Soil Organic Matter

Question 1

What 4 basic things does SOM provide for the living soil?

Answer 1

1. food
2. water
3. shelter
4. air

Question 2

What is SOM?

Answer 2

a vast array of carbon compounds in the soil

originates from plants, microbes, and other organisms and plays a variety of roles in nutrient, water, and biological cycles

Question 3

What are the two major categories of SOM?

Answer 3

1. Stabilized: high decomposed and stable (humus)

2. Active: being actively used and transformed by living organisms

Question 4

What are the other 2 categories of SOM?

Answer 4

3. Living organisms

4. Fresh residue: freshly deceased organisms

Question 5

Define stabilized OM

Answer 5

OM that has either been broken down into such small, complex compounds that most organisms cannot break them down further OR they are physically protected by being inside a soil aggregate and cannot be reached

Question 6

How to mend a soil with a low clay content?

Answer 6

Add more OM to make the soil richer and more capable of holding onto nutrients

Question 7

what are the 4 changing forms of OM

Answer 7

1. Additions: when roots and leaves die, they become part of the SOM
2. transformations: microorganisms are constantly changing organic compounds from one into another
3. microbes feed plants: some wastes released by microbes are nutrients that can be used by plants
4. stabilization of OM: eventually SOM compounds will become too complex to broken down further and become stable and resistant to further change

Question 8

Why are regular additions of OM important for soil organisms?

Answer 8

1. Provides food
2. Provides habitat
3. can degrade potential pollutants
4. helps control disease
5. binds soil particles into larger aggregates enhancing root penetration, water infiltration, makes the soil easier to work, and reduces erosion

Question 9

T or F: adding SOM in a large quantity one time is sufficient

Answer 9

FALSE because it will get decomposed, it must be continually added to soil

Question 10

What are 2 ways very fresh OM can cause problems for crops?

Answer 10

1. Nitrogen tie-up:
   temporary N deficiency for crops can occur if the OM is low in N (high C:N ratio)
   - microbes will take up the N before the plants can use it
   - too much OM can be difficult to break down because there is too much carbon
2. allelopathy: chemicals formed when some residues decay that can inhibit plant growth

Question 11

T or F: OM and fertilizer are the same

Answer 11

FALSE
OM feeds the soil
fertilizer feeds plants

Question 12

The 3 most important compounds that make up OM?

Answer 12

1. carbohydrates
2. lignins
3. proteins

all 3 include carbon

Question 13

7 ways OM improves soil conditions

Answer 13

1. increases sandy soils water holding and nutrient holding capacity
2. improves clay soils by loosening them and improving their workability
3. stores nutrients used by plants in humus particles and in carbon chains for when they need them
4. humus makes nutrients more available
5. builds aggregation
6. improves soil structure
7. improves water infiltration

Question 14

What are the 5 functions of SOM?

Answer 14

1. improves Water storage
2. improves nutrient storage
3. improves soil aggregation = soil resistance
4. prevents erosion
5. carbon sequestration

Question 15

7 major factors that affect the content of OM in the soil?

Answer 15

1. vegetation type
2. climate
3. soil texture
4. drainage
5. tillage
6. rotations/residue
7. policy

Question 16

What are some management practices to improve/increase SOM content in the soil?

Answer 16

1. conservation tillage/no till
2. consistently adding OM
3. decreasing practices that compact soil (less traffic, traffic in the same places with wide wheels, no livestock grazing)
4. cover crops
5. diverse crop rotations
6. no synthetic fertilizers/pesticides
7. not over or under watering soils
8. leaving surface residue
9. mulching