Soils 205-General Soils

study questions- Quiz 4

 

  1. What are the five soil forming factors?

The soil forming factors provide a conceptual framework by which we can attempt to understand and predict soil properties across a landscape.  The model states that soil properties (s) are a function of climate (cl), organisms (o), relief or topography (r), parent material (p), and time (t).  S = f(cl,o,r,p,t).

 

 

  1. How does rock type influence soil texture and fertility?

Rock type, which falls under parent material (above), influences both the texture and fertility of the soil that forms in it.  Coarse textured rocks such as granite and gneiss tend to give rise to coarse textured soils, while fine textured rocks like basalt tend to form fine-textured soils (smaller particle size).  Acidic rocks with high Si and low base cation content tend to form soils that are low in base cations and have a low native fertility.  Soils formed from rocks high in base cations such as limestone tend to have higher native fertility.

 

  1. List and describe the types of parent materials that are transported by water, ice, and wind.

 

1.  Water-

Alluvium- deposited by rivers, it is well sorted.  The particle size distribution reflects the water of the energy.  Alluvial soils are some of the most important

Lacustrine- deposited by lakes (low energy) so the particle size is often fine (silty)

 

2.  Ice-

Glacial till- deposited in contact with the ice, either below it or on the sides.  It is generally poorly sorted (contains a wide range of particle sizes)

Glacial outwash- deposited by glacial melt water, so it is more like alluvium than glacial till.

 

3.  Wind (eolian)

Dune sand- medium to fine grained sand, well sorted, moved by saltation

Loess- coarse silts and very fine sands, they are actually suspended in the air during transportation; soils formed in loess tend to have high water holding capacity due to the predominance of silt.  Soils formed in loess are susceptible to erosion due to the lack of cohesion between silt grains.

Volcanic ash- light and porous material (high silt content), good water holding capacity.  Soils formed in volcanic ash are productive due to the high amount of available water, but are highly susceptible to erosion.  Therefore, Andisols tend to make better forest soils than they do agricultural soils.

 

  1. Draw graphs showing the influence of temperature or precipitation on clay formation, depth to carbonates, and % organic matter.

Clay

Formation

Rate
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


  1. Why are A horizons more common in grassland soils than forest soils?

In grasslands, most of the organic matter is added directly to the mineral soil through root death.  In grasslands, a significant amount of biomass is found underground.  In forested ecosystems, above-ground biomass is larger and organic material is added to the surface of the soil where it tends to decompose before it is incorporated into the soil.

 

  1. A

     

     

    Bg1

     

    Bg2
     

    A

     

    Bt

     

     

    BC
     

    A

     

    BC

     

    C
     
    Match the soil profiles below to the following landscape positions.  Explain your answer. 

     A.  Summit            B.  toeslope           C.  backslope

 

 

 

 

 

 

 

 

 

 

A                                 B                                      C

 

The toeslope soil receives direct precipitation plus runoff from the backslope position, the soil with the gleyed horizons (Bg1 and Bg2), therefore, would be found in the toeslope position.  It also has the thickest A horizon due to higher water content (slow decomposition) and the addition of A horizon material eroded from the backslope soil.  The soil at the summit has the Bt or clay enriched horizon.  The Bt horizon takes a relatively long time to form and therefore reflects a stable position.  The backslope soil has the thinnest A horizon due to erosion.

 

  1. What are the four soil forming processes?

The four soil forming processes are:

Additions-  the addition of organic material, for example

Transfers- the downward movement of soluble material with water.  An example is the redistribution of calcium carbonate in a soil profile.  Note that while the distribution of material with depth has changed, the material is not completely lost form the soil.

Transformations- the weathering of primary minerals to secondary minerals or the decomposition of organic matter

Removals- The complete loss of material from the soil profile.  The loss of calcium carbonate in a soil forming in a humid environment, for example.

 

  

  1. If all four processes are active in all soils, why aren’t all soils alike?

All four processes can and do occur at the same time in any given soil.  It is the balance of these processes that determines the type of soil that forms.  The balance of the four processes is controlled by the five soil forming factors.

