Soil waterSoil water

Types of water in soil:Types of water in soil: 1. Adhesion water1. Adhesion water

““HYGROSCOPIC WATER”HYGROSCOPIC WATER”

Remove by oven dryingRemove by oven drying Not available to plantsNot available to plants

2. Cohesion water2. Cohesion water ““CAPILLARY WATER”CAPILLARY WATER”

Remove by air dryingRemove by air drying Most is available to plants Most is available to plants

some unavailable to plants (especially in some unavailable to plants (especially in clay or high OM soils)clay or high OM soils)

15 – 20 molecules thick15 – 20 molecules thick

Difference between wilting point Difference between wilting point and hygroscopic coefficienand hygroscopic coefficient:t:

MoistMoist Dry to touchDry to touchCan’t squeeze waterCan’t squeeze water Air-driedAir-driedPlant can’t get water Plant can’t get water Can be oven dried to Can be oven dried to

remove waterremove water

at wilting point

at hygroscopic coefficient

3. Gravitational water3. Gravitational water

Not available to plantsNot available to plants Drains through soil under influence of Drains through soil under influence of

gravitygravity Through large poresThrough large pores

Small pores can hold water against pull of Small pores can hold water against pull of gravity through capillaritygravity through capillarity

Gravitationalwater

Plant-availableWater(capillary)

Plant-unavailable water(capillary)

Adhesion water

Field capacityWilting pointHydroscopic

coefficientOven dry

No water

(Air dry)

capillaritycapillarity

Height water will rise in cylinder depends on diameter of tube; due to adhesion of water and tube

Plastic Glass

Critical levels of water in Critical levels of water in soil:soil:

Field capacityField capacity Wilting pointWilting point Hygroscopic coefficientHygroscopic coefficient

Field CapacityField Capacity Amount of water in soil after free Amount of water in soil after free

drainage has removed gravitational drainage has removed gravitational water (2 – 3 days)water (2 – 3 days)

Soil is holding maximum amount of Soil is holding maximum amount of water available to plantswater available to plants

Optimal aeration (Optimal aeration (microporesmicropores filled filled with water; with water; macroporesmacropores with air) with air)

Wilting PointWilting Point Amount of water in Amount of water in

soil when plants soil when plants begin to wilt.begin to wilt.

Plant available Plant available waterwater is between is between field capacity and field capacity and wilting point.wilting point.

Hygroscopic coefficientHygroscopic coefficient

Amount of moisture in air dry soilAmount of moisture in air dry soil

Difference between air dry and oven Difference between air dry and oven dry amountsdry amounts

Not all capillary water is equally Not all capillary water is equally available to plantsavailable to plants

Plants can extract water easily from Plants can extract water easily from soils that are near field capacitysoils that are near field capacity Sponge exampleSponge example

Wilting point is not the same for all Wilting point is not the same for all plantsplants Sunflowers can extract more water from Sunflowers can extract more water from

soil than cornsoil than corn Sponge exampleSponge example

Wilting point Field Capacity

Adhesion water Micropores full;macropores have air

Gravitational water

All pores full

Hydraulic pressure of soil Hydraulic pressure of soil waterwater

Pressure = force / areaPressure = force / area

Open body of water

“0”at surface

increases with depth

Hydraulic pressure

Same in saturated soilSame in saturated soil

“0” at surface

increases with depth

Capillary pressureCapillary pressure Thin tube in open pan waterThin tube in open pan water

Pressure in tubedecreases awayfrom water surface

0

-10

-20 g/cm3

(Adhesion to walls of tube;cohesion in center of tube;therefore thin tube only)

Same in unsaturated soil:Same in unsaturated soil:

Capillary water is water in small Capillary water is water in small pores continuously connected to free pores continuously connected to free water surface (soil water table)water surface (soil water table)

Capillary water(continuous film)

Soil water table

Saturated soil

0

-10

-20

+10

the smaller the pore space, the higher the smaller the pore space, the higher capillary water will rise in profilecapillary water will rise in profile

Smaller pore space, tighter water is Smaller pore space, tighter water is held to particle surfaces against held to particle surfaces against gravity (i.e., higher field capacity)gravity (i.e., higher field capacity)

Pan of water

clay silt sand

atat Insert Fig 9.6Insert Fig 9.6

Energy status of soil Energy status of soil waterwater

Energy statusEnergy status Things move to lower energy statesThings move to lower energy states

