Soil water potential refers to:

the energy status of soil water and is made up of:

– Gravitational potential energy
- Matric potential energy
- Osmotic potential energy

 Measuring soil moisture content:

Gravimetric water content–
Volumetric water content–
Resistance method–
Neutron scattering

Measuring soil moisture potential:

– Tensiometer

Water moves from

a higher to a lower potential energy

Gravimetric or mass water content (θm)

 – ratio of weight of soil water to weight of dry soil
θm = (wt of H2O)/(wt of dry soil)
– dry soil weight is determined after oven drying.

Gravimetric = mass

Volumetric water content (θv)

fraction of soil volume occupied by water
– gravimetric moisture times bulk density
θv = θm x Db

Gravimetric Soil water content

Volumetric Soil Water Content (θv)

Ρb = Db

soil bulk density

Ρp = Dp

 soil particle density = 2.65 g/cm3

θg = θm

 gravimetric water content

θv

volumetric water content

Soil water content is commonly expressed as

a depth of water

depth of water = θv x (depth of soil)
= 0.425 x 24 inches = 10.2 inches of water

A common depth unit is

Why is energy is needed to remove water from soil?

Adhesion + Cohesion =

“Capillarity”

Capillary rise in soil is related to:

pore radius (r) and to soil texture

because of adhesive and cohesive forces,

water in soils is not:

free to move

other way to think of this is that energy (pressure or suction) is usually needed to remove water from soil.

The soil water potential (energy) is the: 

work soil water can do when it moves to a defined reference state (a pool of water)
– This is almost always a negative number for soil water

soil water has less free energy than free water. – This means that energy needs to be applied to extract this water.

• Plants roots apply suction to extract water

• Similarly, to remove water from a sponge you apply pressure by squeezing

Attraction to the soil matrix, and the presence of solutes

both decrease soil water energy

Gravitational Potential (Ψg)

Matric Potential (Ψm)

Osmotic Potential (Ψo)

Soil water potential (SWP)

Tensiometer

In most situations, ψg
and  Ψo are small, therefore:

ΨT≈ Ψm
• Water always moves in soils from high to low potential energy (usually from wet to dry).
• Water is held tightly on particle surfaces.  As distance from surface increases, water is held more weakly
(free energy increases).

Saturated flow

– “saturated” means that all soil pores are filled with water. 
• Flow is relatively rapid. 
• Flow is downward.

Unsaturated flow

– “unsaturated” means that some pores contain air.  
• Flow can be in any direction.

• Water movement is through relatively small pores (large pores are mostly empty)
• Hydraulic conductivity is a function of soil water content and soil texture

For unsaturated flow, at higher water contents:

conductivity is greater in coarser textured soils

For unsaturated flow, at lower water contents

conductivity is greater in finer textured soils

Water infiltration into soils decreases with time because:

– Soil clays swell, decreasing pore size, closing cracks
– Air bubbles become entrapped, effectively closing pores

Decreasing infiltration rate increases

risk of runoff and erosion in prolonged precipitation
events .

Effects of Stratification (layering)

Water Movement in Layered Soil

Soil pores contain

air, water, or some of both
Ideally, soil pores should contain sufficient water for plant growth, and sufficient air for their needs.  However, this situation is dynamic.

A well‐aerated soil has 

rapid gas exchange between atmosphere and soil.  Too much water will slow down gas exchange.

Factors Regulating Aeration

• Soil macroporosity
– Macropores are mostly responsible for air movement.
• Soil water content
– Water‐filled pores transmit oxygen very slowly

• Oxygen consumption/CO2 evolution
– By roots and microorganisms

Macropores

are mostly responsible for air movement.

Major gases in soil

Aeration refers to

the oxygen supply of the soil

What is Oxygen’s crucial role in the soil?

the most important electron acceptor.

A shortage of  _____  shuts down aerobic respiration; favors anaerobic respiration

O₂

What is this reaction?

CH₂O + O₂ → CO₂ + H₂O

Respiration/Oxidation-Reduction

Oxygen is consumed in soils by plant roots and aerobic microorganisms in aerobic respiration

All plants and many microorganisms

must have ______ for respiration.

Oxygen

Compared to the atmosphere, soil air has:

– more CO2
– less O2
– greater moisture (relative humidity)

In wet soils, O₂ may be consumed by

 plant roots and microbes faster than it can be replenished from the air. 

This can cause anaerobic conditions (no O₂).

