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(B) Soil Science > Essential Soil Organic Matter > Flashcards

Essential Soil Organic Matter Flashcards

1
Q

Key…

A

OM= Organic Matter

CEC = Cation exchange capacity

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2
Q

What are the two solid constituents of soil?

A

Mineral materials, organic material.

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3
Q

Much of the activity of the soil occurs where?

A

The interface between the void space and the surface of the soil matrix (mineral and organic material) is where much of the chemical and biological activity occurs.

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4
Q

Benefits of organic material in sandy soils?

A

Source of nitrogen water retention and release binding of single grain sands into aggregates.

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5
Q

Benefits of organic material in clay-based soils?

A

Has an ‘opening up effect’ breaking large blocky aggregates into smaller structures.

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6
Q

Define Organic material?

A

All organic material within the soil living and dead. Sports turf relates more to the remains of plant and animal matter such as leaf fall and clippings.

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7
Q

Soil as a carbon sink?

A

Reserves of carbon held out of the atmosphere, and the carbon cycle represents around 10 billion tonnes stored as organic matter.

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8
Q

What is the typical composition of plant material?

A

50% Cellulose 20% Hemicellulose 15% Lignin 10% Protein + amino acids 1% waxes

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9
Q

Cellulose and Hemicellulose are…

A

Long-chain sugar molecules that are major components of cell walls

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10
Q

Proteins and amino acids are…

A

Compounds that contain alot of N, useful to a lot of the photosynthetic systems of the plant. When these structures are returned to the soil they are rapidly decomposed and provide excellent nutrition for soil micro-organisms.

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11
Q

Lignin is…

A

A complex polymer containing lots of carbon. Used in bonding and strengthening within the plant. They are tough to break down and accumulate in the soil as humus.

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12
Q

Thatch is…

A

The accumulation of excess lignified tissues especially rhizomes and stolons.

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13
Q

Fats, waxes, oils and tannins…

A

All resistant to decay, when decaying release acids that lower pH of the soil. Resembles a small % of organic material.

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14
Q

If minerals (sand,silt,clay) are the skeleton of the soil.

A

Organic matter is the flesh. .

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15
Q

Primary Colonisers are…

A

Hardy plants such as mosses and Lichen.

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16
Q

Lichen forms a symbiotic relationship with…

A

algea and fungi

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17
Q

Lichens have the ability to overcome what deficiency in pure mineral soil?

A

N deficiency

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18
Q

Lichen in symbiosis with algae and fungi fix what from the atmosphere?

A

Nitrogen (N2)

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19
Q

Not all primary colonisers fix nitrogen but…

A

… they do represent the first input of organic matter into mineral soil.

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20
Q

The breakdown of rock to form the mineral components of soil is called?

A

Weathering

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21
Q

Decomposition of organic material releasing molecules and elements is called?

A

Mineralisation.

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22
Q

Chemical weathering and mineralisation go further than breaking down mineral and organic material…

A

… these processes can ultimately ‘transform’ mineral and organic material.

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23
Q

Transformed components will…

A

… determine the specific properties of the soil.

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24
Q

Mineralisation Stage 1.1

A

Organic material is deposited, decay is slow and dominated by micro-organisms.

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25
Q

Mineralisation Stage 1.2

A

Organic material is incorporated and mixed into the soil, eaten, digested and excreted.

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26
Q

Mineralisation Stage 1.3

A

Significant reduction in size but increase in surface area.

27
Q

Mineralisation Stage 1.4

A

Some initial activity by bacteria, fungi and actinomycetes.

28
Q

Mineralisation Stage 2.1

A

The rapid increase in soil micro-organisms, particularly bacteria.

29
Q

Mineralisation Stage 2.2

A

Most easily decayed compounds (carbs and proteins) are consumed.

30
Q

Mineralisation Stage 2.3

A

Longer chain molecules found in cell walls (cellulose) are split into shorter chains.

31
Q

Mineralisation Stage 2.4

A

Lots of carbon dioxide, water and heat are released. (aerobic process)

32
Q

OM mineralisation equation…

A

OM + soil organisms + O2 = CO2+H2O+ heat

33
Q

Humification is?

