Water movement in the soil

Summary

This educational video script explores the dynamics of water movement in soil, emphasizing the role of potential differences and soil structure. It uses time-lapse photography to illustrate capillarity, the forces of adhesion and cohesion, and how they counteract gravity. The script also discusses the impact of soil types, such as sandy and clay loams, on water infiltration and retention. It highlights how soil layers, such as coarse sand or clay pans, can act as barriers to water flow, affecting agricultural practices and soil management. The demonstrations serve to educate on the principles of unsaturated water flow, crucial for understanding irrigation, drainage, and soil conservation in agriculture.

Takeaways

• 💧 Water movement in soil is driven by potential differences, with water moving from areas of high potential to low potential.
• 🌱 The potential at any point in soil is influenced by forces such as gravity and capillarity, which cause water to move downward or upward.
• 🔍 Time-lapse photography in the video demonstrates how water moves into soil, showing processes that would take hours in nature in just minutes.
• 📏 The model used in the video, with soil between glass plates, represents a vertical cross-section of soil and allows for visualizing water movement.
• 🌿 Capillarity is a key principle where water moves into dry porous materials due to the attraction of solid mineral surfaces and cohesion of water molecules.
• 🏺 The height to which water rises in capillary action is influenced by the closeness of surfaces and the internal tension in the water.
• 🌳 Soil texture greatly affects water infiltration and retention, with sandy soils allowing more rapid infiltration but less retention compared to clay soils.
• 🌾 Sandy soils are better for irrigated areas due to their good infiltration properties, while clay soils are more suitable for dryland farming due to their water retention.
• 🚧 The presence of layers such as coarse sand or fine clay in soil can significantly impact water movement, acting as barriers or check valves.
• 🌱 The structure of soil, including the presence of aggregates and organic matter, plays a crucial role in water infiltration and the soil's ability to support plant growth.
• 🌱 Proper soil management, such as maintaining good tilth and avoiding compaction, can enhance water infiltration and reduce erosion.

Q & A

What determines the movement of water in soil?

Water movement in soil is determined by the distribution of potential within the soil profile, with water tending to move from areas of higher potential to areas of lower potential.

What happens if there is no potential difference in the soil?

If there is no potential difference, no water movement will occur.

What are the main forces that affect water potential at any point in the soil?

The main forces affecting water potential are gravitational and matric forces, which cause water to move downward due to gravity and upward due to the attraction of solid surfaces, known as capillarity.

How does the soil model in the video represent a real-world scenario?

The soil model, held between glass plates, represents a vertical cross-section of soil, allowing viewers to observe water movement as it would occur in nature, albeit accelerated due to time-lapse photography.

What principle is demonstrated when water rises between two closely spaced plastic plates?

The principle of capillarity is demonstrated, where water is pulled upward against gravity due to the adhesive forces between the plastic and water and the cohesive forces between water molecules.

Why does the height of water rise differ when plastic plates are pinched together?

The height of water rise is greatest when the plastic plates are pinched tightly together due to the increased adhesive forces and the resulting internal tension in the water.

How does the size of soil pores affect the rate of water flow and retention?

The finer the soil pores, the more restricted the rate of water flow, and the greater the water retention. Sandy soils have larger pores and allow for greater initial penetration, while clay soils with finer pores retain water more effectively.

What is the significance of soil texture in irrigation and agricultural practices?

Soil texture is significant because it affects the infiltration properties and water-holding capacity, which in turn influence irrigation needs and the suitability of the soil for different agricultural practices.

How does a layer of coarse sand in soil affect water movement?

A layer of coarse sand can act like a check valve, holding water back until the overlying soil becomes very wet, at which point it allows excess water to pass through, demonstrating the principle of unsaturated flow.

What is the impact of a fine clay layer within otherwise uniform soil?

A fine clay layer can restrict root growth and water penetration, often leading to water table buildup above such layers. It can impose limitations on agricultural use due to its resistance to water flow and its impact on soil drainage.

How do soil aggregates and their pores influence water movement?

Soil aggregates with large pores can transmit water readily under saturated conditions, but under unsaturated conditions, water moves into and through the soil due to the attraction of solid surfaces, with the rate of movement being restricted by the number of contacts between aggregates.

