chapter 7

Question 1

soil organic matter

Answer 1

broad term for soil organic materials, including living and dead animals, plants, microorganisms, insects, pests, manures, and crop residues and other organic materials

Question 2

5 fractions of OM

Answer 2

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

Question 3

characteristics of humus (1-6)

Answer 3

1. dark brown
2. complex structure
3. amorphous
4. colloidal
5. resistant to further microbial action
6. modified lignin, polysaccharides

Question 4

characteristics of humus (7-12)

Answer 4

7. high specific surface area
8. colloidal surface are negatively charged (pH dependent)
9. at high pH, CEC is high (150-300)
10. OM has 4-5 times higher water-holding capacity
11. low plasticity and cohesion
12. C:N ratio ranges bet. 10:1 and 12:1

Question 5

physical benefits of OM

Answer 5

1. enhanced soil aggregate stability, water infiltration, aeration, reduce run-off
2. improve water-holding capacity
3. reduces stickiness, easy to till
4. reduce surface crusting, seedbed preparation

Question 6

chemical benefits

Answer 6

1. increase soil CEC
2. improve buffering capacity
3. accelerate decomposition of soil minerals
4. increase N supply capacity
5. holds organic nutrients (N,P, S) and makes them available to plants
6. reduces NH3 volatilization in alkaline soils

Question 7

biological benefits

Answer 7

1. provides foods for organisms
2. enhances soil microbial biodiversity and activity, helps disease and pests suppression

Question 8

in summary OM:

Answer 8

increases soil fertility
improves soil physical condition
improves microbiological health of soil

Question 9

sources of OM

Answer 9

1. crop residue
2. animal manure
3. compost
4. cover crops (green manure)
5. perennial grasses and legumes

Question 10

factors that affect rate of decomposition

Answer 10

1. nature of organic material
2. availability of nutrients, particularly N
3. moisture
4. aeration
5. temperature
6. pH

Question 11

moisture
aeration
temperature
pH

Answer 11

field capacity
aerobes
30-40 C
neutral pH

Question 12

what destroys OM

Answer 12

1. decrease in biomass production
2. decrease in OM supply
3. increased decomposition rates

Question 13

decrease in biomass production

Answer 13

1. replacement of perennial vegetation
2. replacement of mixed vegetation with monoculture pf crops and pastures
3. high harvest index
4. use of bare fallow

Question 14

decrease in OM supply

Answer 14

1. burning of natural vegetation and crop residues
2. overgrazing
3. removal of crop residues

Question 15

increased decomposition rates

Answer 15

1. tillage practices
2. drainage- slow on poorly aerated soils
3. fertilizers and pesticide use

Question 16

OM Management

Answer 16

1. use conservation tillage practices
2. rotation of annual row crops with perennial grasses or legumes
3. establishment of legume cover crops
4. avoid soil compaction
5. zero tillage
6. agro-forestry
7. composting

Question 17

relationship between C:N ratio and decomposition rate

Answer 17

higher C:N ratio, slower decomposition rate
lower C:N ration, faster decomposition rate