have a very high pH. When an acid (like vinegar) is squirted on this part of the soil, the calcium carbonate will "effervesce" or bubble as it reacts with the acid. A hard cemented layer of calcium carbonate like this is also known as "caliche".
At one time, caliche was mined for building material by Native Americans. Many ruins made from caliche bricks are still standing, even after hundreds of years. Also, notice that there is only a very thin A horizon, which is mostly red in color. This is because there is very little organic matter contributed by the vegetation growing above the soil, which is what usually produces the dark A horizon color. What else do you notice about this soil?
