Chapter 12 Summary – Soil Organic Matter
Components of (dry) organic matter
Elements: 42% carbon, 42% oxygen, hydrogen 8%, ash 8%
Matter: cellulose, sugars/starches, hemicellulose, lignin,
polyphenols, fats and waxes, proteins.
Lignin – in plant cell walls, structurally important
Effect of Lignin
High lignin/phenol = slow decomposition, poor quality resource for
carbon-cycling organisms
Polysaccharides – long chains of sugars, (ie starch and cellulose) cellulose especially difficult to break down, needs cellulase to start
process
R and k specialist microorganisms
R’s prosper when abundance of mat, then die off, even
provide food for the slower metabolism K’s
o
Process of K-selected consuming R-selected
remains is mineralization
Aerobic soils decomposition – three main reactions
- oxidation of carbon -> CO2 H20, and biomass
- release/holding of nitrogen, phosphorous, sulphur
- creates stable material resistant to breakdown
Anaerobic soils – wet, low-oxygen, reduced decomposition
A)
B)
Environmental conditions of soil, and
Quality of residue as food the two factors in
decomposition and mineralization
Carbon/nitrogen ratio important for:
- competition between micro orgs. over these nutrients
- determines rate of decay and availability of nitrogen
matter richer in protein = lower C/N ratio, fungi’s – higher
Arable soil ratio range – 8:1 – 15:1
Soil microbes need carbon and nitrogen:
with overabundance of C, hard to find N in soil
with high N content, have high and fast decomp rates
Soil organic matter separated into 3 categories:
- Living biomass
- Dead roots and litter
- Non-identifiable tissue, amorphous and colloidal = humus
Classification of humus based on solubility
Fulvic acid – lowest molecular weight, lightest colour,
soluble in alkali and acid, most vulnerable to attack by
microbes
Humic acid – middle weight and colour, soluble in alkali,
medium resistance to attack
Humin – highest weigt and darkest colour, insoluble, most
resistant to attack
All forms relatively stable compared to other matter
Colloidal properties of humus:
High surface area, negative charge
Very high water holding capacity and CEC
Promotes soil stability and aggregation
By combining humus with clay = higher organic matter
content
Allelochemicals – chems secreted by plants that affect
microorgs or other plants, can be pos or neg
‘Pools’ of Organic Matter
Active plant matter – easily decomposed matter, lasts a few
days/ years
Slow – intermediate resistence, part of cycle
Passive – stable matt – hundreds/thousands of years.
Change in organic matter tied to quantity of carbon
Factors affecting soil organic levels
Differences between soil orders, affect amount of
naturally endowed nutrients
Net gain or loss of organic carbon
Climatic factors, colder – less chem reactions, less
micro orgs, moisture – soil org C and N increase with
more moisture
Natural vegetation – more org matt in grasslands than
forests, more in roots proportionally
Texture and drainage, affects water holding
o Finer texture – more organic matter

Produce more plant biomass

Lose less org matt, more aerated

Protected from loss due to clayhumus content
o Drainage – more moisture, less leaching,
more organic matter
Agricultural effects; tillage and management
o These areas have less org matt than natural
vegetated, loss of organic matter due to
cultivation, does not return to soil
Rotation, residue, plant nutrients
o Rotation to reduce loss of nutrients, while
still cropping,
o Lime and fertilizers help soil fertility
Dilemma of need for balance of use and conservation
o Farmers need to make profit, but if the
organic content is overdrawn, soil becomes
useless
Ways to manage loss
o Perennial veg
o Assure adequate nitrogen
o Continuous plant residue, no periods of
residue drought, limits erosion etc...