Transcript Soil Colloids and Soil Chemistry

Soil Organic Matter
Section C
Soil Fertility and Plant Nutrition
Review - Soil organisms
• Bacteria
– Most numerous, smallest
– Aerobic and anaerobic
• Actinomycetes
– Share characteristics of bacteria and fungi
– Active in degradation of resistant compounds
• Fungi
– Aerobic only, filamentous
– Active in degradation of resistant compounds
Major Soil Organisms
Bacteria 108/gram
Actinomycetes
107/gram
Fungi 106/gram
Soil Microorganisms
• Can be classified according to C and energy
sources and their oxygen requirement:
– photoautotrophs
• Energy from sunlight & C from CO2
• Some bacteria and algae only
– chemoautotrophs
• Energy from oxidizing inorganic material, C from CO2
• Some bacteria only
– chemoheterotrophs
• Energy and C from oxidation of organic materials
• Most bacteria, all fungi and actinomycetes
Soil Microorganisms
• Oxygen requirement
– aerobic
• Require free O2 for respiration
• All fungi and actinomycetes, most bacteria
– anaerobic
• Must use alternative electron acceptors instead of O2
– NO3 -, SO4 2-, Fe3+ , CO2
• Some bacteria are anaerobic
– facultative
• Can be aerobic or anaerobic. Some bacteria
Decomposition of Plant Residues
(Under aerobic conditions)
CO2
Plant
Residues
+
Dead
Microorganisms
More microbial biomass
NH4+, SO42-, etc. (inorganic waste)
Humus (organic waste)
Soil Organic Matter
• Soil organic matter: all organic matter
in the soil, including humus, microbial
biomass, and plant and animal residues
in various stages of decomposition.
– Composed of a wide range of organic
materials, from highly decomposable to
resistant to decomposition.
Roles of Soil Organic Matter
•
•
•
•
•
Microbial substrate
Nutrient reserve (esp. N, P, S)
CEC
Water-Holding capacity
Soil structure
Humus
• The stable portion of soil organic matter that
results from microbial degradation of
residues.
– Dark colored
– About 58% C, 5% N
– Complex chemical structure, aromatic plus
aliphatic functional groups
– Difficult to break down because of structure
– high CEC
Humus
• The major organic “waste” by-product of
OM degradation.
• The percentage of a residue that will
become humus is approx. proportional to
its lignin content.
Lignin
Humus
Carbon
Hydrogen
Oxygen
Nitrogen
Decomposition of Organic
Matter
• Organic materials are decomposed by
heterotrophic microorganisms. The
carbon
organic matter is a source of _______,
energy
nutrients
__________,
and _____________
to
these organisms.
Humus and Nutrients
• Humus contains about 58% C, 5%N, 0.6%
P, and 0.6% S
• How much humus in soils?
An Aridisol might contain 0.5% SOM by weight, a Mollisol 3-5% by weight
• How much OM does this represent?
An Aridisol with 0.5% SOM in the top 30 cm will contain
3000 m3/ha x 1500 kg/m3 x 0.005 = 22,500 kg/ha (top 30 cm)
A Mollisol with 5.0% SOM in the top 30 cm will contain
3000 m3/ha x 1500 kg/m3 x 0.05 = 225,000 kg/ha (top 30 cm)
Decomposition of Humus
• The rate of decomposition of humus is most strongly
affected by soil moisture and temperature (<1 to
>5%/yr).
• Humus is chemically complex and has a C:N ratio of
about 11:1
• High soil temperatures, abundant (but not excessive)
moisture encourages “rapid” humus breakdown
• In soils where OM content is not decreasing, synthesis
of “new” humus approximately equals decomposition of
“old” humus.
Decomposition (Mineralization) of
Humus
• Releases N as NH4+ , available for plants
• If 2.5% of the N in SOM is mineralized each year,
how much N would be released for plant uptake?
• Aridisol (from previous example)
– 22,500 kg SOM/ha x 0.05 kg N/kg SOM x 0.025 (% min)
= 28 kg N/ha
• Mollisol (from previous example)
– 225,000 kg SOM/ha x 0.05 kg N/kg SOM x 0.025 (%
min) = 280 kg N/ha
Decomposition of Plant Residues
(Under aerobic conditions)
CO2
Plant
Residues
+
Dead
Microorganisms
More microbial biomass
NH4+, SO42-, etc. (inorganic waste)
Humus (organic waste)
What Happens to Residues?