It takes work to keep them from doing soIt takes work to keep them from doing so E.g. keeping something from falling in response to E.g. keeping something from falling in response to

gravitygravity

Influences water movementInfluences water movement E.g. adhesion attracts water to soil particles E.g. adhesion attracts water to soil particles

so particles close to soil are at lower energy so particles close to soil are at lower energy statestate

Forces on soil water:Forces on soil water: AdhesionAdhesion

Attracts water to soil particlesAttracts water to soil particles Holds adhesion(hygroscopic) water and Holds adhesion(hygroscopic) water and

cohesion (capillary) watercohesion (capillary) water Called Called “matric force”“matric force”

Ions in solutionIons in solution Attracts water to ionsAttracts water to ions Called Called “osmotic force”“osmotic force”

GravityGravity Pulls water downwardPulls water downward ““gravitational force”gravitational force”

Soil water potentialSoil water potential Amount of work required to move waterAmount of work required to move water Expressed in Expressed in barsbars or or PascalsPascals Similar to Similar to soil water tensionsoil water tension

Water is held at various tensions/attractions

potentials

Water is removed by various potentials

Water moves from areas of Water moves from areas of higherhigher water potential (wetter) to areas of water potential (wetter) to areas of lowerlower water potential (drier). water potential (drier).

PotentialsPotentials

MatricMatric GravitationalGravitational HydrostaticHydrostatic OsmoticOsmotic TotalTotal

MatricMatric potential potential Work required to remove water held Work required to remove water held

by adhesion to soil surface and by adhesion to soil surface and cohesion in capillary pores.cohesion in capillary pores.

HygroscopicHygroscopic and and capillarycapillary water water

ZeroZero (if saturated) or (if saturated) or negativenegative

GravitationalGravitational potential potential Work required to draw water down in Work required to draw water down in

response to gravity response to gravity Applies to gravitational water onlyApplies to gravitational water only

Increases with increasing elevation Increases with increasing elevation above soil water tableabove soil water table PositivePositive

HydrostaticHydrostatic Potential Potential Work required to move water below Work required to move water below

the water able; applies only to the water able; applies only to saturated conditionssaturated conditions

Osmotic Osmotic potentialpotential If there are solutes in the solution, If there are solutes in the solution,

water will group around them and water will group around them and reduce the freedom of water reduce the freedom of water movement, i.e., lowering the movement, i.e., lowering the potential.potential.

Osmotic Osmotic potentialpotential Water containing Water containing saltssalts is less able to do is less able to do

work than pure waterwork than pure water e.g., cannot boil at standard boiling pointe.g., cannot boil at standard boiling point

The more salts, the lower (higher absolute The more salts, the lower (higher absolute value) the potentialvalue) the potential negativenegative

Important for plant uptakeImportant for plant uptake In salty soil, potential in soil solution may be In salty soil, potential in soil solution may be

lower than inside plant root cells, impeding lower than inside plant root cells, impeding ability of water to pass into plantability of water to pass into plant

Total water potential=Total water potential=

Matric + osmotic + gravitational + Matric + osmotic + gravitational + hydrostatichydrostatic

Unsaturated flowUnsaturated flow: water movement : water movement in soils at less than saturationin soils at less than saturation Water moves in response to water Water moves in response to water

potential gradient (high to low)potential gradient (high to low)

Saturated flow: Saturated flow: moves according to moves according to gravitational potential onlygravitational potential only

Hydraulic conductivityHydraulic conductivity Ability of a soil to transmit waterAbility of a soil to transmit water Depends on :Depends on :

Pore sizePore size Coarse grained soil has higher cond. than Coarse grained soil has higher cond. than

fine-grained because movement through fine-grained because movement through large pores is fasterlarge pores is faster

Amount of water in soilAmount of water in soil Cond. decreases as water content increasesCond. decreases as water content increases

Water moves through largest pores firstWater moves through largest pores first

Water uptake by plantsWater uptake by plants >90% by >90% by passive absorption:passive absorption:

““Domino effect” of water in a continuous Domino effect” of water in a continuous film being drawn up column from soil film being drawn up column from soil through plant cells, as water is lost by through plant cells, as water is lost by transpirationtranspiration

No energy requiredNo energy required

Active absorptionActive absorption