A

The decay of single molecules that are ‘humified’ and more resistant to further decay.

34
Q

Mineralisation Stage 3.1

A

Readily digestible organic material has been broken down and may be incorporated into tissues of soil fauna and flora. Remaining is humus and lignified material resistant to further decay.

35
Q

Mineralisation Stage 3.2

A

Population and activity of micro-organisms declines. Production of CO2, H2O and heat reduces. Fungi dominate mineralisation as they’re are more able to utilise lignified and humic material.

36
Q

Mineralisation Stage 3.3

A

Assuming no more OM is added the soil system will be left with enhanced humus levels. This too will eventually decompose in aerobic conditions.

37
Q

Satge 3 OM equation…

A

Humus+ Fungi + O2 -> CO2+ H2O

38
Q

Humus is…

A

… a collection of highly complex organic compounds based on humic acid and fluvic acid.

39
Q

Humus size is?

A

Around 0.002mm Forms an intimate mix with clay structures known as the clay/humus complex.

40
Q

What are the 3 classes of soil Humus?

A

Mull Mor Moder

41
Q

Describe Mull Humus…

A

pH range of 6.5-8.0 Low rainfall/ aerobic conditions organic matter is fully decomposed Typically high CEC

42
Q

Describe Mor Humus…

A

-pH range -<5 -high rainfall/strongly leached soil environment poorly decomposed OM that is poorly incorporated in the soil. -Exhibits as a dark, thin (4-5 cm) of compact, poorly drained material. - Low CEC

43
Q

Describe Morder Humus…

A

Falls between the charecteristics of Mor and Mull.

44
Q

Is Soil organic material Carbon based?

45
Q

Increased N/C ration will influence?

A

The rate at which the organic matter can be decomposed.

46
Q

What is the typical C:N ratio in microbial cells?

47
Q

What happens when the C:N ratio is very high?

A

Micro-organisms in the soil cant maintain their 6:1 ratio and need to find other nitrogen sources.

48
Q

What is the continued effect of low a high C:N ratio in soil micro-organisms?

A

Their rate of growth and productivity reduces and the process of humification is retarded or stopped.

49
Q

C:N ratios of common material…

A
  • Clover leaves 16:1
  • Grass Leaves 18:1
  • Rhizomes and Stolons 50:1
  • Garden Compost 18:1
  • Mull Humus 11:1
  • Mor Humus 30:1
50
Q

Additions to the soil of organic material with a high C:N ratio will result in…

A
  1. A Slow in decay and breakdown
  2. Induce N deficiency in the soil. Microbes instead draw on ‘free’ nitrogen sources in the soil to make up for the low nitrogen content of the deposited matter.
51
Q

Essential process ‘Mineralisation’

A
  1. Micro-organisms derive energy by decomposing complex organic molecules in the OM
  2. During decomposition, essential elements (e.g N,P,S) are converted from complex organic, to simple inorganic forms.
52
Q

What are the forms of the essential nutrients that mineralised from OM that are then able to be used by the plant?

A
  1. NH4+
  2. H2PO4-
  3. SO42-
53
Q

Mineralisation products include …

A

… CO2 & H2O

Both essential for plant growth

54
Q

Carbon and other nutrients essential for plant growth are incorporated into the body components of the decomposing organisms. What is this process called?

A

Assimilation

55
Q

Assimilation means that…

A

Until the organisms themselves die and are mineralised those nutrients are unavailable to the plant.

56
Q

Soils containing more than 20% of OM are categorized as?

A

Organic soils.

57
Q

Soil texture classification with the inclusion of OM description.

58
Q

If the O Horizon is coley OM and is >= 40cm deep, what is the soil classed as?

59
Q

Properties of OM depends largely on?

A

Its stage of decay.

60
Q

The humus/clay relationship is known as?

A

The Clay/humus complex.

61
Q

The CEC is a key feature of the clay/humus complex. What main benefit is associated with the CEC of soils?

A

Nutrient availability.

62
Q

Humified material can hold how many times its on weight in water?

63
Q

What are the two colloidal fractions of soil?

A

Humus and Clay

(B) Soil Science (6 decks)

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