Chemically simple
residues
Chemically complex
residues
CO2
CO2
Biomass
Biomass
Waste
Waste
Decomposition of Plant Material
• The rate of decomposition of plant
residues is governed mostly by:
– Chemical makeup of the residue
– C:N ratio
– Available soil N
– Temperature, moisture, oxygen, and other
environmental conditions that affect microbial
growth
Chemical Composition of Plant Residues
Sugars
Simple proteins
Starchs
Complex proteins
Hemicellulose
Increasing chemical complexity
Increasing rate of decomposition
Cellulose
Waxes
Lignin
C:N Ratio
• Why is the C:N ratio important?
– Microorganisms need C and N in fixed ratios,
because C and N are used to synthesize proteins,
nucleic acids, etc.
– Bacterial cell C:N is 5:1 to 8:1. Since about 50% of
the C in an organic material is converted to CO2,
they need roughly a C:N of 10:1 to 16:1 in the
residue they consume.
– Fungi need a C:N of about 40:1 in their diet
C:N Ratio
decomposition
50 g C
20 g as CO2
10 g as waste
20 g as biomass
Therefore, if the residue
containing 50 g of C
contains < 2 g of N
(C:N>25:1), it will have
insufficient N for microbial
needs. What about
>2 g N (C:N <25:1)
Microbial biomass has an average
C:N of 10:1, therefore how much N
is needed to balance the new biomass
C?
2g
C:N Ratios
• High C:N material:
– Woody
– Grain crop residue
– Mature plant tissues
• Low C:N material:
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–
–
–
–
Green
Young plant tissues
Legume residues
Composts
Manures
C:N Ratio and Residue Mgmt.
• What are the implications of the C:N ratio
of crop residues for nutrient management?
Immobilization
The conversion of inorganic (available) N (NH4+, NO3-)
to microbial biomass N. Results from...
CO2 release
NH4+ and NO3-)
C:N ratio of residues
Time
Mineralization
The conversion of organic (unavailable) N to NH4+ .
Results from...
C:N ratio of residues
NH4+
CO2 release
Time
Soil Organic Matter Content
• In “undisturbed” soils:
SOM = f (I, O)
– Inputs = plant residues
– Outputs = decomposition, erosion
• In managed soils:
SOM = f (I, O, M)
– M = management practices such as tillage,
cultivation ,residue management, etc.
Soil Organic Carbon
Soil Organic Matter Content
• The amount of organic matter in a soil tends
to be difficult to change, and reflects an
equilibrium between additions and losses
over long periods of time.
• In the absence of changes in management or
climate, soil organic matter content tends to
remain relatively constant (steady state). In
this case, the low amounts broken down each
year are replaced by new humus.
Management Effects on SOM
• Agricultural management of soils usually
decreases
_____________
amounts of SOM (compared to
undisturbed soils) because:
– tillage increases aeration and aerobic microbial
activity
– liming, where practiced, increases microbial activity
– irrigation may increase microbial activity
– erosion
Effects of Cropping on SOM - Oklahoma
2.5
Soil % C
2
1.5
1
0.5
0
1880
1900
1920
1940
Unfertilized Wheat
1960
1980
Wheat + manure
2000
Conserving SOM
• Management practices that can help
conserve or build SOM:
– Reduced (minimum) tillage
– Cover crops
– Growing high residue crops
– Adding organic materials to soils
– Practicing crop rotation
Effect of Cropping Practices
Effect of Fertilizers
Manitoba
Illinois
Organic Materials
• Animal Manures
– Solids, liquids
• Human Manures
– Solids (sewage sludge, biosolids)
– Liquids (effluent)
• Composts
• Reasons for applying to soils:
–
–
Animal Manures
• Were a major source of plant nutrients
P
N and _____)
(especially ____
before widespread
use of commercial fertilizers
• Manures average 0.5 to 1% N, 0.25 to 0.5% P
• Significant environmental problems are
associated with storage and disposal of animal
manures.
Human Waste
• In some parts of the world, have historically
been an important fertilizer source
• Average 4% N, 3% P, 0.3% K
• Soil disposal is one of the few options for
disposal
• Use is becoming more common
Composting
• Compost is formed from the aerobic
breakdown of organic materials which
results in a mass of partly decomposed
organic matter.
• Can be a valuable soil amendment. Most
valuable for organic-matter building in
soils. Not nutrient-rich.
“Sustainable Agriculture”
• A general term that is often applied to
agricultural practices deemed “organic”.
Usually means that organic fertilizer
sources are emphasized.
• “Organic” agriculture means that only
organic fertilizer sources are used.
• In organic agriculture, the proper use and
management of organic inputs is critical