Life in the Soil
Ch 5
Soil teems with life . . .
1 teaspoon of fertile soil could contain:
• 100 nematodes
• 250,000 algae
• 300,000 amoeba
• 450,000 fungi
• 11,700,000 actinomycetes
• 100,000,000 bacteria
Carbon Cycle
Part of the food chain . . .
organisms in the soil, including plants,
participate
• Producers – Plants are primary producers
• Consumers – Primary and Secondary
Carbon and Energy move up the food chain
from plants to the highest level
Detrital Food Chain
Breaks down and recycles dead organisms
• detritus – dead organisms or their
products
• decomposers – consume organic matter,
release CO2, and leave humus
Detrital Food Chain
Releases plant nutrients that were tied up in
bodies of plants and animals
• Soil is the location of the Detrital Food
Chain
Thus . . .
• Soil life functions to recycle Carbon
and Nutrients
Ecology of the Soil
Five ecological roles of interest to soil
students
• Autotrophs (producers) and
Heterotrophs (consumers)
• Parasites
• Predators
• Saprophytes
• Symbionts
Ecology of the Soil
Additional classifications:
• Preferred environment . . .
e.g. oxygen (aerobic), no oxygen (anaerobic)
Note: for an overview of specific organisms
refer to additional “Ch 11” handout
Nutrient Cycling
• Nutrients in living bodies or fresh O.M. are
said to be immobilized
• Microbes decompose immobilized
nutrients through mineralization
Nutrient Cycling
• Immobilization and Mineralization are
opposite processes
• Most soil sulfur comes from the
weathering of sulfur-containing minerals
(see p. 77, fig. 5-8)
Nitrogen Fixation
Soil air contains ~ 78% N2 gas, however . . .
Plants cannot use Nitrogen in the N2 form
Nitrogen Fixation
Soil microbes can absorb N2 gas, converting it
to ammonia that plants can use
• This process is called Nitrogen Fixation
• Rhizobium bacteria enter symbiotic
relationship with legume plants
Nitrification and Denitrification
Nitrogen fixed by microbes is immobilized in
the bodies of microbes or their host plant
This organic nitrogen is mineralized to
ammonium ions (NH4+) when the microbe
or plant dies
Nitrification and Denitrification
While some NH4 + may be absorbed by
plants, most ammonium is oxidized to
Nitrites (NO2 -) then to Nitrates (NO3 -)
first by:
Nitrosomonas
then by: Nitrobacter bacteria
Nitrates are then taken up by plants or by
other microbes
This completes the process of Nitrification
Nitrification and Denitrification
• Denitrification occurs when nitrates are
changed by anaerobic bacteria back to N2
gas
• These denitrifying bacteria use nitrates
instead of oxygen
• Denitrification occurs most rapidly in
wet soils
(see p. 79 for series of steps)
Mycorrhizae
“fungus root”
• Fungi that act as extensions of plant roots
Form symbiotic relationships:
fungi get sugars produced by plants (~15%)
plants get Phosphorus
additionally: water, zinc, copper, longer life,
protection from disease/toxins
Mycorrhizae
• Two types:
• Ectomycorrhizae
– Outer cells
– Tree seedlings
• Endomycorrhizae
– Inner cells of most plants
– VAMs (vesicular-arbuscular mycorrhizae)
Microorganisms
• Additional functions:
– Break down chemicals (bioremediation)
– Certain anaerobics produce methane while
others oxidize the methane to carbon dioxide
– Organic acids produced dissolve minerals
– Rhizospere microbes may produce plant
hormones and vitamins, improve nutrient
uptake or suppress root disease
Promoting Soil Organisms
• Enhance plant functions by:
Inoculation
puposely infecting soil with useful organisms
- Mycorrhizae inoculants improve transplants
- Legumes inoculated with Rhizobium for
nitrogen fixation
Soil Conditions
• Healthy soils provide good microorganism
growth
• Fresh O.M. is needed
• Good aeration for most
• Adequate moisture/temperature
• Neutral pH
• Nutrient requirements (note Nitrogen need)
Soil Animals
• Nematodes
- Some harmful, some beneficial
- Pierce with stylet-type mouthparts
- May lead to subsequent pathogen infection
• Arthropods – mix soil
• Earthworms – benefit most agriculture
• Mammals – mix soil