R2102 2.1 Describe the role of organic matter in the soil

Question 1

What are the main types of organic matter in the soil

Answer 1

1. Living organisms
2. Dead organic matter
3. Humus

Question 2

What is living organic matter (soil biomass)

Answer 2

Living plant and animals and micro-organisms which live in the soil and on plant material and other animals in the soil.

Question 3

Benefits of living organisms (4)

Answer 3

1. Plant and animal debris and converted to minerals and humus.
2. Rhizoba and Azotobacter spp. fix gaseous nitrogen
3. Soild structure improved by activity of plant roots, earthworms and other burrowing organisms
4. Detoxification of harmful organic materals such as pesticides is undertaken by many bacteria species.

Question 4

What is dead organic matter?

Answer 4

Decomposing plants, residues from leaf litter and roots to tree trunks. Animal tissue including dead soil micro-organisms.

Gradually decomposed by active mirco-organisms

Question 5

Benefits of dead organic matter in the soil

Answer 5

1. Microbial activity is increased because it is food for soil organisms
2. Soil is physically opened up and aeration improved by the dead but recoginisable organic matter
3. Water holding capacity of the soil is improved by fine (unrecognisable) organic matter
4. It is a dilute source of slow-release nutrients as the organic matter decomposes and release minerals

Question 6

Green organic matter (4)

Answer 6

1. Succulent and leafy
2. Decomposes very rapidly
3. Short term physical effect
4. Yields nutrients, especially nitrogen

Question 6

Green organic matter (4)

Answer 6

1. Succulent and leafy
2. Decomposes very rapidly
3. Short term physical effect
4. Yields nutrients, especially nitrogen

Question 7

Brown organic matter (4)

Answer 7

1. Fibrous / woody
2. Decomposes very slowly
3. Physical effect persists
4. Little nutirent contribution

Question 8

What is buffering?

Answer 8

The ability of the soil to retain nutrients, including lime, against loss by leaching

Question 9

What is humus?

Answer 9

Complex organic substances no longer identifiable as tissues, a black colloidal material which coats soil particles but seperating and binding them.

Question 10

Benefits of humus in soil? (5)

Answer 10

1. In sandy and silty soils it helps to form stable crumbs
2. In clays the surface charges on humus are capable of combining with the clay particles, thereby making them less sticky and more friable
3. Buffering capacity is increased
4. Soil warms up more quickly in the spring as darker soils absorb more of the Sun’s radiation
5. Improves water holding capacity

Question 11

Fungi (3)

Answer 11

1. One group can decompost lignin
2. Can tolerate acid conditions and low calcium levels
3. Adapted to survive in dry soils

Question 12

Soil bacteria (4)

Answer 12

1. Inactive below 6c and above 35c
2. Killed at 82c
3. Thrive in ph range 5.5-7.5
4. Leads to release of nutrients and hence more vigorous root growth as well as the creation of humus that improves soil crumb.
5. They attack minerals leading to the weathering of rock debris

Question 13

Earthworms in soil (3)

Answer 13

1. Create a network of burrows which improve aeration and water movement
2. Many species feed on soil and digest the organic matter in it before returning the waste into the tunnel they created.
3. These worm casts have a higher nutirent content than the bulk soil

Question 13

Earthworms in soil (3)

Answer 13

1. Create a network of burrows which improve aeration and water movement
2. Many species feed on soil and digest the organic matter in it before returning the waste into the tunnel they created.
3. These worm casts have a higher nutirent content than the bulk soil

Question 14

Plant roots effect on soil

Answer 14

1. Create tunnels that improve areation and water movement
2. When they dies the channels they have created are stabilsed by the coating of their decomposted remains
3. Roots take up water and as the heavier soils dry out they shrink and crack thereby helping to develop and improve their structure

Question 15

How does organic matter influence soil structure? (5)

Answer 15

1. OM opens out a soils structure and improves structural stability.
2. Helps light sandy soil to hold more water
3. Reduces the chance of compactation on clay
4. As it increase stability of soils structure it is easier to work and extends the cultivation window
5. OM decreases from soil surface downwards - 1%-6% in topsoil, >2% in subsoil