W4, Soil Organic Matter (SOM)

Question 1

Soil organic matter (SOM) can be said to improve soil fertility because it contains and retains nutrients. Comment on this distinction and identify at least two nutrients elements that are influenced by each of these mechanisms.

Answer 1

Soil organic matter is made up of dead living organisms - mostly plants - which have previously taken up the nutrients that they’ve required to survive, which are the same nutrients that living/future plants need to survive. This is the basis of nutrient cycling.

• Can’t be taken up by plants in organic form - have to be mineralised by microbes first.
• SOM represents a large pool of some nutrients (particularly N, P, and S)

SOM plays an important role in the retention of nutrients, mostly due to its high cation exchange capacity - far higher than clay.

• CEC of SOM is difficult to estimate because it varies among different SOM fractions (particulate OM, humus, etc.) and is strongly pH dependent.

Question 2

Explain why the C:N ratio of organic matter decreases over time.

Answer 2

As microbes consume the organic matter, they respire carbon and turn over the nitrogen over in their biomass. Because carbon is being lost and the nitrogen isn’t, over time the C:N decreases.

Question 3

How much OM would you expect to be mineralised within a growing season?

Answer 3

Only ~3% (in other words, OM is quite stable).

Obviously depends a lot on the environment and soil.

Question 4

Why is the balance between mineralisation and immobilisation complex (and difficult to predict)?

Answer 4

• C:nutrient ratios vary
• Different microbes are involved (and have different nutrient requirements)
• Environmental conditions have a strong influence on the time dimension
• Multiple pools of OM are present and decompose at different rates (leaves, bark, wood, roots, fruit, etc.)
• Other additions and losses are occurring

Question 5

List some other benefits, or indirect effects on plant nutrition, of soil organic matter (SOM).

Answer 5

MOSTLY ON SOIL PHYSICAL PROPERTIES:

• ↑ aggregate stability (↓ slaking)
  
  • in most soils, SOM is the key determinant of structural stability
• ↓ bulk density (soil strength/resistance)
• ↑ permeability
• ↑ water infiltration rate
• ↑ drainage
• ↑ nutrient mobility
• ↑ water holding capacity
• ↑ aeration
• ↑ biological activity