Soil water

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

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

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

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

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

Field Capacity 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 Optimal aeration (micropores filled with water; macropores with air)

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

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

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

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

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

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

Capillary pressure Thin tube in open pan water Pressure in tube (Adhesion to walls of tube; cohesion in center of tube; therefore thin tube only) -20 g/cm3 Pressure in tube decreases away from water surface -10

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

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 Insert Fig 9.6

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

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

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

Water is held at various tensions/attractions

potentials Water is removed by various potentials

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

Potentials Matric Gravitational Hydrostatic Osmotic Total

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

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

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

Osmotic potential 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 Water containing salts is less able to do work than pure water e.g., cannot boil at standard boiling point The more salts, the lower (higher absolute value) the potential negative Important for plant uptake 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

Unsaturated flow: water movement in soils at less than saturation 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 Depends on : Pore size Coarse grained soil has higher cond. than fine-grained because movement through large pores is faster Amount of water in soil Cond. decreases as water content increases Water moves through largest pores first

Water uptake by plants >90% by passive absorption: “Domino effect” of water in a continuous film being drawn up column from soil through plant cells, as water is lost by transpiration No energy required

Active absorption