Soil water

Question 1

How can you increase the water storage capacity of a soil with low SA?

Answer 1

Adding OM to stabilize the aggregates and create more available pore space

Question 2

Is water storage capacity high, medium, or low for clays, silts, and sands?

Answer 2

Clays = high
silts = medium - high
sands = low

Question 3

What are the 2 forces involved in the movement of soils?

Answer 3

1. gravity

2. capillary action (cohesion and adhesion)

Question 4

How does water move through soil?/simple rule of physics

Answer 4

Water moves from areas of high concentration to areas of low concentration

Question 5

Rate sandy loams, loams, and clay loams from best to worst for irrigation agriculture?
why?

Answer 5

Sandy loam

loam

Clay loam

sandy loams will drain the quickest and will not become over-saturated or build up a water puddle on the surface

Question 6

Rate sandy loams, loams, and clay loams from best to worst for dry land agriculture (no irrigation)?

why?

Answer 6

clay loams

loams

sandy loams

clay loams have the best capacity to store water due to their small particle sizes and their multiple negatively charged surfaces

Question 7

For what reason are clay loams difficult to irrigate?

Answer 7

they have low infiltration rates so water will move very slowly through their small pores

Question 8

Briefly describe the issues of irrigating clay soils?

Answer 8

• not enough water movement through the soil could put water stress on plants
• water can puddle at the surface and run-off due to lack of infiltration
• the adhesion of water to clay particles is very strong and may be difficult for plants to obtain

Question 9

Define adhesion vs. cohesion?

Answer 9

Adhesion is the bond between water and soil particles

Cohesion is the bond between water molecules and other water molecules

Question 10

Describe the flow of water in a sandy soil with a clay layer in the middle

Answer 10

Water will infiltrate and percolate downward with gravity very quickly as sand particles are very large and have a small SA so water cannot easily adhere to the particles.
When water reaches the clay layer, it will immediately slow down, but because the pores of clay are very tiny, they can use capillary action to bring in water from the saturated soil above

Question 11

What is capillary action and how does it work?

Answer 11

It is adhesion and cohesion working together to move water against gravity in the soil

water flows from high to low, so when there are small soil particles (ie. clay), water will bind to itself (cohesion) and then to the particles (adhesion) to move to an area of low concentration

Question 12

T or F: sandy/coarse soils will have less capillary action?

Answer 12

TRUE because they have larger pore sizes, so the main force of water movement would be gravity

Question 13

What is soil water potential?

Answer 13

the measurement of the forces of water

it’s on a scale that moves negatively from 0

Question 14

What is pure water’s water potential?

Answer 14

zero

Question 15

T or F: the higher the water potential, the more free it is?

Answer 15

TRUE

Question 16

What is an example of a low water potential?

Answer 16

-1500 kpa

Question 17

What are the 3 forces of water potential?

Answer 17

1. matric - (MAIN FORCE)
2. osmotic -
3. gravitational +

Question 18

What is matric potential?

Answer 18

the attraction of soil water for soil particles

Question 19

T or F: Sand and silts can have the same water potential, but different levels of water content

Answer 19

TRUE
they can both be at -10kpa water potential, but finer textured soils have more pore space filled with water than a coarse textured soil, a coarse soil would have more air pore space.

fine textured soils have much greater water content than coarse

Question 20

Which soil is drier? one at -30kpa or -10kpa?

Answer 20

-30kpa

Question 21

Define unsaturated flow of water?

Answer 21

water movement in soils that are not saturated - capillary action required

Question 22

define saturated flow of water?

Answer 22

water that will move through saturated soils by gravity

Question 23

What is field capacity?

Answer 23

Matric potential and gravitational potential are balanced = the soils capacity to hold water against gravity
ideal for plant growth

Question 24

What is the kpa of field capacity?

Answer 24

around -33kpa

Question 25

What is wilting point?

Answer 25

When there is no more available water for plants and they wilt

Question 26

What is permanent wilting point?

Answer 26

When most cohesion water is gone and plants cannot get anymore water, they will die and this cannot be undone

Question 27

What is the kpa of PWP for most plants?

Answer 27

-1500kpa

Question 28

What is hygroscopic water?

Answer 28

Air dried soil, when most cohesion water has evaporated

Question 29

What is available water?

Answer 29

The water between the field capacity and wilting point

it can be absorbed by plant roots and it is gravitationally drained

Question 30

T or F: in saturated soils, capillary action dominates?

Answer 30

FALSE gravitational action will dominate in saturated soils