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Soil Genesis and Development, Lesson 3 - Soil Forming Factors

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3.4 - Effects of Organisms on Soil Formation

Soil organisms play a vital role in the degradation of organic matter and subsequent soil humus formation. When plants die, leaves are dropped onto the soil surface where microorganisms can “attack” and decay plant tissue. The organic matter is used as an energy source for microorganisms, increasing their population in the soil. These organisms utilize easily digestible materials (like simple sugars and carbohydrates) found in the plant material, leaving more resistant materials (such as fats and waxes) behind. The material left behind is not easily decomposed; it comprises the humus found in soil.  Humus acts as a gluing agent, essentially holding primary soil particles (sand, silt, clay) together to form secondary aggregates or ‘peds’. These organisms and the humus they help create aid in the  soil development and the formation of soil horizons.

Figure 4 below illustrates the effect soil organisms, specifically vegetation, have on the creation of humus and soil formation. The figure shows the percentage of humus content tends to be greater in grassland soils, as compared to coniferous forest soils. The reason behind this observation is quite simple; dead grassland plants tend to have a somewhat neutral pH as compared to forest needles, which tend to have an acidic pH. The relatively basic pH of the grassland plants makes them easier for microorganisms to degrade and turn into humus. Oppositely, needles are more difficult for microorganisms to degrade; thus, the humus content of coniferous forest soils tends to be less than grassland soils. The acidic nature of the forest litter, however, causes acids to flow through the soil profile and help develop horizons quicker than a grassland soil. The acids can dissolve soil materials and redeposit them deeper in the soil, which helps to more quickly create horizons. Figure 4 also illustrates that humus content decreases with soil depth. This makes sense, because humus is derived from decaying plant material which originates at or near the soil surface.

Figure 4.  Humus per depth in centimeters (cm).
Image courtesy of Jim Ippolito

Figure 5.  Organic matter in forest vs. prairie soils.
Image courtesy of Jim Ippolito and Paul McDaniel

The following questions refer to Figure 5, above. Consider the surface as the top of the soil profile, note where the profile is marked as “0 depth”.  You should also note the forest soil has more horizons, thus it is more mature. 

Question 8: What causes the difference between the surface horizon in the coniferous forest  soil, (i.e., the black portion) versus grassland soil‏ (i.e., the dark brown upper portion)?

A.  Flat soil surface.
Forest litter is acidic and more resistant to degradation by microorganisms.
C.  Wind has not blown the forest litter away.
D.  The forest litter has not been disturbed by man, unlike the grassland soil.


Question 9: What causes the coniferous forest soil to develop quicker than the grassland soil‏?

A.  The forest litter has not been disturbed by man, unlike the grassland soil.
B.  The soil looked that way initially.
C.  Soils of different colors were laid down on top of one another, creating the horizons.
D.  The forest litter released some acidity, which flowed through the soil and created horizons quicker.

E.  Grassland soils do not really develop.



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