Mass Flow

Diffusion

– Movement due to differences in partial pressure (concentration) of individual gases.

Most gas movement in soil occurs by diffusion

The most important gas diffusion process in soils is the movement of ________
in response to_______ depletion by respiration

O₂, O₂

O₂ diffusion through water is _________ as rapid as through air

1/10,000th

If water‐filled porosity is > 60‐80%

• O2 diffusion slows down

• Soil will become O2‐limited

Total porosity ‐ θv =

air filled porosity

Soil redox potential is:

a measure of the tendency of an environment to oxidize or
reduce chemicals

Tell 3 things about Oxidation‐Reduction Rxn's

• Oxidation state – The oxidation number of an element corresponds to the number of electrons shared, lost, or gained during a reaction

• Transfer of electrons – Oxidation is loss of electrons
• Valence increases– Reduction is gain of electrons
• Valence (electrical charge) decreases

• Reactions are coupled
– Electron goes from one atom to another

In redox reactions, both oxidation and reduction occur.  Redox reactions are
important because they can:

– Change the solubility and availability of plant nutrients.
– Change the solubility and mobility of environmental contaminants.
– Affect microbial populations and functions.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Represents the ______of organic matterin soils. 

decomposition

– C in C₆H₁₂O₆ is oxidized in this process
– O in O₂ is reduced in the reaction
• The O₂ is called the “electron acceptor ”

If a soil becomes anaerobic because of ___________, O₂
is not present, so another electron acceptor is needed.

 waterlogging

Denitrification

This reaction represents the decomposition of organic matter in an anaerobic soil. 
– C in CH₂O is oxidized in the reaction –
N in NO₃‐ is reduced in the reaction 

The NO₃ is called the “electron acceptor ”

Electron Acceptors in Soils

• Aerobic Soils         • Anaerobic Soils   – CO₂ → CH₄
– O₂ → H₂O                                    – NO₃^‐ → N₂O, N₂
                                                   – SO₄^2‐ → H₂S, S2‐

– Polluted soils:                           – Fe³⁺ → Fe²⁺
• Cr₆+ → Cr ³⁺                           – Mn⁴⁺  → Mn²⁺
• AsO₅ → AsO₃                         – CH₃COOH → CH₄     

Poor Soil Aeration/ If soils become anaerobic:

– Most plants do not grow  – Microorganisms that are able will switch to anaerobic metabolism (yields less energy)
– Toxic organic molecules may begin to build up in the soil. – Metals in soil may become more soluble

(Fe²⁺, Mn²⁺) (Mn ⁺² can reach toxic levels )
– Nitrate (plant nutrient) is lost as a gas

What causes poor soil aeration?

What is a hydric soil?

soils that are anaerobic for considerable periods of time.
 

What is Mottling?

“Mottling” is mixtures of reddish (oxidized Fe) and
gray (reduced Fe), and indicates alternating
aerobic and anaerobic conditions.

What would you find in the swamps/wetlands ?

Define:

If wetlands are drained, drying and aerobic respirationwith result in organic matter loss.

Subsidence

Waterlogged Soils:

Why is Soil Temperature important?

• Affects plants – Plants are more sensitive to soil Temp. than air Temp– Seed germination completely dependent on soil temp
• Microbial processes – Nearly cease below 5°C

– For every 10°C increase in T between 10 and 40oC, microbial activity roughly doubles.
– “Solarization” using clear plastic covers is used to raise soil temperatures to kill pathogens

Tell me about Solar Energy Absorption and Loss:

Albedo?

is a measure of the ability of a soil to reflect solar energy.

– Low albedo (0.1‐0.2) is characteristic of dark surfaces that do not reflect much energy.
– High albedo (0.5) represents light‐colored surfaces that reflect much solar energy.

At a depth of several feet and greater the temperature of a soil is ___________.

Constant

Average Soil Temperature

Normally 1°C warmer than the average air temperature
– soil absorbs sun’s heat more readily than does air

Soil temperature fluctuate when?

What are some factors influencing soil temperature?

How much heat energy reaches the soil surface
– More in arid than humid regions

– Sun angle and day length (winter versus summer)

–Slope   –Soil color (light reflects more than dark)

– Soil cover (mulches, crop residues)

What happens to energy in the soil?

In general _______ colored objects absorb more heat and become _________ than lighter colored objects.

darker, warmer

 Heat capacity of water =

1 cal/g

Heat capacity of dry soil =

0.2 cal/g