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 (sandsiltclay) 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)?  

Looks Good! Correct: The accumulation of organic matter in the forest soil surface is due to the acidic nature of the needles inhibiting microbial attack and breakdown. The grassland plants tend to have a neutral or basic pH which microorganisms can readily decompose. It is true, the soil is flat. But it is also flat in the grassland photo. Most animals probably do not eat either type of plant. Wind has not blown away the forest litter, but it has not blown away the grassland litter either, because the plant canopy (tops of plants) force wind up and above the soil surface. If man disturbed the forest soil, it definitely would not look like the photo.

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

Looks Good! Correct: Although forest organic matter is more resistant to microbial decomposition, when it does decompose it releases some acidity which flows through the soil and creates more horizons quicker. If the forest were disturbed by man, the soil would probably be mixed and horizons would be indistinguishable. The soil did not look that way initially; it developed from parent material which is relatively uniform looking. Some soils may have various colored horizons due to wind deposited material, or being formed under lakes or oceans, but they would not have organic matter accumulation on the surface. And finally, grassland soils do develop, but they take longer to mature than forest soils.