Properties Of The Liquid Fraction (Soil Water)

Question 1

What does water do within the plant?

Answer 1

Acts as a structural constituent
Acts as a solute for biochemical reaction
Acts as a transport medium for nutrients and metabolites
Acts as a constituent in photosynthesis
Water is involved in hydration
Water is involved in transpiration

Question 2

What does water do within the soil?

Answer 2

Solvent for various minerals to dissolve e.g. fertilisers, soil and rock minerals

Question 3

What effect does gravity have on water in soils?

Answer 3

Natural tendency to drain through soils

Question 4

What are the two forces that effect water retention in soils?

Answer 4

Adhesion - attraction of water molecules to soil solid surfaces.
Cohesion - attraction of water molecules for each other because of hydrogen bonds between each water molecule.Allows successive layers of water to be absorbed onto the solid surface.As a water film gets thicker the force of cohesion decreases and gravity can remove the water from the solid surface, that is, the water closest to the solid surface is held strongly and each successive layer from the surface is held less strongly.

Question 5

What does water retention in small pores do?

Answer 5

In small pores filled with water, the strength of cohesion be- tween water molecules (x) at the centre of the pore and the neighbouring water molecule is greater than the force exerted by gravity. Therefore the pore shall remain full of water and shall not free drain

Question 6

What does water retention in large pores do?

Answer 6

In large pores which are filled with water, the strength of the cohesion between water molecules at the centre and their neighbour water molecules is less than the force of gravity. Therefore this molecule will move out of the pore under gravity

Question 7

What are the two points to remember on small and large soil pores?

Answer 7

• small pores hold water

* large pores hold mainly air with some water around the out- side.

Question 8

When is the soil saturated?

Answer 8

Initially, if water can be added to soil fast enough so that all pores are filled with water and none contain air,
the soil is said to be ‘Saturated’.

Question 9

What is meant by Field Capacity?

Answer 9

The water will drain until a point is reached where the maximum amount of water which can be retained against gravity occurs (rapid drainage takes 1-3 days depending on texture). At this point, gravitational forces become equal in magnitude to cohesive forces and drainage ceases.
The soil is at ‘Field Capacity’.

Question 10

What is meant by the wilting point?

Answer 10

When plants are present, water will be gradually removed from the soil. The layer of water most distant from the solid
surface is held weakly and taken up by plants easily. Because the strength of retention increases as each successive layer of water is removed, more energy is required for uptake of the water. A moisture content is reached where the energy re- quired to remove the water from the soil solid is greater than that which can be exerted via the plant. At this content, no more water can be removed from the soil by the plant and the plant will wilt.
The soil is said to be at ‘Wilting Point’.

Question 11

What is meant by plant available water?

Answer 11

The quantity of water held in the soil between field capacity
and wilting point is called the ‘Plant Available Water’.

Question 12

What is meant by permanent wilting point?

Answer 12

Permanent Wilting Point ? a water content at which plants wilt and cannot re- cover, that is, they die.
This shows that water is still in the soil at permanent wilting point. In fact for many soils about half the water in the soil at field capacity is left in the soil when plants cease to grow.

Question 13

What is meant by soil structure?

Answer 13

The structure relates to the way in which individual particles are grouped together and how groups of particles are them- selves grouped into larger aggregates. This means that a well aggregated (or structured) soil will have a greater internal sur- face area than it would if it were poorly structured where its particles were closely packed. In the well structured condi- tion, the soil would have a higher percentage pore space that would lead to a greater ability to hold water.

Question 14

How does organic matter effect the holding capacity of water in soil?

Answer 14

The presence of organic matter has both an indirect effect and a direct effect. Indirectly, organic matter can increase a soil’s water holding capacity through any effect the organic matter will have on the soil structure in the long term. Organic mat- ter has a direct effect in that it itself can hold many times its own weight of water. Peats are used in potting mixes to im- prove water retention

Question 15

What is meant by soil texture?

Answer 15

Texture can give an indication of the presence of micropores. The finer the texture, the finer the pores and the higher the percentage of micropores.

Question 16

What is Soil Water Content?

Answer 16

The soil water content or concentration in the soil is ex- pressed as either, on a weight (gravimetric) or volume basis.

Question 17

What is moisture content by weight?

Answer 17

Moisture content on a weight basis is the weight of water per weight of soil solids, for example, kilograms of water per 100 kilograms of oven dry soil (the term ‘oven dry soil’ or O.D soil is used.

Question 18

What is a Tensionmeter?

Answer 18

A tensiometer consists of a hollow tube, usually perspex. One end is covered with a porous ceramic tip with the large pores being able to hold water at a potential of about - 100 kPa. The top has a screw cap to enable the tube to be filled with water. In the side at the top a vacuum gauge is inserted.

Question 19

What is the Water Potential Gradient?

Answer 19

Water potential gradient is the force responsible for moving the water through soil. The water potential gradient, (d /dx), is the change in water potential (d ) per unit dis- tance (dx) along the direction of flow

Question 20

What is Hydraulic Conductivity?

