Soil Moisture RetentionSoil Moisture Retention

Laboratory #5Laboratory #5

ObjectivesObjectives

• Know the definitions of oven dry, Know the definitions of oven dry, saturation, evapotranspiration, permanent saturation, evapotranspiration, permanent wilting point, field capacity, macropore, wilting point, field capacity, macropore, micropore, and available water content.micropore, and available water content.

• Know how to calculate bulk density, soil Know how to calculate bulk density, soil water content (by weight and by volume), water content (by weight and by volume), available water percentage, percent pore available water percentage, percent pore space, volume of macropores and space, volume of macropores and micropores.micropores.

Soil MoistureSoil Moisture

• There are three moisture terms that There are three moisture terms that you must be familiar with in order to you must be familiar with in order to understand the relationship between understand the relationship between soil water and plant growth: oven soil water and plant growth: oven dry, saturated, and field capacity.dry, saturated, and field capacity.

Oven DryOven Dry

• Soil consists of soil particles and pore Soil consists of soil particles and pore spaces, which are filled with gases such as spaces, which are filled with gases such as oxygen (Ooxygen (O22), carbon dioxide (CO), carbon dioxide (CO22) and ) and dinitrogen (Ndinitrogen (N22). ).

• When all of the pore space is filled with When all of the pore space is filled with gases, the soil is said to be oven dry. gases, the soil is said to be oven dry.

• An An oven dryoven dry soil is defined as a soil that soil is defined as a soil that has been dried at 105°C until it reaches has been dried at 105°C until it reaches constant weight and contains no water.constant weight and contains no water.

SaturatedSaturated

• A A saturatedsaturated soil has soil has all of the pore space all of the pore space filled with water. filled with water.

• At this point the soil At this point the soil is at its maximum is at its maximum retentive capacity.retentive capacity.

http://www.css.cornell.edu/faculty/hmv1/watrsoil/CDI32F6.gif

Field CapacityField Capacity

• Following a rain or irrigation, a portion Following a rain or irrigation, a portion of the water from saturated soils will of the water from saturated soils will drain from the soil due to gravity. drain from the soil due to gravity.

• After two to three days the gravitational After two to three days the gravitational drainage will become negligible. drainage will become negligible.

• At this time the soil is said to be atAt this time the soil is said to be at field field capacitycapacity..

Field Capacity & PoresField Capacity & Pores

• The remaining water is found in the The remaining water is found in the micropores and the water drained from micropores and the water drained from the soil was lost from the macropores. the soil was lost from the macropores.

• The micropores are small enough that The micropores are small enough that the adhesive and cohesive forces the adhesive and cohesive forces holding the water to the pore wall are holding the water to the pore wall are stronger than the gravitational force stronger than the gravitational force trying to drain the soil. trying to drain the soil.

Micropores & MacroporesMicropores & Macropores

• Although there is no Although there is no clear size specification clear size specification of the pores, generally of the pores, generally pores larger than 0.06 pores larger than 0.06 mm are considered mm are considered macroporesmacropores, and , and those smaller than 0.06 those smaller than 0.06 mm are mm are microporesmicropores..

http://www.landfood.ubc.ca/soil200/images/16images/16.1.1macro&micropores.jpg

Volume of MacroporesVolume of Macropores

• The volume of the The volume of the macroporesmacropores is equal is equal to the volume of the water that has to the volume of the water that has drained drained from the saturated soilfrom the saturated soil to reach to reach field capacity. field capacity.

• For example, you have 100 cmFor example, you have 100 cm33 in a in a saturated soil but when the soil reaches saturated soil but when the soil reaches field capacity, you are left with 65 cmfield capacity, you are left with 65 cm33..

• What is the volume of macropores? 35 What is the volume of macropores? 35 cmcm33

Volume of MicroporesVolume of Micropores

• The volume of The volume of microporesmicropores equals the equals the volume of water remaining in the volume of water remaining in the soil soil at field capacityat field capacity..

• In the previous example, we had 65 In the previous example, we had 65 cmcm33 of water remaining in the soil at of water remaining in the soil at field capacity.field capacity.

• What is the volume of micropores? What is the volume of micropores? 65 cm65 cm33

EvapotranspirationEvapotranspiration

• Most of the water that plants absorb Most of the water that plants absorb from the soil is lost through from the soil is lost through evaporation at the leaf surfaces. evaporation at the leaf surfaces.

• Simultaneously water is evaporated Simultaneously water is evaporated from the soil. from the soil.

• The combined loss of water from the The combined loss of water from the soil and from plants is termed soil and from plants is termed evapotranspirationevapotranspiration. .

EvapotranspirationEvapotranspiration

T=Transpiration=The water loss from plant leaves

E=Evaporation=The water loss due to the change of water from a liquid state to a vapor state

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WiltingWilting

• As the soil dries, plant As the soil dries, plant available water available water decreases. decreases.

• The initial response of The initial response of plants is wilting. plants is wilting.

• At the first onset of At the first onset of wilting, most plants can wilting, most plants can recover during times of recover during times of reduced reduced evapotranspiration (i.e. evapotranspiration (i.e. night). night). http://creatures.ifas.ufl.edu/field/less_corn06.htm

Permanent Wilting PointPermanent Wilting Point

• As the soil continues to dry, the plants reach As the soil continues to dry, the plants reach a point at which they cannot recover during a point at which they cannot recover during periods of reduced evapotranspiration. periods of reduced evapotranspiration.

• The plants are then in a permanently wilted The plants are then in a permanently wilted condition. condition.

• The soil moisture content of the soil when The soil moisture content of the soil when plants no longer can recover from daytime plants no longer can recover from daytime wilting is called the wilting is called the permanent wilting pointpermanent wilting point..

RelationshipsRelationships

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Plant Available WaterPlant Available Water

• Plant available waterPlant available water is exactly as is exactly as the name implies, it is the unbound the name implies, it is the unbound water that is available to plants for water that is available to plants for uptake.uptake.

• This is calculated by subtracting the This is calculated by subtracting the water content at field capacity from water content at field capacity from the soil water content at the the soil water content at the permanent wilting point. permanent wilting point.

Plant Available Water Plant Available Water ExampleExample

• If we have 65 cmIf we have 65 cm33 of water at field of water at field capacity, and are left with 13 cmcapacity, and are left with 13 cm33 at at the permanent wilting point, what is the permanent wilting point, what is our plant available water? 52 cmour plant available water? 52 cm33

Plant Available WaterPlant Available Water

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RelationshipsRelationships

-31 bars

-15 bars

-1/3 bars

0 barsOven Dry Drained 2

DaysSaturated

Oven Dry

Air Dry Wilt. Point

Field Capacity Saturated

Micropores Macropores

Unavailable for Plants

Available for Plants Unavailable

Dry Wet

Note on CalculationsNote on Calculations• Soil water calculations may be done on either a Soil water calculations may be done on either a

weight or volume basis. weight or volume basis. • Most of the calculations are first done on a weight Most of the calculations are first done on a weight

basis and then converted to a volume basis. basis and then converted to a volume basis. • Volume measurements are important because a Volume measurements are important because a

plant does not grow in a weight of soil, it grows in plant does not grow in a weight of soil, it grows in a volume of soil. a volume of soil.

• Volume measurements are also important Volume measurements are also important because when we are dealing with pore space, because when we are dealing with pore space, we are working with volume of pores, not weight we are working with volume of pores, not weight of pores. of pores.

Questions?Questions?