soil organic matter
broad term for soil organic materials, including living and dead animals, plants, microorganisms, insects, pests, manures, and crop residues and other organic materials
5 fractions of OM
fresh and partly decomposed plant and animal remains
lignin- part of cell wall plants
OM adsorbed on colloids
biomass of microbial cells and by-products
humus
characteristics of humus (1-6)
- dark brown
- complex structure
- amorphous
- colloidal
- resistant to further microbial action
- modified lignin, polysaccharides
characteristics of humus (7-12)
- high specific surface area
- colloidal surface are negatively charged (pH dependent)
- at high pH, CEC is high (150-300)
- OM has 4-5 times higher water-holding capacity
- low plasticity and cohesion
- C:N ratio ranges bet. 10:1 and 12:1
physical benefits of OM
- enhanced soil aggregate stability, water infiltration, aeration, reduce run-off
- improve water-holding capacity
- reduces stickiness, easy to till
- reduce surface crusting, seedbed preparation
chemical benefits
- increase soil CEC
- improve buffering capacity
- accelerate decomposition of soil minerals
- increase N supply capacity
- holds organic nutrients (N,P, S) and makes them available to plants
- reduces NH3 volatilization in alkaline soils
biological benefits
- provides foods for organisms
- enhances soil microbial biodiversity and activity, helps disease and pests suppression
in summary OM:
increases soil fertility
improves soil physical condition
improves microbiological health of soil
sources of OM
- crop residue
- animal manure
- compost
- cover crops (green manure)
- perennial grasses and legumes
factors that affect rate of decomposition
- nature of organic material
- availability of nutrients, particularly N
- moisture
- aeration
- temperature
- pH
moisture
aeration
temperature
pH
field capacity
aerobes
30-40 C
neutral pH
what destroys OM
- decrease in biomass production
- decrease in OM supply
- increased decomposition rates
decrease in biomass production
- replacement of perennial vegetation
- replacement of mixed vegetation with monoculture pf crops and pastures
- high harvest index
- use of bare fallow
decrease in OM supply
- burning of natural vegetation and crop residues
- overgrazing
- removal of crop residues
increased decomposition rates
- tillage practices
- drainage- slow on poorly aerated soils
- fertilizers and pesticide use
OM Management
- use conservation tillage practices
- rotation of annual row crops with perennial grasses or legumes
- establishment of legume cover crops
- avoid soil compaction
- zero tillage
- agro-forestry
- composting
relationship between C:N ratio and decomposition rate
higher C:N ratio, slower decomposition rate
lower C:N ration, faster decomposition rate