Answer 20

A measure of the ease of flow water through the soil at a given soil water potential.

Question 21

What so soils with low hydraulic conductivity have?

Answer 21

Soils with low hydraulic conductivity generally have:
• mainly of small pores;
• little continuity between pores; and
• tortuosity is high (the pores are not linear but are quite twisted).

Question 22

What so soils with high hydraulic conductivity have?

Answer 22

• a high proportion of large pores (macropores);
• good continuity of pores;
• low tortuosity.

Question 23

What is the direction of water flow in the soil?

Answer 23

Water can move in soil in any direction depending on which forces are strongest, that is, adhesion/cohesion or gravity.
If gravity is stronger, movement is normally downward.
If adhesion/cohesion is stronger, movement can be in any direction depending on the direction of the change in water po- tential. The bigger the difference in water potential at the wet- ting front, the faster the movement. This means water moves quickly from very wet to very dry soils but slowly from moist
to slightly moist soil.

Question 24

How does water react in saturated soils?

Answer 24

In saturated soils, water is very weakly held in pores so the force exerted on some of the water molecules by gravity is greater than the strength by which they cohere to other water molecules (cohesion). Under these conditions, the movement of water is normally downward.

Question 25

How does water react in unsaturated soils?

Answer 25

In unsaturated soils, the forces of adhesion/cohesion are stronger than gravity and hence the movement of water can
be in any direction depending on the direction of the water potential gradient. For example if the surface layer of soil is drier (lower water potential) than sub surface layer (high water po- tential) then water will move upward from the wetter layer to the drier surface layer. Also the larger the difference in water potential at the wetting front, the faster the movement of wa- ter. This means that water flows quickly from very wet to very dry soils but slowly from moist to slightly moist soil.

Question 26

What does the term infiltration mean?

Answer 26

Infiltration is the term used for the entry of water into soil. This water may be derived from either rainfall, irrigation or runon. The ability of a soil to accept water is an important property of soil and is particularly useful in determining the potential for a soil to be used for irrigation, or agricultural pro- duction in areas of high rainfall.

Question 27

Why does the infiltration decreases in soils?

Answer 27

• As time progresses, the soil become wet and so the water potential gradient decreases over time; and/or
• As the soil wets up, soil structural breakdown may occur. This results in smaller aggregates or particles clogging water conducting pores, which reduces the hydraulic conductivity.

Question 28

What is redistribution of water in soil?

Answer 28

After water has infiltrated the soil, it continues to move slowly as long as water potential gradients exist. This slow move- ment is known as redistribution and occurs as soils wet up or, when plant roots remove water from a pore, and water redis- tributes from within an aggregate to the pore in response to the removal by the plant.

Question 29

How does evaporation play a part in water in soils?

Answer 29

Water can move upward through a soil and be lost from the surface via evaporation. Under these conditions, water rises from a wet subsurface layer to a drier layer driven by a change in water potential.

Question 30

How does atmospheric factors control evaporation?

Answer 30

Evaporation is the process by which water vapour returns to the atmosphere from the soil. The rate of evaporation is de- pendent on;
• the input of energy into the system. The main source of heat energy for evaporation is solar radiation • the capacity of air to absorb the water. The capacity for air to take up water vapour increases with its dryness (low humidity)

Question 31

How does wind affect evaporation in soils?

Answer 31

Wind to remove the water vapour from the surface. Air above the soil can become saturated. Further evaporation de- pends on wind to move the saturated air away, so evaporation increases with wind velocity.

Question 32

How is evaporation of wet soil governed?

Answer 32

The rate of evaporation from wet soil is governed by the differ- ence in water potential between the soil and the atmosphere. As the soil dries, particularly in areas with high evaporative rates, the rate of water loss soon exceeds the rate of upward water flow from the deeper soil. The surface soil becomes air-dry. As the larger pores become dry first, the hydraulic conductivity (K) decreases rapidly and more than offsets the increase in the potential gradient (d dx). Under these condi- tions, the rate of evaporation is controlled by soil properties and decreases with time.

Question 33

What if there is no water table in the soil?

Answer 33

Where there is no water table in the soil, water rarely moves to the surface from depths greater than 30-50 cm (dependent on soil structure and texture).

Question 34

What if there is a water table in soil?

Answer 34

Where a water table is present, water can move upward from a depth of between 1 and 3 m (sand and clays respectively). Capillary rise is very dependent on the pore size and pore size distribution of the soil, however a water table within reach of the soil surface can lead to excessive evapora- tive losses. It is this process that is responsible for the salinisation from salty water tables.

Question 35

What is meant by water balance in soils?

Answer 35

Water comes to the soil via precipitation (rain, irrigation and snow).
Of the rain reaching the soil surface;
• some is lost from the soil surface by evaporation (E);
• some infiltrates the soil (infiltration, I)
•any remaining on the surface runs off (surface runoff,
RO).
Some of the water that infiltrates moves downward to the ground water (deep percolation, DP).

Question 36

What is evapotranspiration?

Answer 36

The combined loss from soil by soil evaporation plus plant transpiration is called evapotranspiration (ET).