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Biophysical Lab Exam > Lab 5 > Flashcards

Lab 5 Flashcards

1
Q

Soil texture refers to

A

size of inorganic soil primary particles

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2
Q

what are many chemical reactions influenced by?

A

particle size range of the soil

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3
Q

What gives rise to soil structures?

A

formation of aggregates

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4
Q

What are the 2 main textural classes (based on particle size)

A
  1. Coarse particles or non-earth fraction (stone and gravel) - diameter > 2mm
  2. Fine earth (sand, silt, clay) – diameters < 2mm
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5
Q

What is changed about the nomenclature if particles > 2mm are present in significant amounts?

A

gravelly / stony

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6
Q

sand diameter

A

2.00 - 0.05

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7
Q

silt diameter

A

0.05 - 0.002

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8
Q

clay diameter

A

< 0.002

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9
Q

sand characteristics

A

Gritty feeling. Not plastic or sticky when moist. Promotes aeration in soils

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10
Q

silt characteristics

A

Smooth, powdery when dry. Not sticky when moist. Provides poor physical structure, rapid capillary rise of water.

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11
Q

clay characteristics

A

Smooth, sticky, and plastic when moist. Very hard clods when dry. Absorbs water and nutrients for biological organisms in the soil

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12
Q

How can the sand, silt and clay % composition of each separate be determined without physically separating each one out?

A

via Particle-Size Analysis by the Hydrometer Method

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13
Q

What is Particle-Size Analysis by the Hydrometer Method?

A

when mineral particles of a soil are suspended in water, they settle at different rates depending on their surface area per unit volume & their density. Clay = high surface area so settle slowly. Sand = small surface area so settle faster.

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14
Q

How can the rate at which particles settle in water be calculated?

A

stoke’s law

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15
Q

what is stoke’s law?

A

a law which governs the rate of settling of spherical particles in a viscous medium

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16
Q

what are the 2 steps in Particle-Size Analysis by the Hydrometer Method

A
  1. separation of all particles from each other

2. determination of the amounts of sand/silt/clay present in the sample

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17
Q

What are the 2 types of dispersion methods used in Particle-Size Analysis by the Hydrometer Method?

A
  1. Chemical: add sodium hexametaphosphate. This overcomes the flocculating cations (Ca, Mg) in soil
  2. Physical: rapid stirring in a milkshake machine cup
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18
Q

After placing soil + water in milkshake cup, what method did we do?

A

hydrometer method

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19
Q

Where is the hydrometer placed and what does it read?

A

placed in soil suspension and reads the average density of the soil + particles.

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20
Q

What was the hydrometer calibrated to read?

A

grams of suspended solid material per litre of solution

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21
Q

When should the hydrometer be used?

A

when sand particles (2-0.05 mm) fall below it in 40 seconds and silt particles within 8 hours

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22
Q

What happens from the moment the milkshake machine is done stirring to the moment you use the hydrometer at 40 seconds?

A

Pour soil solution into 1000 mL cylindrical tube. Add distilled water until solution reaches 1000 mL mark. A stirring plunger is pushed up and down 20 times to get all soil particles in suspension. Place hydrometer in suspensions 20 seconds before reading needs to take place

23
Q

What should you add to the soil solution if there is foam at the top after using the plunger?

A

iso-amyl alcohol

24
Q

Why is it important to record the temperature of the soil mixture as you read the hydrometer? What must you do if the temperature changed?

A

viscosity and density of water changes with temperature

for every degree above 20C add 0.36 g/L
for every degree below 20C subtract 0.36 g/L

25
Q

What does the hydrometer measure?

A

grams of solids per litre left in suspension

26
Q

After the 40 seconds and all sand has settled out, what does the hydrometer reading show?

A

grams of silt and clay left in 1 litre of suspension

27
Q

% sand =

A

(sample size – 40s cor. hydro.)/(sample size) x 100

28
Q

% clay =

A

8hr cor. hydro/sample size x 100

29
Q

% silt =

A

100 – % sand – % clay

30
Q

How much must you wet your soil sample when determining its texture by hand?

A

consistency of putty

31
Q

Determining Texture by Hand: what does each soil type form?

  1. sand/sandy loam
  2. Loams
  3. Silt loams
  4. Clay loams
  5. Silty clay loams
  6. Clays
A
  1. a cast that is easily broken, have gritty feeling
  2. a cast upon squeezing that can be handled freely without breaking, considerable grittiness
  3. a cast that can be handled freely, moist soil has a soapy feeling
  4. the cast is quite malleable, resistant to breaking
  5. as in clay loams (quite malleable, resistant to breaking), but a soapier feel
  6. cast is plastic, sticky, and highly malleable
32
Q

How does soil organic matter affect the plasticity of soils?

A

reduces it

33
Q

Ah horizons normally have what structure?

A

granular - highly porous

34
Q

What structure is considered best for most plants?

A

granular - highly porous

35
Q

How are soil aggregates formed?

A

when primary particles are drawn together and cemented into clumps

36
Q

What is the major stabilizing structure of soil aggregates?

A

colloidal fraction - act as binding agents for primary particles

37
Q

What are the 3 main groups of colloidal matter that act as binding agents for primary particles? (most important to least)

A
  1. colloidal organic matter (including microbial gums)
  2. colloidal oxides of iron and aluminum
  3. clay minerals
38
Q

Why are medium sized, water-stable soil aggregates important in fine and coarse-textured soils? (4)

A
  1. increase porosity
  2. aid in soil aeration
  3. facilitate downward movement of water in the profile
  4. increase the resistance of the soil to wind and water erosion
39
Q

What did Tisdall and Oades develop?

A

a hierarchical model of how soil aggregates are constructed and stabilized

40
Q

According to Tisdall and Oades, what are larger stable aggregates made of?

A

smaller aggregates

41
Q

What size are the smaller aggregates (micro aggregates) that make up the larger ones?

A

2 um - 250 um (diameter)

42
Q

How are the smaller aggregates that make up the larger aggregates stabilized?

A

humic compounds

43
Q

what are humic compounds?

A

persistent organic materials from microbial processed plant and microbial debris which are resistant to further degradation

44
Q

What are particles < 2 um diameter held together by?

A

electrostatic bonding and cation bridging

45
Q

what happens to micro aggregates above 250 um diameter?

A

they are packed into macro-aggregates by network of fungal hyphae and fine roots

46
Q

What does the stability of macro-aggregates depend on?how much time can it take to break this down? build back up?

A

fungal mycelium and fine roots

1 growing season, but can be quickly increased by establishment of plants producing abundant fine roots and mycorrhizal mycelium

47
Q

What does the stability of micro-aggregates depend on? how much time can it take to break this down?

A

organic compounds and electrostatic forces

several decades

48
Q

What is the most abundant fungi in soils?

A

mycorrhizal fungi

49
Q

What is the function of mycelium? What produces them?

A

mycorrhizal fungi produces mycelium

function: form an extensive web entangling and tying together micro-aggregates on the top 10-15 cm soil layer

50
Q

What is the most well known method of encouraging soil granulation?

A

add OM

51
Q

What do bacteria and fungi produce as they decompose OM?

A

polysaccharide gums

52
Q

What does the decomposition of plant roots produce?

A

microbial gums

53
Q

Why does the action of earthworms have a large effect on soil aeration and structural stability?

A

because they secrete mucus on the walls of their tunnels as they burrow

54
Q

reread plage 46

A

reread page 46

Biophysical Lab Exam (5 decks)

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