Soil Water Concepts in Agriculture

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Questions and Answers

What is the term used for the upward movement of a liquid in narrow spaces due to capillary action?

Capillary Tension

Capillary Rise (correct)

Capillary Flow

Capillary Action

How is the available water in soil defined?

All water present in the soil

Amount of water stored and usable by plants (correct)

Total water a plant can absorb

Water retained regardless of plant absorption

What determines the variation in available water in soil?

Pore space size (correct)

Amount of organic matter

Soil density

Soil temperature

Which equation represents the relationship for available water (AW) in soil?

AW = Field Capacity - Permanent Wilting Point Signup and view all the answers

What is the main factor enabling capillarity in soil?

Adhesion and cohesion forces Signup and view all the answers

What is hygroscopic water primarily defined by?

Water held on the surface of soil particles by electrical forces Signup and view all the answers

Which category of water is defined as water that cannot be removed by air drying?

Water at permanent wilting point Signup and view all the answers

What characterizes gravitational water in soil?

It can be drained by gravity and is not useful to plants Signup and view all the answers

What is the definition of capillary water?

Water retained in the soil due to surface tension Signup and view all the answers

How is chemically-combined water uniquely characterized?

It is found within the crystal structure and not removable by conventional drying Signup and view all the answers

What primarily allows water to rise in small soil pores despite gravity?

Capillary forces Signup and view all the answers

Which type of water movement occurs due to gravity at low suction levels?

Saturated flow Signup and view all the answers

What causes water molecules to hold onto each other in soil?

Cohesion Signup and view all the answers

Which force is greater in smaller soil pores compared to larger ones?

Capillary force Signup and view all the answers

Which of the following factors contributes to the energy state of soil water?

Matric potential Signup and view all the answers

In which type of soil texture are the capillarity effects expected to be the lowest?

Fine sand Signup and view all the answers

How do plant roots primarily access water from the soil profile?

Through capillarity from wet soil layers Signup and view all the answers

What can increase as the size of soil pores decreases?

Water retention ability Signup and view all the answers

What happens to water when cohesive forces in soil are insufficient?

Water flows out of the soil through large pores. Signup and view all the answers

How does osmosis relate to osmotic potential in soil?

Water moves from regions of higher potential to regions of lower potential. Signup and view all the answers

What is matric potential primarily associated with?

The attraction of water to soil solids. Signup and view all the answers

Which of the following most accurately describes the energy state of water in relation to permeability?

Water moves toward lower energy states, minimizing energy loss. Signup and view all the answers

What factor contributes to the effectiveness of water infiltration into soils?

Soil texture and structure. Signup and view all the answers

What must a plant root overcome to extract water from moist soil?

All three forces: gravitational, osmotic, and matric potential. Signup and view all the answers

When is gravitational potential most significant?

When soils are saturated. Signup and view all the answers

How does an increase in solutes affect osmotic potential?

It decreases osmotic potential. Signup and view all the answers

What is the primary limitation of the electrical resistance blocks method for measuring soil moisture?

Limited accuracy and range of moisture content Signup and view all the answers

How does a neutron scattering probe measure soil water content?

By counting slow neutrons that collide with H atoms Signup and view all the answers

What is the main function of the porous ceramic cup in a tensiometer?

To allow water to move into the soil Signup and view all the answers

Which statement is true about the operation of a tensiometer?

It allows for continued water movement until equilibrium is reached. Signup and view all the answers

What type of neutrons are used in a neutron scattering method to measure soil water content?

Fast neutrons Signup and view all the answers

Why might one choose to use electrical resistance blocks for soil moisture measurement despite their limitations?

They are inexpensive and suitable for basic estimation. Signup and view all the answers

What does the tensiometer measure in relation to soil?

Matric water potential Signup and view all the answers

Which of the following best describes the interaction between fast neutrons and hydrogen atoms in the neutron scattering method?

Fast neutrons slow down and scatter upon collision. Signup and view all the answers

Study Notes

Soil Water

Five categories of water in the soil:
- Adsorbed Water (Hygroscopic Water): tightly held by soil particles due to electrical attraction.
- Water at Permanent Wilting Point: water that can be removed by oven drying but not by air drying.
- Capillary Water (Water at Field Capacity): held by surface tension. Removeable by air drying.
- Gravitational Water: Water that drains under the influence of gravity; not readily available to plants.
- Chemically-Combined Water: Water within the crystal structure of soil minerals, not removable by oven drying.

Soil Water Content

Available Water: Water stored in the soil that plants can use.
Available Water Calculation: Field Capacity - Permanent Wilting Point

Capillarity

Refers to the ability of a liquid to flow against gravity.
Due to adhesion (water molecules attracted to soil particles) and cohesion (water molecules attracted to each other).
Capillary Rise: The upward movement of water in small pores due to capillary action.
Capillary Forces: Stronger in smaller pores.

Water Movement in Soils

Saturated Flow: Water movement through soil due to gravity when the soil is saturated.
Unsaturated Flow: Water movement at higher suction pressures, driven by capillary action.

Energy State of Soil Water

Gravitational Potential: Water's ability to flow downward due to gravity; significant when soil is saturated.
Osmotic Potential: Water's tendency to move from regions of higher water potential to lower potential due to the presence of solutes.
Matric Potential: Water's potential energy due to its attraction to soil solids; operational in unsaturated soil.

Soil Water Content Measurement

Electrical Resistance Blocks Method: Inexpensive method to measure approximate changes in soil moisture, but limited accuracy.
Neutron Scattering Method: Uses a neutron scattering probe to measure soil water content by measuring the scattering of neutrons by hydrogen atoms.
Tensiometer Method: Measures matric water potential by measuring the tension in a water-filled tube with a porous ceramic cup in the soil.

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Description

Explore the different categories of water in soil and their significance in agriculture. This quiz covers topics such as adsorbed water, capillary water, and water availability for plants, providing a comprehensive understanding of soil water dynamics.