 

  1. Draw a representative profile for each of the soil orders.

Gelisol Histosol            Spodosol       Andisol        Oxisol        Vertisol

 

O

 
 
 

 

 

 

 

 

 


Aridisols           Ultisol                              Alfisol                                Mollisol

Bt has relatively high base saturation
 
 

 

 

 

 

 

 

 


Inseptisol                      Entisol

 

 

 

 

 

 

 

 


  1.   What is the major factor limiting land use possibilities for each of the following soils? 
    1. Aridisols – low soil moisture levels; high pH and salts
    2. Vertisols – high shrink swell potential, poor for construction of homes and roads; difficult to till
    3. Ultisols – low base saturation (low fertility)
    4. Gelisols – low soil temperatures; permafrost
    5. Andisols – P and S sorption, erosion hazard
    6. Oxisols – low pH and base saturation
    7. Spodosol – low pH and base saturation
    8. Histosols – organic soils are oftentimes wet and make poor foundations

 

  1. What is humus?  Humus is a complex, dark colored material remaining after the major portions of added plant and animal residues have decomposed.  It is relatively stable.
  2. How is the soil organic matter related to global warming?

The soil contains more C than the atmosphere and the world’s vegetation put together.  Through photosynthesis, plants turn CO2 into biomass that can eventually find its way into the soil.  Within the soil it can be stored as humus or decomposed (returned to the atmosphere as CO2).  Practices such as conventional tillage result in increased oxidation of organic matter, increasing the CO2 (a green house gas) returned to the atmosphere.  The soil has a large potential to store C and thereby offsetting the increase in atmospheric CO2 levels.

 

  1. What are the colloidal characteristics of humus?  How does humus influence soil?

Despite the fact that the organic fraction makes up a small portion of the total volume of a soil, it has a large impact on soil properties. It serves as an important source of nutrients (especially N).  It has a high CEC compared to clay minerals.  Humus has a high water holding capacity (4-5 times greater than clays), and therefore makes a substantial contribution to water retention.  Humus also enhances aggregation and soil structure. 

 

 

  1. Give an example of an important member of the macrofauna, mesofauna, and microfauna; explain how humans benefit from the activities of each of these groups.

Macrofauna- earthworms

Earthworms are very mobile within the soil/plant system.  They bring organic material from the surface into the mineral soil where microorganisms are available to carry out decomposition.  While earthworms do not really chemically alter organic matter, they decrease the particle size by grinding the material.  This increases the surface area available for microbial attack.  Humans benefit from the incorporation of organic matter into soils by earthworms.  This increases fertility and promotes good physical properties.

 

Mesofauna- mites

Mites, like earthworms, physically alter the size of organic debris, facilitating microbial decomposition.  Mesofauna tend to have lower N requirements than microorganisms.  This results in the release of excess N when mesofauna feed on microbes.

 

Microfauna- nematodes

Nematodes are restricted to water filled pores and are not as active in the physical breakdown of organic matter as the macro and mesofauna.  They do actively graze the microbial communities and by doing so release excess N.

 

  1. Describe an activity carried out by fungi, bacteria, and actinomycetes (one for each group) that benefits human beings.

 

Fungi- Decomposition of a variety of compounds including lignin (they are extremely important in forested ecosystems).  They form symbiotic relationships with plants (mycorrhiza) which helps plants take up P and possibly water.

 

Bacteria- the great diversity of bacteria in soil is due to their ability to survive long periods by producing spores and their rapid reproduction.  This diversity ensures that there will be microorganisms capable of degrading practically anything in any one soil.  We have used this diversity to our advantage in the field of bioremediation.  Some bacteria are also N-fixers.

 

Actinomycetes- Actinomycetes are capable of degrading very complex natural and anthropogenic compounds.  They are important N fixers in forested ecosystems and produce many antibiotic compounds that are used by humans.

 

  1. After adding sawdust mulch to his field, a farmer’s plants turn yellow and die.  Why, in terms of N availability, did this happen?

 

The farmer applied a material that has a very high C:N ratio (about 80:1).  Microbes need to maintain a C:N ratio of about 8:1.  Since about 2/3 or the C will be lost through respiration, they require a food with a C:N ratio of about 24:1.  To decompose the sawdust, the microbes will start to take available soil N resulting in competition between the microbes and higher plants for available N.  Eventually, when the microbes take enough N from the available pool, and as C is lost as CO2, the C:N ratio will fall, the microbes will die off, and the N will become plant available.

 

 

  1. How does soil drainage affect organic matter accumulation?

Decomposition is much more rapid under aerobic conditions.  If the soil is poorly drained, water will accumulate in the pores and the oxygen level will fall.  Decomposition, which is an oxidation process, will decrease and organic matter will accumulate.

