Soil Organic Matter

Question 1

What is SOM?

Answer 1

SOM includes live plants and animals (5%), and MO’s and decomposition products (95%)

Question 2

What are some examples of active, readily degradable compounds?

Answer 2

Acids, lipids, monosaccharides, polysaccharides produced by roots and microorganisms

Question 3

What are some characteristics of stable humic matter?

Answer 3

Slow turnover, heterogeneous, Polymeric aliphatic chains, eg. lignin-like polysaccharides, amino acids linked to form peptides. It is stable.

Question 4

Does humic matter have large or small molecules?

Answer 4

Large molecules

Question 5

What is the surface area and charge like for humic matter?

Answer 5

Large surface area and generally negative with high CEC

Question 6

What are the three classifications of SOM?

Answer 6

1. Humin (H) = alkaline insoluble
2. Humic Acid (HA) = alkaline soluble
3. fulvic acid (FA) = Alkali soluble.

Question 7

Which one of the three SOM classifications makes up most of the humus?

Answer 7

HA comprises 50 - 80% of humus.

Question 8

What are the three different rates of turnover in soil?

Answer 8

1. active biomass (1.7 years)
2. moderate stable humus ( 06 y)
3. stable humus (1400 y)

Question 9

How much humus does active biomass, moderate stable humus and stable humus account for?

Answer 9

15, 65 and 20%

Question 10

What 3 things have resulted in 40 - 50% of Prairie soil’s original OM loss?

Answer 10

1. Cultivation
2. Summer fallow
3. Annual cropping

Question 11

What kinds of functions does SOM have?

Answer 11

1. Biological
2. Physical
3. Chemical

Question 12

What are the 6 main roles of SOM?

Answer 12

1. Decomposition of SOM
2. Soil aggregation
3. Water-holding capacity
4. Formation of soluble complex in soil solutions
5. Immobilization and mineralization
6. Adsorption and decomposition of nutrients by surface complexes, ion exchange reactions.

Question 13

Describe decomposition.

Answer 13

1. It is a form of respiration
2. Aided by microorganisms
3. An oxidation process.

Question 14

Write substances in order of rapid decomposition to slow decomposition.

Answer 14

1. Sugars, starches and simple proteins
2. Crude protein
3. Hemicellulose
4. Cellulose
5. Fats and waxes
6. Lignin, oils

Question 15

What is the rate of decomposition affected by?

Answer 15

1. Temp.
2. Moisture
3. Aggregation (O2 availability)
4. pH
5. Aggregate
6. Organic matter quality

Question 16

How does organic matter quality affect decomposition rate?

Answer 16

Materials with low C:N ratio break down much more easily than materials with high C:N ratio.