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Factors Affecting Water Factors Affecting Water Availability In SoilAvailability In Soil

Topic:Topic:

ContentsContents

• Classification Of Soil Water

• Movement Of Water In Soil

• Soil Water Availability To Plants

• Indicators Of Plant Water Stress• Development Of Plant Water Deficiency

• Water Availability Factors

• Water Availability In Pakistan

• Conclusion4

Water

• Makes up approximately 90% of a plant's mass and performs many functions:

1. Required for seed germination

2. Carries minerals into and through the plant.

3. Transports photosynthates and other biochemicals

4. Cools the plant by transpiration

5. Involved in photosynthesis5

Classes of Soil Water

1.Gravitational water: move through gravity

2. Hygroscopic water: move in vapor form

3. Capillary water: available

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Movement Of Water Within SoilsMovement Of Water Within Soils

Moves along gradients

• Water in the liquid phase:

– flows through the water filled pore space

• Water in vapour phase:

– moves through the air filled pore spaces

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Water Movement And Retention In SoilWater Movement And Retention In Soil

Three forces are responsible for water movement within the soil.

1- Gravity (Downward movement)2- Adhesion (soil particles and water). 3- Cohesion (water and water).

(Adhesion+cohesion) Principal forces that

move water in an unsaturated soil.10

Capillary RiseCapillary Rise

The upward movement of water.• Responsible for the loss of water from the

soil surface by evaporation• As soil dries, the water film surrounding

each soil particle thins. Consequently, the adhesive and cohesive forces of attraction increase rapidly, making it more difficult for the plant to extract water that is held tightly in soil particles.

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Soil Water Availability To PlantsSoil Water Availability To Plants

The pore spaces of soil are always filled with water, air or a mixture of both.

Normal condition Saturation condition Too dry condition

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Indicators Of Plant Water StressIndicators Of Plant Water Stress

Soil water potential

Leaf stomatal conductance

Leaf water potential

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Development Of Plant Water DeficiencyDevelopment Of Plant Water Deficiency

Saturated Field

Gravitational Water

Field Capacity

Plant absorption &

Evapo-Transpiration

No recharge of water:

No rain, No irrigation

Soil water deficit

Depends on1- Rate of evapotranspiration2- Physical & chemical properties of soil

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Water Availability Factors

• Available water• (FC --- PWP)

i. FC---Saturation

ii. Below PWP

• Cohesion and adhesion forces

• Presence of salt = Plasmolysis

• Soil porositya) Clay soils

b) Sandy soils

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Water Availability Factors

• Root system

• Water table

• Hardpan

• Slope

• Saturation

• Field Capacity

• Permanent Wilting Point

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Water Availability Factors

• Hindrance

• Type of soil

• Water holding capacity

• Water potential

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Response Of Water To Different SoilsResponse Of Water To Different Soils

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Water holding capacityWater holding capacity

The ability of the soil to retain water is called its water holding capacity.

• It differs from soil to soil• Clay soils = smaller but more pores• Sandy soils= larger but less pores Thus, an equal volume of clay soil holds

more water than a sandy soil when the pores are filled.

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Water-Holding Capacity of SoilWater-Holding Capacity of Soil

Coarse SandCoarse Sand Silty Clay Loam Silty Clay Loam

Gravitational WaterGravitational Water

Water Holding CapacityWater Holding Capacity

Available WaterAvailable Water

Unavailable WaterUnavailable Water

Dry SoilDry Soil

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Soil Water potentialSoil Water potential

Difference between chemical potential of pure water to that of osomatically bound water.

Describes • How tightly water is bound in the soil• Availability of water for biological processes Defines the flow of water in all systems

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Soil Water PotentialSoil Water Potential

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Reclamation Of SaltsReclamation Of Salts

o Leaching

o Flushing

o Scrapping

o Strip Cropping

o Soil amendments and water treatments

o Lowering soil pH25

Reclamation Of SaltsReclamation Of Salts

o Fertilizer management

o Irrigation management

o High transpiring plants

o Salt tolerant crops

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Water Availability in Pakistan Is Water Availability in Pakistan Is Declining Rapidly!!!!!!Declining Rapidly!!!!!!

• Over exploitation of water resources

• Deforestation at the rate of 69,600 ha year -

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• Growing demand of water

• Ground water depletion at the rate of 10 m year-1

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ConclusionConclusion

The soil should be carefully studied with regard to the following:

(a) Size of soil particles

(b) Compactness

(c) Depth of water table

(d) Organic matter content

(e) Effective irrigation method

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• All the above aspects influence the depth of available water that the irrigator can store in the root zone of soil in a single application of water and hence influence the required frequency of watering.

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