 

  1. Explain the following pathways in the nitrogen cycle.  What are the beginning and end products, and the conditions favoring each?

Mineralization- conversion of organic to inorganic N (plant available form).  It is favored by the presence of oxygen and a low C:N ratio.

Immobilization- conversion of inorganic N to organic form.  It is favored by C:N ratio greater than 24.

Nitrification- oxidation of ammonium (NH4+ ) to nitrate (NO3-).  This reaction is favored by:

-         neutral to alkaline pH

-         high Ca

-         supply of NH4+

-         the presence of oxygen (aerobic conditions)

 

Ammonia volatilization- conversion of NH4+ to NH3(g).  This reaction is favored by high pH and dry conditions.  Loss of ammonia gas can be high when growers apply ammonium containing fertilizers to the surface of dry, alkaline soil.  To help avoid losses, the fertilizer should be incorporated into the soil with tillage.

 

NH4+ fixation- ammonium is fixed within the interlayer space of certain 2:1 minerals such as vermiculite.  This is favored when there is a high content of 2:1 layer silicate clays.

Denitrificaiton- reduction of NO3- to N2(g).  This reaction is carried out by facultative anaerobes that use nitrate as a terminal electron acceptor in the absence of oxygen.  For this reason the reaction is favored by anaerobic soil conditions.  These organisms are also hetertrophs, meaning they need organic matter for an energy source.

 

Leaching- the loss of N with water moving down through the soil profile.  It is favored when NO3- is present and there is adequate water for leaching.

           

20.  List two ways in which humans have impacted the N cycle. (any two of the following will do)

Production/application of N fertilizers

Factory and automobile emissions

Burning of biomass (clearing of rainforests)

Planting of legumes

 

21.  What are three impacts of the activities in question 20? 

Acidification- the oxidation of ammonium in fertilizers produces acidity

Loss of nutrients- H+ produced from the oxidation of ammonium fertilizers can displace cations that are plant essential nutrients such as Ca and Mg.  These nutrients are then free to leach out of the root zone.

Atmospheric chemistry- acid rain, photochemical smog, ozone depletion, greenhouse affect.

 

    22.  How are deficiency symptoms of S different from those due to a deficiency of N?

Both N and S deficiency symptoms include chlorosis and stunted growth.  Nitrogen is mobile in the plant and is retranslocated from older leaves to new growth.  For this reason, the symptoms show up in the older leaves first.  Sulfur is relatively immobile within plants and the symptoms tend to show up in the younger leaves first.

 

    23.  Compare the sources (increases in available nutrients) and sinks (a loss of available nutrients) of soil N, P, and S.

Atmospheric pool - Nitrogen and sulfur both have important gaseous phases so the atmosphere is an important source and sink of these elements.  They are both lost from the soil through volatilization.  Since P doesn’t have a gaseous form, the atmosphere is not an important source or sink for this element.   

 

Organic matter- The majority of soil N (~95%) and S (90-95%) is found in the organic form.  For P, organic forms are important in humid environments, but inorganic forms tend to be predominant in semi-arid and arid environments.  All three elements can be mineralized through microbial oxidation of the organic matter.  

 

Leaching- Inorganic, anionic forms such as NO3- and SO42- can be lost through leaching.  Since P is quickly fixed with soil colloids, it is rarely lost through leaching.

Erosion- Erosion is especially important for P since it is predominately associated with the soil colloids.

 

vegetation removal- important loss for all three nutrients in agronomic (especially with high yielding crops and when no residue is left to decompose on the surface) and forested systems.

 

Fixation- Ammonium fixation is generally a small sink for N.  For P, fixation is the major sink.

 

24.  What forms of N, P, and S are considered plant available?

N- nitrate and ammonium

P- all forms in soil solution

S- sulfate

25.  If the three most common forms of P in soils are plant available, why are P         deficiencies so common?

P deficiency is common because the amounts of P present in soils is generally low, the P that is present is relatively insoluble and only becomes plant available over long periods, and any P added to the soil is quickly “fixed” (no longer plant available) by forming insoluble forms with Ca or Al.

 

26.  Why are N, P, and S so important to plant nutrition? 

All three of these nutrients are components of important biomolecules that play a role in photosynthesis, the transfer of energy and genetic information, protein and enzyme activity, etc. and are therefore essential for plant growth.