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AGRI - 51 > LECTURE 4D: SOIL WATER > Flashcards

LECTURE 4D: SOIL WATER Flashcards

1
Q

1 of most variable charac of soil

A

water content

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2
Q

reservoir for water (and nutrients); making it avail for plants when needed

A

soil

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3
Q

part of global hydrological cycle (continuous circulation of water in earth-atmosphere system)

A

soil water

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4
Q

how well soil stores water is of great importance to

A

crop production and vitality to land

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5
Q

water causes soil particles to: 3 bullets

A
  1. swell and shrink
  2. adhere to each other
  3. form structural aggregate
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6
Q

water is essential to plants for 4 reasons:

A
  1. constitutes 85% - 95% of plant’s protoplasm (physical basis of life)
  2. essential for photosynthesis
  3. solvent in which nutrients are carried into
  4. provides turgidity (keeps plants in proper position)
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7
Q

5 categories of water “surrounding” a clay or soil particle

A
  1. adsorbed water (hygroscopic water)
  2. water at permanent wilting point
  3. capillary water (water at field capacity)
  4. gravitational water
  5. chemically-combined water
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8
Q

category of water surrounding clay/soil particle: held on surface of particle by powerful forces of attraction; solid state of very small thickness (0.005um); can’t be removed by oven drying at 110C; considered as part of solid soil grain

A

adsorbed water (hygroscopic water)

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9
Q

water that can be removed by oven drying but not air drying; not so tightly held

A

water at permanent wilting point

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10
Q

what state is soil in when mesopores (2-50 nm) are filled with water and macropores are drained of water

A

soil at field capacity

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11
Q

category of water surrounding clay/soil particle: held by surface tension, generally removed by air drying

A

capillary water (water at field capacity)

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12
Q
  • category of water surrounding clay/soil particle: removable by drainage, can move in voids between soil grains.
  • flooded field will drain gravi water under influence of gravity until water’s adhesive and cohesive forces resist further drainage and finally reaches FC
  • not much use to plants
A

gravitational water

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13
Q

in form of water of hydration within chemical structure; not removable by oven drying

A

chemically-combined water

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14
Q

4 qualitative descriptions of soil wetness

A
  1. maximum retentive capacity
  2. field capacity
  3. permanent wilting point
  4. available water storage capacity (AWSC)
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15
Q
  • all soil pores are filled with water and soil is saturated
  • matric potential is close to zero and volumetric water content is same as total porosity
  • soil will remain at ________ as long as water continues to ______________, bcs water held in largest pores perlocate downward under influence of gravi forces
A

it is at maximum retentive capacity; infiltrate

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15
Q
  • moisture content of soil after gravity removed all water it can (occurs 1 - 3 days after a rain)
  • smaller pores = ; intermediate pores = remain full of water; partially filled; large pores = nearly emplty
  • plants must exert suction of 1/3 bar to obtain water at this
A

field capacity (FC)

15
Q
  • soil moisture % at which plants cannot obtain enough moisture to continue growing
  • large and medium pores = completely empty; water held very tightly by soil’s mineral and particles
  • exert suction force of ____ bars in trying to obtain water
  • although not yet dead, plants below this cannot recover even if water is provided
A

permanent wilting point (PWP); 15 bars

16
Q
  • amount of liquid water stored in soil and released for use by plants
  • varies with amount of pore spaces
  • pore spaces wider than 10 micrometers = drain more water readily; narrower than 0.2 micrometer = release retained water too slowly
  • formula?
A
  • available water storage capacity (awsc)
  • awsc = field capacity - permanent wilting point
17
Q

water moves thru soil due to 3 forces:

A
  • gravity
  • osmosis
  • capillarity
18
Q

at 0 - 1/3 bar suction, water moves thru soil due to gravity called

A

saturated flow

19
Q

at higher suction, water movement is called

A

unsaturated flow

20
Q

water flows thru ground unevenly, called _______, bcs of surface tension between water particles

A

gravity fingers

21
Q

3 main types of forces which contribute energy state of soil water

A
  1. gravitational potential
  2. osmotic potential
  3. matric potential
22
Q
  • water has positive energy and can flow out of soil through large pores
  • point when cohesive forces are not large enough to hold onto water
  • significant only when soils are saturated
A

gravitational potential

23
Q

movement of water from regions of higher potential (activity) to regions of lower potential (activity)

A

osmosis

24
Q
  • potential is due to attraction that salts have for water through higher to lower
  • attributable to presence of solutes in soil solution. greater concentration of solutes, lower _______
  • this energy is relative to free water
A

osmotic potential

25
Q
  • potential energy of water attracted to soil solids
  • operational in unsaturated soil above water table
  • plants must overcome energy of this to extract water from soil
A

matric potential

26
Q
  • tenacity which soil holds water
  • soil water increases, this decreases
  • this increases, amount of energy exerted by plant to remove water from soil also increases
  • is negative pressure and expressed in units of bars
A

soil moisture tension

27
Q

2 forces that allow water to move thru soil that act simultaneously

A
  1. gravitational forces
  2. capillary forces
28
Q

true or false: gravity pulls water downward when water it not held by capillary action and influences water in saturated soils

A

both true

29
Q

2 types of attractions of capillary action

A
  1. cohesion
  2. adhesion
30
Q

true or false: capillary forces are greater in small pores than in large pores

A

true

31
Q
  • enables plant roots to make use of water from wetter portions of soil profile (particularly from a water table)
  • true or false: smaller pores, higher water will rise ABOVE water table
  • true or false: higher rise, tighter water will be held to soil particles to overcome force of gravity
A

capillary movement of soil water

32
Q
  • faster method but prone to error since subjective
  • acceptable if observer has experience and is comfortable
  • allow for more locs to be sampled
A

hand-feel method

33
Q
  • direct measurement of soil water content
  • sample of moist soil weighed then dried, then weighed again
  • weight loss = soil water
A

gravimetric method

34
Q
  • uses small blocks of porous gypsum, nylon, or fiberglass (embedded with electrodes)
  • when placed in moist soil, absorbs water in proportion to soil moisture content
  • inexpensive and can be used to measure during one / more cropping seasons ‘
  • accuracy and range is limited
A

electrical resistance blocks method

35
Q
  • contains a source of fast neutrons and detector for slow neutrons
  • when fast neutrons collide with H atoms, neutrons slow down and scatter
  • number of slow neutrons counted corresponds to soil water content
A

neutron scattering method

36
Q
  • is a water-filled tube closed at bottom with porous ceramic cup at top with airtight seal
A

tensiometer method

AGRI - 51 (1 decks)

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