LECTURE 4D: SOIL WATER

Question 1

1 of most variable charac of soil

Answer 1

water content

Question 2

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

Answer 2

soil

Question 3

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

Answer 3

soil water

Question 4

how well soil stores water is of great importance to

Answer 4

crop production and vitality to land

Question 5

water causes soil particles to: 3 bullets

Answer 5

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

Question 6

water is essential to plants for 4 reasons:

Answer 6

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)

Question 7

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

Answer 7

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

Question 8

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

Answer 8

adsorbed water (hygroscopic water)

Question 9

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

Answer 9

water at permanent wilting point

Question 10

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

Answer 10

soil at field capacity

Question 11

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

Answer 11

capillary water (water at field capacity)

Question 12

• 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

Answer 12

gravitational water

Question 13

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

Answer 13

chemically-combined water

Question 14

4 qualitative descriptions of soil wetness

Answer 14

1. maximum retentive capacity
2. field capacity
3. permanent wilting point
4. available water storage capacity (AWSC)

Question 15

• 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

Answer 15

it is at maximum retentive capacity; infiltrate

Question 15

• 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

Answer 15

field capacity (FC)

Question 15

• 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

Answer 15

permanent wilting point (PWP); 15 bars

Question 16

• 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?

Answer 16

• available water storage capacity (awsc)
• awsc = field capacity - permanent wilting point

Question 17

water moves thru soil due to 3 forces:

Answer 17

• gravity
• osmosis
• capillarity

Question 18

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

Answer 18

saturated flow

Question 19

at higher suction, water movement is called

Answer 19

unsaturated flow

Question 20

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

Answer 20

gravity fingers

Question 21

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

Answer 21

1. gravitational potential
2. osmotic potential
3. matric potential

Question 22

• 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

Answer 22

gravitational potential

Question 23

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

Answer 23

osmosis

Question 24

• 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

Answer 24

osmotic potential

Question 25

• 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

Answer 25

matric potential

Question 26

• 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

Answer 26

soil moisture tension

Question 27

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

Answer 27

1. gravitational forces
2. capillary forces

Question 28

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

Answer 28

both true

Question 29

2 types of attractions of capillary action

Answer 29

1. cohesion
2. adhesion

Question 30

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

Answer 30

true

Question 31

• 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

Answer 31

capillary movement of soil water

Question 32

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

Answer 32

hand-feel method

Question 33

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

Answer 33

gravimetric method

Question 34

• 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

Answer 34

electrical resistance blocks method

Question 35

• 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

Answer 35

neutron scattering method

Question 36

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

Answer 36

tensiometer method