Outlines

00:00

💧 Water Movement in Soil

This paragraph discusses the principles governing water movement in soil. Water in soil moves due to potential differences, influenced by forces like gravity and capillarity. The video uses a model with soil between glass plates to demonstrate these principles. The model represents a vertical cross-section of soil, and time-lapse photography is used to speed up the observation of water movement. The soil used in the demonstration is air-dried loam with sand, clay, and aggregates to simulate non-uniform bodies. The video illustrates capillarity, where water is pulled into dry porous materials due to the attraction of solid mineral surfaces. The height of water rise is greatest when plastic plates are closely spaced, demonstrating tension in the water. The paragraph also compares the water retention and transmission abilities of sandy and clayey soils, explaining their suitability for different agricultural practices.

05:02

🌱 Soil Stratification and Water Flow

This paragraph explores how water enters the soil and the role of soil stratification in water movement. It emphasizes that water movement is influenced more by the attraction of solid surfaces than by gravity alone. As the soil becomes wetter, gravity plays a more significant role. The video demonstrates how a layer of coarse sand acts as a check valve, holding water back until the soil is very wet. The paragraph also discusses how soil layers, such as a clay pan, can restrict root growth and water penetration, leading to water table buildup. The text compares different soil types and their impact on drainage and plant growth, using examples from agricultural land in Belgium. It concludes by explaining how the principles observed in the model apply to field soils with layers of sand and gravel.

10:25

🌾 Water Flow and Agricultural Practices

This paragraph delves into the practical applications of water flow principles in dryland agriculture. It explains how water movement in soil is affected by the size of soil pores and the presence of free water or water under pressure. The video shows how soil aggregates and their contact points influence water movement, with large pores facilitating rapid water entry under saturated conditions. The paragraph contrasts the effects of organic matter on soil structure, with well-aggregated soil allowing for better water infiltration compared to soil with channels or cracks. It also discusses the placement of tile drains for effective water management, emphasizing the need for them to be below the water table in wet soils. The video concludes by demonstrating how certain soil conditions, such as a straw layer, can hinder water infiltration and increase soil erosion, highlighting the importance of understanding water flow principles for sustainable agriculture.

15:37

🌱 Unsaturated Flow and Agricultural Land

This final paragraph summarizes the principles of unsaturated water flow in soil and porous materials. It reiterates that water movement is driven by the attraction of solid surfaces for water and the cohesive forces between water molecules. The paragraph emphasizes that the nature of water movement is dependent on the characteristics of the soil pores and the porosity of the soil. The video demonstrates how water moves differently under unsaturated conditions compared to saturated conditions, with the former being influenced by the tension in the soil. The paragraph concludes by stating that these principles are applicable to agricultural land where crops are grown, and understanding them is crucial for effective water management in farming practices.

Keywords

💡Water movement

Water movement in soil is a fundamental concept in the video, referring to the process by which water travels through the soil profile. This movement is driven by differences in potential energy within the soil, typically from areas of high potential to areas of low potential. The video explains that without a potential difference, water movement does not occur. This concept is crucial for understanding irrigation, drainage, and the natural water cycle within soil ecosystems.

💡Potential

Potential, in the context of the video, refers to the energy status that influences the direction and rate of water movement in soil. It is determined by various forces, including gravitational and matric forces. The video illustrates how water moves from areas of higher potential to lower potential, and how this concept is key to understanding water distribution in soil. For instance, water tends to move downward due to gravity and upward due to the capillary action caused by the attraction of solid surfaces.

💡Capillarity

Capillarity is the upward movement of water against gravity due to the adhesive forces between water and solid surfaces, and the cohesive forces between water molecules. The video demonstrates this principle by showing how water rises between two closely spaced plastic plates. This phenomenon is critical in the context of water movement in soil, especially in dry porous materials where water is pulled into the soil by the attraction of solid mineral surfaces.

💡Adhesion and Cohesion

Adhesion is the attraction between different types of molecules, such as between water and solid surfaces, while cohesion is the attraction between similar types of molecules, like water molecules for each other. These forces are responsible for capillary action, as explained in the video, where water moves upward against gravity. The script uses the example of water rising between plastic plates to illustrate how adhesion and cohesion work together to move water.

💡Soil texture

Soil texture refers to the relative proportions of different-sized mineral particles (sand, silt, and clay) in the soil. The video discusses how soil texture affects water infiltration and retention. Sandy soils with larger pores allow water to penetrate deeply, whereas clay soils with finer pores retain more water but limit infiltration. The video uses time-lapse photography to show the differences in water penetration in soils of varying textures, highlighting the importance of soil texture in agricultural practices.

💡Infiltration

Infiltration is the process by which water enters the soil surface and moves into the soil. The video explains that the rate of infiltration depends on soil texture and structure. Sandy soils typically have higher infiltration rates due to their larger pore spaces, while clay soils have lower rates because of their smaller pores. The video uses the example of water moving out from an irrigation furrow to illustrate how the movement is influenced more by the attraction of solid surfaces rather than gravity alone.

💡Stratification

Stratification in soils refers to the layering of different soil textures or compositions. The video discusses how stratification can affect water movement and retention. For example, a layer of coarse sand can act as a check valve, holding water back until the overlying soil is very wet. This concept is important for understanding how water moves through complex soil structures and how it can impact agricultural practices and soil management.

💡Water table

The water table is the upper surface of the zone of saturation in the soil, where all the pores are filled with water. The video explains how the presence of certain soil layers, such as a clay pan, can affect the water table by restricting the downward movement of water. This can lead to the buildup of water tables above the land surface during wet seasons, which can impose limitations on agricultural use. Understanding the water table is crucial for managing water resources in agricultural and environmental contexts.

💡Aggregates

Soil aggregates are groups of soil particles held together by organic materials, such as plant roots or microorganisms. The video discusses how the presence of aggregates can influence water movement in soil. Aggregates contain fine pores that can hold water, but the large pores between aggregates can restrict water flow. The video uses the example of a layer of coarse sand and aggregates to illustrate how water moves through these structures, emphasizing the importance of soil aggregation for water infiltration and retention.

💡Erosion

Erosion is the process of wearing away rocks and soil by natural agents like wind, water, and ice. The video connects soil structure and water movement to erosion, particularly in the context of agricultural land. It explains how certain soil practices, such as creating aggregates, can help reduce erosion by improving water infiltration and reducing surface runoff. Conversely, the presence of channels or cracks in the soil can accelerate erosion by promoting water runoff and reducing the soil's ability to hold water.

💡Unsaturated flow

Unsaturated flow refers to the movement of water through soil when not all the pores are filled with water, which is the typical condition in agricultural soils. The video explains that unsaturated flow is driven by the attraction of solid surfaces for water and the cohesive forces between water molecules. This concept is important for understanding how water moves in soils under normal conditions, where water is held in the soil by tension rather than by positive pressure.

Highlights

Water movement in soil is driven by potential differences, with water moving from higher to lower potential areas.

The potential at any point in soil is influenced by gravitational and matric forces, causing water to move downward due to gravity and upward due to capillarity.

A time-lapse photographic process is used to observe water movement in soil, accelerating natural processes for easier viewing.

Demonstrations show water rising between plastic plates due to adhesive and cohesive forces, illustrating the principle of capillarity.

The height of water rise due to capillarity is greatest when plastic plates are closely spaced, demonstrating the effect of tension.

Different soil textures (sand, silt, clay) exhibit varying abilities to transmit and retain water, with sand allowing the deepest penetration and clay the best retention.

Sandy soils have good infiltration properties but poor water retention, making them suitable for irrigated areas.

Clay soils are difficult to irrigate due to low infiltration rates but are suitable for dryland farming due to their water retention capabilities.

The rate at which water enters the soil is crucial for designing irrigation systems and cultural practices for erosion control.

Water movement in soil is primarily due to the attraction of solid surfaces rather than gravity, especially when the soil is not completely saturated.

A coarse sand layer in soil acts like a check valve, holding water back until the soil above is very wet, then allowing excess to pass through.

Soil layers with sand and gravel can affect drainage and plant growth, as seen in Belgian soils with loam overlying brolan sands.

A fine clay layer in soil can restrict root growth and water penetration, leading to water table buildup if present at shallow depths.

The resistance to water flow in fine pores of restrictive layers is so great that little water is transmitted through them over weeks or months.

Coarse materials with large pores aid in water movement only under conditions of free water or positive water pressure.

Principles of water flow are applied to Dryland agriculture, showing the importance of proper soil tilth and organic matter for water infiltration.

Channels or cracks in soil do not assist in water movement unless connected to a source of free water, which is also true for tile drains.

A straw layer can check downward water flow, leaving more water to run off the surface and making the soil more vulnerable to erosion.

Unsaturated flow of water in soil is driven by the attraction of solid surfaces for water and the cohesion of water molecules.

Transcripts

water movement in a soil is determined

by the distribution of potential within

the soil

profile water tends to move from areas

of higher potential to ones of lower

potential

if there is no potential difference no

movement will occur the potential at any

point results from the action of

different forces mainly gravitational

and matric water tends to move downward

due to gravity and upward due to

attraction of Solid Surfaces called

capillarity through a series of small

experiences this video will show the

basic principles governing water flows

in these demonstrations soil is held

between glass plates so that you can

watch what happens as the soil is wetted

the glass plates are 30 cm high and 60

cm wide with about 2.5 cm of space

between for soil think of this model as

representing a vertical cross-section

through the soil thanks to time-lapse

photographic processes actions that

would require many hours in nature will

be observable in just a few minutes

using a motion picture camera single

pictures are taken every second as the

water moves into the soil the completed

Motion Picture film is then projected

and speeded up 25

times because water movement is usually

very rapid when water is first applied

and very slow at later times the speed

up Factor might change during a sequence

this will be indicated on the screen the

soil used is an air dried LOM which has

been passed through a fine screen sand

clay and Aggregates are used to simulate

non-uniform bodies in a salt profile

water will be added in a Farrow with a

funnel connected to a to which keeps the

water level at any desired

depth before the sequences using

time-lapse photography here is a

demonstration that illustrates the

principle of

capillarity this principle is involved

when water moves into dry porous

materials liquid is pulled because of

the attraction of solid mineral surfaces

for water adhesion and attraction of

water molecules for each other cohesion

adhesive and cohesive forces are

responsible for moving water upward

against the downward force of gravity in

this demonstration water rises between

two closely spaced plastic plates

because of the adhesive forces between

plastic and water and cohesive forces

between water

molecules the height of Rise is greatest

when the plastic plates are pinched

tightly

together the pressure in the water above

the free water surface in the container

is less an atmospheric pressure this is

called

tension the higher water rises the

greater the internal

tension now to the models and time-lapse

pictures

before analyzing the problems of

stratification it is important to

illustrate differences among uniform

Souls with respect to their ability to

transmit and retain

water note that the depth of penetration

at any given time is greatest for the

sandom which has the largest pores and

least for the clay which has the finest

pores the finer the pores the more the

rate of water flow is

restricted retention of water after the

source is removed is greatest in a clay

Loom which has the finest pores and

least in a Sandy Loom despite this

however the net useful storage is

greatest in a clay Loom and least in a

Sandy Loom although Sandy Loom retains

less use for water than does the clay

LOM it is a good soil in an irrigated

area where lack of water holding

capacity can be compensated by

irrigation the infiltration properties

are generally good clay Looms on the

other hand are often difficult to

irrigate because of low infiltration

rates in dry climates where there is no

irrigation and Sandy Loom would not hold

enough water to carry agricultural

plants through the growing season a clay

Loom by contrast would retain more water

over a longer period of time hence

dryland farming on fine Tex Ed soils is

practical the rates at which water would

enter the soil is an important factor to

consider when designing an irrigation

system or when deciding on cultural

practices for use and erosion control

watch as the water moves out from an

irrigation Farrow note that the movement

outward is almost as great as that

downward this is added evidence that the

force responsible for this type of water

movement is mainly due not to

gravitation but to the attraction of

Solid Surfaces as the soil becomes

wetter and wetter however gravitation

plays a stronger role and if the soil

becomes comes completely saturated then

gravitational forces

predominate the horizontal layer you see

is coarse sand one of the important

principles of unsaturated flow is

described as you witness what happens as

the wedding front Encounters this layer

of coarsed

sand the pores in the soil are many

times smaller than those between sand

grains water is held in these small

pores by large adhesive and cohesive

forces the pores in the oil are like the

pores in a piece of blooding paper used

to soak up ink the huge pores in the

sand cannot hold water at the tension

which exists in a wetted soil above so

the water does not move readily into the

sand however as the soil above the sand

becomes very wet the water eventually

moves into the sand just as ink would

drip from a bladder which is wet

excessively the sand layer does ACT

something like a check valve holding the

water back until the soil becomes very

wet and then letting the excess pass

through what happens to water and soil

containing a sand layer is typical in

principle of what happens to water in

field soils where sand and graval occur

as layers in finer soil materials a

great deal of agricultural land is

layered in this fashion in Belgium you

can find soil composed of lome overlying

brolan Sands in the woon Bren Province

this layout greatly affects the drainage

and the ability to support plant growth

as more water can be retained this is

one of the best soils in our

regions now in this sequence you see a

layer of fine clay in otherwise uniform

soil this clay layer is similar to a

clay pan or any type of layer in which

the pores are extremely fine compared to

the pores in the overlying

soil these layers often restrict root

and depths of plants and are

particularly known for the trouble they

cause in preventing downward penetration

of water when excess water is added to

the soil water tables are often built up

over such layers if they occur at Shel

low depth water tables often rise above

the land surface during wet Seasons

imposing serious limitations on

agricultural

use despite the fact that a clay pan

hinders downward movement of water it

does absorb water readily as the soil

above is wetted observe the wetting

front as it moves into the clay pen the

pores in the clay are much finer than

those in the overlying soil so they can

pull water from the soil water tables

are not built up over clay pans because

of the inability of water to enter them

instead water tables result from slow

transmission of water the resistance to

water flow in these extremely fine pores

of layers like these is sufficiently

great that even over periods of weeks

and months little water is transmitted

through them into the soil

below the poison restrictive layers

found in nature are quite variable they

range all the way from fine pores that

allow almost no water to pass up to

pores that are almost as large as those

of the overlying soil the extent to

which downward flow is restricted and

water storage is altered depends on the

Finesse of these pore and the thickness

of the restricting

layer this is in contrast to what was

shown earlier in soil overlying Co sand

layers there the downward movement of

water was temporarily checked but water

tables could not be built up so as long

as their opportunity for free drainage

into the course materials was

possible this model has a sun layer on

the left and the layer of Cor Aggregates

on the right the pores between sand

grains and those between Aggregates are

large but Aggregates are made up of soil

particles like those of the surrounding

soil water movement in soil materials

which wet readily depends upon paracity

and not upon the chemical or

minerological nature of the so material

unless it influences its

porosity each individual soil aggregate

contains numerous fine pores of a size

similar to the pores in the surrounding

soil as water approaches these

Aggregates note as they wet out as soon

as the water reaches them however pores

between Aggregates are too large to hold

water at the existing tensions hence

they remain empty all the water must

therefore move first to the finer pores

of an Aggregate and then across the

point of contact with the adjoining

Aggregates the small number of contacts

between Aggregates restricts the rate at

which water can move

if free water is supplied directly to a

layer of coarse sand water rushes in

rapidly filling all the pores these are

conditions of saturated flow the moving

force is due to positive pressure from

the water in the Farrow under saturated

conditions large pors can transmit water

readily with a rate of transmission in a

given material depending only upon the

hydrostatic pressure of the water supply

the energy derived from this positive

pressure is dissipated rapidly over very

short distance in a fine pores giving

way to absorb these forces in a drier

soil water moves out into the soil from

the sand layer under unsaturated

conditions

it is pulled into and through the soil

because of the attraction for water of

the middle surfaces making up the fine

pores of the

soil the sand in the layer at the left

is the same kind of sand through which

water is Flowing at the right here

however the layer is not in contact with

free water or water under positive

pressure the surrounding soil is wetted

under unsaturated conditions where the

water is present only under tension this

sand layer cannot wet until the water

tension in the surrounding soil becomes

very low which means that the soil

becomes very wet as this happens the

layer takes water coarse materials with

very large pores Aid in water movement

only under conditions where a contact

free water or water under pressure where

water exists only under tension such

materials stop or materially

water flow

the Practical applications of principles

of water flow to Dryland agriculture are

shown here water moves rapidly into cils

with good

D proper til practices on the soil on

the left have produced numerous small

Aggregates which have been stabilized by

decomposing organic materials the

resulting large pores which remain open

all the way to the surface take water

readily thus the infiltration rate

remains High the same amount of organic

material when turned under in a layer

does little to improve soil to and if

anything makes conditions worse on the

right a channel filled with coarse sand

simulates an open Channel left by

borrowing worms or angle worms or

perhaps a channel left by decaying roots

of straw

such channels or cracks do not assist in

water movement when they are not

connected to a source of free

water the principle involved also

applies to tile drains water can move

into such drains only if positive water

pressures exist in the surrounding soil

hence stle drains placed in wet soils

must be located below the water table if

they are to carry away unwanted water

and make the land

tillable the straw layer like a sand

layer checks downward flow of water in

this case not only does less of the

rainfall penetrate into the root Zone

leaving more water to run on the surface

but wet conditions in a plot Zone make

the soil even more vulnerable to damage

such as that caused by the impact of

falling

raindrops thus you can understand why

soil and water loss by erosion is

accelerated

these demonstrations emphasize the

principles of water flow under

unsaturated conditions conditions under

which crops are grown on agricultural

land each demonstration has its

counterpart in nature where it may be

less dramatic but the principles hold

and can be seen in operation if one

observes

carefully in summary the then

unsaturated flow of water in soil and

other porous material takes place

because of the attraction of solid

surfaces for water and of water

molecules for each other how the water

moves depends upon the nature of the

pores and the porosity changes in the

porous system