3.3 - Climate's Effect on Soil Formation

pH = higher ------------------------------------------------- > pH= lower

Figure 1. The general effect of climate on soil pH, CaCO3 and humus accumulation, from the western to eastern United States.

Western to Eastern U.S.

Generally, as one moves from west to east in the U.S., the soil profile thickness becomes greater. In the western U.S., lack of moisture and high temperatures promote evaporation and thus CaCO3(lime) accumulation near the surface and throughout the soil profile, making soil pH higher. Moving eastward, soils receive more precipitation, which promotes CaCO3 dissolution and movement downward into the subsurface, where it reprecipitates (reforms as a solid). This removal of CaCO3by water reduces the pH of the soil profile, so eastern soil surfaces will have a lower pH, while subsoil pH will be higher.

Concurrently, an increase in moisture promotes more plant and microbial growth. Plants, especially those in the Midwest, die every year. Plant litter falls to the soil surface where it is decomposed by microorganisms and turned into humus (soil organic matter). We see humus content of soils increase as we move from west to east, with the greatest amount of humus accumulation generally in the eastern United States. This accumulation of humus generally decreases soil pH.

Northern to Southern U.S.

Generally, as one moves from the northern to southern U.S., soil temperatures increase. This increase causes microorganism activity to increase, which speeds up the breakdown of humus. Thus, we generally observe  more humus accumulation in cooler areas and less accumulation in hotter areas.

Figure 2. World soil moisture regimes. Generally, red/purple = low moisture, yellow = moderate moisture, green = high moisture, and blue/gray/whites = frozen or partially frozen. Image courtesy of USDA-NRCS

U.S. Soil moisture regimes. (yellow/pale yellow/red/orange/orange brown = low moisture, peach/dark brown = moderate moisture, green/blue = high moisture.) Image courtesy of the USDA-NRCS National Soil Survey Center

 Figure 3. World soil temperature regimes. Generally, red/purple/orange = high temperature, yellow = moderate temperature, green = mild temperature, and blue/gray/white = near or below freezing. Image courtesy of USDA-NRCS 

U.S. soil temperature regimes. (light purple = high temperature, purple = moderate temperature, yellow = mild temperature, light blue/dark blue = slightly above freezing to below freezing). Image courtesy of the USDA-NRCS National Soil Survey Center

Quiz

Question

Question 4: From the above USDA-NRCS soil temperature and moisture regime maps, identify which of the following locations in the United States would have CaCO3 accumulation.‏

Looks Good! Correct: You would most likely find CaCO3 accumulation in Arizona, because it is an arid environment (lacking moisture) and has high temperatures that promote evaporation, causing CaCO3 accumulation near the surface and throughout the soil profile. Michigan, Southwestern Oregon, and North Carolina have increased rainfall,promoting the breakdown of CaCO3, thus leaching it from the soil.
Question

Question 5: From the above USDA-NRCS soil temperature and moisture regime maps, identify which of the following locations in the World would have CaCO3 accumulation.

Looks Good! Correct: You would most likely find CaCO3 accumulation in Australia and the Western United States, because both regions consist of an arid environment (lacking moisture) and have high temperatures promoting evaporation, causing CaCO3 accumulation near the surface and throughout the soil profile. Europe, Central Canada, and Eastern South America have increased rainfall, promoting the breakdown of CaCO3, thus leaching it from the soil. Central Canada may have some CaCO3 accumulation, but most likely not as much as Australia or the Western US.
Question

Question 6:  From the above USDA-NRCS soil temperature and moisture regime maps, where in the US do you think peat bogs will be located‏?  Peat soils contain large quantities of organic matter (humus) and in general can be found under conditions of excessive moisture and cool/cold temperatures. Some peat soils can be several meters thick with organic matter.

Looks Good! Correct: You would most likely find peat bogs in the Upper Peninsula of Michigan, because the temperatures are cool/cold and the area falls within a udic (humid) moisture regime. Arizona is too hot and dry to accumulate much soil organic matter. Both Southwestern Oregon and North Carolina receive enough moisture, but are relatively warm, thus preventing large quantities of organic matter accumulation.
Question

Question 7:  From the above USDA-NRCS soil temperature and moisture regime maps, where in the World do you think peat bogs will be located‏?  Peat soils contain large quantities of organic matter (humus) and in general can be found under conditions of excessive moisture and cool/cold temperatures. Some peat soils can be several meters thick with organic matter accumulation.

Looks Good! Correct: Both Northern Europe and Northeast Canada have cool/cold temperatures and the areas fall within udic (humid) moisture regimes. Central Australia and the Western US are too hot and dry to promote tremendous organic matter accumulation. Eastern South America, although relatively wet, has elevated temperatures which promote organic matter breakdown, and peat bogs would not form.
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