Soil Microbiology

1. A bacterium can reproduce once every hour under optimum conditions.  Find the number of bacteria
present if this doubling process continues unimpeded for one day.

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For a view of important soil organisms choose any of the following links.
 

Bacteria shapes    Bactertia Arrangments    Nematodes    Rotifers   Blue Green Algae   Ciliate

Collembola   Difflugia    Euglena    Fusarium    Mildew     Mite   Penicillium   Yeast   Aspergillus
 

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Dilution factors, a brief explanation
 
  1.    If 5 grams of sugar is placed in 100 ml of water
  it is diluted by a factor of 100/5  =  20/1, or 20X.
 
  If 4 ml of this sugar solution is placed in 200 mls,
  the second sugar solution dilution factor becomes
  200/4  =  50X.

  The sugar has been diluted by a factor of
  20X  x  50X  =  1000X.    1 mg sugar/ml

  2.    Conversely, a 100 ml sugar solution was analyzed
  for sugar content by removing 4.0 ml of the
  solution and diluting it to 200 ml.  The diluted
  solution contained 1 mg sugar/ml.  Find the
  concentration of the original solution.

  1 mg/ml  X  200/4   =   50mg/ml  or  5000mg/100ml
  or 5g/100ml of sugar in the original solution.

2. A gram of moist soil from a garden is placed in a five liter flask of sterile water and mixed thoroughly.
Two ml of this solution is removed and mixed in a 500 ml volume of sterile chicken broth.  Five ml of the
broth is mixed in warm agar, poured into a petri dish, covered and placed in a warm incubator.  After
24 hours,  the number of colonies of microbes growing in the petri dish is 125.  Find the number of
microbes in the original gram of soil.  (Hint use dilution factors).
 
 
 

 3. Construct a diagram that illustrates the distribution of soil microbes in the top 15 cm of soil as to both numbers and biomass. (See Ch. 10 pp323 in the text and pp 229 in the Lab Manual.)

 4. Find the definition for the following terms in the nitrogen cycle:

 N2 fixation
 

 Nitrification
 

 Mineralization
 

 Ammonification
 

 Nitrate reduction
 

 Denitrification
 

 Immobilization
 

5. How do bacteria classed as chemoheterotrophs differ from chemoautotrophs?
 
 
 

6. Describe the two types of symbiotic Mycorrhizae found associated with plant roots.
 
 

 7.  Earthworms have an impact on soils.   A warm, moist, productive soil may contain about 700 pounds of
earthworms.  If each worm weighs five grams and ingests one half gram of soil per day, find the time needed
for earthworms to pass an acre furrow slice (2,000,000 lbs.) of soil through their digestive system.

8. How might the process in question 7 affect soil fertility, mineral weathering, and plant production?

A. Soil fertility
 

Mineral weathering
 

Plant nutrition
 

9. Draw a diagram of the carbon cycle.  Explain why the earth's atmosphere does not become enriched in CO₂.
 

10.  Can you give a reason why the atmosphere of the planet Venus is enriched with CO₂?
 

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