Soil Organic Matter Flashcards
(19 cards)
What is Soil Organic Matter?
- All material in soil that contains (reduced) carbon.
- SOM is derived from
- Plant residue (both litter and roots)
- Animal remains and excreta
- Living soil microbes (microbial biomass)
- Over time microbes transform fresh organic material into stable soil organic matter
What is so important about organic matter?
Physical
- SOM Improves Soil Physical Properties
- Increased granulation and aggregate stability
- Makes heavy soils easier to work
- Increases water infiltration rates
- Increases water holding capacity
- Decreases erosion
What is so important about organic matter?
Chemical
-SOM Improves Soil Chemical Properties
Increases Cation Exchange Capacity so the soil is better able to store and supply plant nutrients
-Increases pH buffering so the soil resists changes in pH
-Reduces Aluminum, Iron, and Manganese toxicity in acidic soil
What is so important about organic matter?
Biological
- SOM Improves Soil Biological Properties
- Greater abundance, diversity and activity of soil microbes
- Increased nutrient cycling
- Increased root elongation and abundance
- Increased access to water and nutrients
Global Carbon Pools and Fluxes
petagrams (1015 g) in pool or annual flux
Atmospheric: 760 Vegetation: 550 Soil: 2,400 Fossil Fuel: 5,000 Oceans and Lakes: 40,000 Carbonate Rocks: 75,000,000
Composition of plant residues
-Fresh plant material is mostly water, 60 – 90% by weight
-Dry matter consists of the following chemical elements
Carbon ~42%
Oxygen ~42%
Hydrogen ~8%
Ash ~8% (minerals, trace elements; Si, Ca, Mg, N, K, P, S …)
-Dry matter consists of the following types of compounds
Cellulose ~45%
Hemicellulose ~18%
Lignin ~20%
Protein ~8%
Sugars and Starches ~5%
Fats and Waxes ~2%
Polyphenols ~2%
Effects of Organic Material Characteristics on Decomposition
Rate of decomposition: Rapid decomposition and nutrient release –>Very slow decomposition
Compound:
- sugars,proteins
- hemicellulose
- cellulose
- fats, waxes
- lignin, polyphenols
- humus
Type of residue
- Green cover crop
- Manure
- Straw, dry leaves
- Wood
- Compost
Factors that control decomposition rates of organic residues
- Residue Properties
- Size and surface area
- C/N ratio of material
- Chemical composition (influence of lignin and polyphenols)
- Soil environmental factors
- Temperature
- Aeration and moisture
- Soil texture
- pH, Ca levels, salinity
- Contact between residue and soil
- Physical protection
- Chemical protection
Influence of C/N ratio on microbial activity and soil mineral N levels
High C/N ratio residue
(>30)
Low C/N ratio residue
(<20)
Mineralization
The overall process whereby simple inorganic products, such as ammonium (NH4+), nitrate (NO3-), sulfate (SO42-) or carbon dioxide (CO2) are produced from organic forms during decomposition of organic materials in soil.
All organic matter in soil is not equal
3 Pools of Organic Matter
Active
Slow
Passive
Active SOM
1-2yrs
C/N ratio 15-30
- Recently deposited organic material
- Rapid decomposition
- 10 – 20% of SOM
Slow SOM
15-100yrs
C/N ratio 10-25
- Intermediate age organic material
- Slow decomposition
- 10 – 20% of SOM
Passive SOM
500-5000 years
C/N ratio 7-10
- Very stable organic material
- Extremely slow decomposition
- 60 – 80% of SOM
Patterns in soil
- There is a constant turnover of organic material in soil.
- The quantity of SOM depends on the balance between inputs and losses of organic material
- If losses increase and inputs remain constant, SOM will decrease
- If inputs increase and losses remain the same, SOM will increase
- But… SOM will not continue to increase or decrease indefinitely
(When inputs or losses are changed, SOM quantity changes to a different level and a new steady state condition is reached.)
What management changes can be made to increase input of organic material?
- Return more crop residues
- Add cover crops
- Diversify crop rotations
- Add other sources of organic material
What management changes can be made to decrease SOM losses?
- Decrease erosion
- Decrease tillage
How does tillage affect SOM decomposition?
- Residues are mixed with soil
- Physically breaks residue into smaller pieces
- Intimate contact between soil and residue
- Aerates soil
- Breaks apart soil aggregates, exposes protected SOM to decomposition
- Promotes erosion losses
- No-till crop production
- Crop residue left on surface
- Protects soil, reduced erosion
- Slowed decomposition of crop residues
- Minimum disturbance of soil structure
Composting
- Controlled, accelerated decomposition of organic materials
- Stabilizes organic materials, allows for storage (avoids problems with storage of putrescible materials like food wastes and manure)
- Stabilizes nutrients, limits loss potential especially of N
- Partial sterilization controls pathogens and reduces viability of weed seeds
- Decomposes potentially toxic organic compounds such as natural plant toxins, pesticides or other anthropogenic contaminants
- When soil-applied composts suppress some soil-borne plant diseases (mainly observed in potting mixes or container media, less effective in field soils)
