NRES 251 - Exam II

Plant Available Water

Difference between the water content of a soil at field capacity and the soil water content at the permanent wilting point.

Field Capacity

The percentage of water remaining in a soil two or three days after its having been saturated and after free drainage has practically ceased.

Permanent Wilting Point

the point where water content isn’t sufficient to sustain plant life. 

Active Acidity

The activity of hydrogen ions in the aqueous phase of a soil. It is measured and expressed as a pH value.

Exchangeable/Salt Replaceable Acidity

The amount of acid cations, aluminum and hydrogen, occupied on the CEC.

• In exchange sites

• Exchangeable with one molar potassium chloride (KCl)

Reserve/Residual Acidity

The combined acid potential of H+, Al+3, AI (OH)+2, and AI (OH)2+1 ions adsorbed on clay colloids.

• Titratable

• Needs a strong base like NaOH. If you keep adding that strong base, it’ll eventually react.

Soil Structure

The arrangement of particles into aggregates.

pH

The negative logarithm (base 10) of the activity of hydronium ions.

Water Potential

The tendency of water to move from one area to another.

Infiltration

The process by which water enters the soil pore spaces and becomes soil water.

Surface Seals (crusts)

A thin layer of fine particles deposited on the surface of a soil that greatly reduces the permeability of the soil surface to water.

Plastic Limit

Point at which increasing the soil’s water content will make it malleable plastic mass and cause shrinking and swelling.

• Measured in percentage of mass water content.

Liquid Limit

Causes soil to transform to viscous liquid that will flow when jarred.

COLE

Coefficient of Linear Extensibility (COLE) quantifies the expansiveness of the soil.

• Measurements: Moisten soil to plastic limit and measure LM. Dry and measure LD. COLE is the % reduction in length of soil bar.

Hue

Dominant wavelength of light. The actual color, so to speak.

Value

Brilliance of color, quantity of light.

Chroma

Relative purity of dominant wavelength. 

Thermal Conductivity

The ease of heat movement through a material. NOT how much heat. (cal/cm² s)

Thermal Conductivity of Air

0.06

Thermal Conductivity of Water

1.4

Thermal Conductivity of Solids

1 to 30

Thermal Conductivity of Dry Sandy Soil

1.0

Thermal Conductivity of Wet Sandy Soil

4.5

Isomorphic Substitution

Replacement of one atom by another of similar size in a crystal structure without disrupting or changing the crystal structure of the mineral

pH-Dependent Charge

(1:1) At low pH, soil charge becomes more positive. At high pH, negative charge increases. Overall charge is usually negative.

Permanent Charge

(2:1) Isomorphic substitution. More CEC, universally. Less charge because of how tightly the charge holds onto the potassium

Mica

Has non-exchangeable potassium in the interlayer.

Mica can weather into…?

Vermiculite or smectite

1:1

Silicate clays.

1:1 Qualities?

— 2 types of sheets tightly held
together by Van Der Waals forces and H-
bonds (non expanding).
— