🌱 Soil Water Movement and Moisture Constants

Water in soil is never completely at rest. It moves downward, upward, sideways, and sometimes in vapour form depending on soil conditions and energy gradients. To manage irrigation properly, we must understand both how soil water moves and how much of it is actually available to plants. This lesson explains those ideas step by step.

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What soil moisture tension means

The source defines soil moisture tension as the tenacity with which water is held by soil, or the force per unit area required to remove water from it.

In simple terms:

• when water is loosely held, plants can extract it easily
• when water is tightly held, plants must work harder

Soil moisture tension is commonly expressed in:

• atmospheres
• bars
• centimeters of water
• millimeters of mercury

The source also refers to the concept of:

• capillary potential
• hydrostatic potential
• hygroscopic potential

These terms describe how tightly water is held under different moisture ranges.

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pF of soil

The source mentions pF, introduced by Scholfield, as the logarithm of soil moisture tension expressed in centimeters of water.

This helps represent very wide ranges of soil water tension on a simpler scale, somewhat like:

• pH in soil chemistry

You do not need to memorize every mathematical detail for revision, but you should remember the idea:

• pF is another way to express soil water tension

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Soil moisture characteristics

The same soil moisture tension does not always mean the same water content in different soils. That is because water retention depends on:

• texture
• structure
• pore size distribution
• organic matter

For example:

• sandy soil drains quickly at low tension
• clay soil may still hold a lot of water even at higher tension

That is why moisture characteristic curves are important. These curves relate:

• soil moisture content
• to soil moisture tension

They help us understand:

• how much water is present
• how much is available to plants
• how much more water the soil can hold before deep percolation begins

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Soil moisture stress

The source explains that plant growth depends not only on soil moisture tension but also on osmotic pressure caused by dissolved salts.

This means total plant stress in saline soils is influenced by:

• soil moisture tension
• osmotic pressure of soil solution

Even if water appears present, high salt concentration can make it difficult for plants to absorb it.

So in irrigated agriculture, successful crop production requires:

• moisture in the root zone
• low enough salinity for roots to function properly

This is why irrigation and salinity management are closely linked.

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Soil moisture constants

The source notes that soil water is always moving, so soil moisture is not truly constant. Still, certain moisture levels are agronomically very important and are treated as soil moisture constants.

1. Saturation capacity

This is the condition when:

• all pores of the soil are filled with water

At saturation:

• tension is almost zero
• water is present in both large and small pores

This condition generally does not remain long in well-drained soils because gravitational water drains out.

2. Field capacity

Field capacity is one of the most important concepts in irrigation.

It is the moisture content after:

• excess gravitational water has drained away
• downward drainage becomes slow
• soil water becomes relatively stable

This usually occurs:

• about 1 to 3 days after heavy rain or irrigation

At field capacity:

• large pores are mostly filled with air
• smaller pores remain filled with water

Field capacity forms the upper limit of available soil moisture for crops.

3. Moisture equivalent

The source also describes moisture equivalent, which is determined in the laboratory by centrifuging saturated soil.

It gives a measure of water retained against a strong centrifugal force and is roughly similar to field capacity in medium-textured soils.

Even if it is less common in routine field explanation today, it remains a useful historical and conceptual constant.

Field capacity is the most practical upper reference point for irrigation scheduling because it represents the soil after free drainage has mostly stopped.

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Hydraulic equilibrium of soil water

The source refers to hydraulic equilibrium as the condition where there is no net movement of liquid or film water in soil because:

• pressure-gradient force
• and gravitational force

balance each other.

This helps explain why movement happens only when there is a potential difference or energy gradient.

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Water movement under saturated conditions

Under saturated conditions:

• all soil pores are filled with water
• movement occurs mainly due to gravitational and pressure forces

The source notes that saturated flow is strongly influenced by pore size and that larger pores permit faster flow.

So, under otherwise similar conditions, the rate of saturated flow generally follows:

• sand > loam > clay

This is because sandy soils have larger pores and therefore greater hydraulic conductivity when fully saturated.

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Water movement under unsaturated conditions

As drainage continues and large pores empty:

• gravitational influence becomes less dominant
• matric forces become more important

This is the normal condition in most cultivated soils between irrigations.

Under unsaturated conditions:

• water moves through capillary films
• hydraulic conductivity decreases rapidly as the soil dries

The source explains that this is why water movement becomes slow when moisture content drops.

For unsaturated flow, the relative order of movement in the moist range is often:

• sand < loam < clay

This is the reverse of saturated flow because fine-textured soils retain more continuous water films under unsaturated conditions.

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Capillary rise and its practical importance

The source notes that capillary movement can supply useful water from a shallow water table, but usually only when free water lies within:

• about 60 to 90 cm of the root zone

This is important in:

• shallow water table areas
• some irrigated regions
• certain situations of subirrigation or groundwater contribution

But under most normal field situations, capillary rise alone cannot meet the full crop demand.

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Water vapour movement

Water in soil can also move as vapour, especially when liquid continuity is broken at low moisture levels.

Vapour movement becomes important when:

• the soil is dry
• open pore space is higher
• there is a vapour-pressure gradient

The source states that:

• vapour movement is negligible in the wet range
• becomes more important in the moist range
• may still exist in the dry range, though total water quantity is then very low

Fine-textured soils may support vapour movement better at higher tensions because they still retain more moisture than coarse-textured soils.

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Why soil water movement matters in irrigation

All of these concepts matter because irrigation is not simply about adding water. It is about understanding:

• how water enters the soil
• how long it stays
• how much drains away
• how much remains available to crops
• how salinity and texture alter plant access to water

Without understanding soil-water movement, irrigation scheduling becomes guesswork.

Summary Cheat Sheet

Topic	Key Point
Soil moisture tension	It expresses how strongly soil holds water and how hard plants must work to extract it.
pF	A logarithmic way of expressing soil moisture tension.
Moisture characteristics	The same tension can correspond to different water contents in different soils.
Soil moisture stress	Plant stress depends on both water tension and osmotic effects from salts.
Saturation capacity	All pores are filled with water; tension is nearly zero.
Field capacity	Soil moisture after free drainage becomes slow; practical upper limit of available water.
Saturated flow	Usually fastest in sand, then loam, then clay.
Unsaturated flow	Governed mainly by matric forces; often more effective in finer soils than sand in the moist range.
Vapour movement	Becomes important when liquid continuity is broken in drier soils.
Irrigation relevance	Soil-water movement determines availability, losses, and scheduling decisions.