Soil water Types of water in soil n

Soil water

Types of water in soil: n 1. Adhesion water n “HYGROSCOPIC WATER” Remove by oven drying n Not available to plants n

2. Cohesion water n “CAPILLARY WATER” 15 – 20 molecules thick n Remove by air drying n Most is available to plants n n some unavailable to plants (especially in clay or high OM soils)

WILTING POINT n n Divides available and unavailable water “the amount of water in the soil when plants have removed all that they can”

Difference between wilting point and hygroscopic coefficient: at wilting point Moist Can’t squeeze water Plant can’t get water at hygroscopic coefficient Dry to touch Air-dried Can be oven dried to remove water

3. Gravitational water Not available to plants n Drains through soil under influence of gravity n Through large pores n n Small pores can hold water against pull of gravity through capillarity

FIELD CAPACITY n n Divides capillary water from gravitational water “amount of water after gravity has removed all freely drained water”

(Air dry) Oven dry Hydroscopic coefficient Wilting point Field capacity No water Adhesion water Plantunavailable water (capillary) Plant-available Water (capillary) Gravitational water

capillarity 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 soil: Field capacity n Wilting point n Hygroscopic coefficient n

Field Capacity n Amount of water in soil after free drainage has removed gravitational water (2 – 3 days) Soil is holding maximum amount of water available to plants n Optimal aeration (micropores filled with water; macropores with air) n

Wilting Point n n Amount of water in soil when plants begin to wilt. Plant available water is between field capacity and wilting point.

Hygroscopic coefficient n n Amount of moisture in air dry soil Difference between air dry and oven dry amounts

Not all capillary water is equally available to plants n Plants can extract water easily from soils that are near field capacity n n Sponge example Wilting point is not the same for all plants n Sunflowers can extract more water from soil than corn n Sponge example

Adhesion water Micropores full; macropores have air Wilting point Field Capacity All pores full Gravitational water

Hydraulic pressure of soil water n Pressure = force / area Hydraulic pressure “ 0” at surface increases with depth Open body of water

n Same in saturated soil “ 0” at surface increases with depth

Capillary pressure n Thin tube in open pan water -20 g/cm 3 -10 0 Pressure in tube decreases away from water surface (Adhesion to walls of tube; cohesion in center of tube; therefore thin tube only)

Same in unsaturated soil: n Capillary water is water in small pores continuously connected to free water surface (soil water table) -20 -10 0 +10 Capillary water (continuous film) Soil water table Saturated soil

n n the smaller the pore space, the higher capillary water will rise in profile Smaller pore space, tighter water is held to particle surfaces against gravity (i. e. , higher field capacity) clay silt sand Pan of water

at n Insert Fig 9. 6

Energy status of soil water n Energy status n Things move to lower energy states n It n n takes work to keep them from doing so E. g. keeping something from falling in response to gravity Influences water movement n E. g. adhesion attracts water to soil particles so particles close to soil are at lower energy state

Forces on soil water: n Adhesion n Attracts water to soil particles n Holds adhesion(hygroscopic) water and cohesion (capillary) water n n Called “matric force” Ions in solution Attracts water to ions n Called “osmotic force” n n Gravity Pulls water downward n “gravitational force” n

n Soil water potential Amount of work required to move water n Expressed in bars or Pascals n Similar to soil water tension n

Water is held at various tensions/attractions

potentials Water is removed by various potentials

n Water moves from areas of higher water potential (wetter) to areas of lower water potential (drier).

Potentials Matric n Gravitational n Hydrostatic n Osmotic n Total n

Matric potential n Work required to remove water held by adhesion to soil surface and cohesion in capillary pores. n Hygroscopic and capillary water n Zero (if saturated) or negative

Gravitational potential n Work required to draw water down in response to gravity n n Applies to gravitational water only Increases with increasing elevation above soil water table n Positive

Hydrostatic Potential n Work required to move water below the water able; applies only to saturated conditions

Osmotic potential n If there are solutes in the solution, water will group around them and reduce the freedom of water movement, i. e. , lowering the potential.

Osmotic potential n Water containing salts is less able to do work than pure water n n The more salts, the lower (higher absolute value) the potential n n e. g. , cannot boil at standard boiling point negative Important for plant uptake n In salty soil, potential in soil solution may be lower than inside plant root cells, impeding ability of water to pass into plant

Total water potential= Matric + osmotic + gravitational + hydrostatic

n Unsaturated flow: water movement in soils at less than saturation n n Water moves in response to water potential gradient (high to low) Saturated flow: moves according to gravitational potential only

Hydraulic conductivity Ability of a soil to transmit water n Depends on : n n Pore size n Coarse grained soil has higher cond. than finegrained because movement through large pores is faster n Amount of water in soil n Cond. n decreases as water content increases Water moves through largest pores first

How do plants get water? n Root hairs are in contact with soil water

n n n Salts (nutrients) are in a more dilute solution in the soil than in roots (plant “sap”) By osmosis, water (with dissolved salts) moves into root hairs. From the roots hairs, water diffuses into other plant cells, eventually to xylem, which is the water conduit of the plant.

n This “root pressure” pushes water up into the plant. n This mechanism is called “active absorption” n (requires energy)

n “passive absorption” : n Plants transpire during the day. n n Transpiration is loss of water vapor to the atmosphere from Stomata (pores) on leaves. In transpiration water is pulled from the plant, beginning at the leaf tip.

n This pull from the leaf tip is transmitted all the way down to the roots. “Domino effect” of water in a continuous film being drawn up column from soil through plant cells, as water is lost by transpiration. n Cohesion holds water together in a continuous film. n n No energy required; roots are passive.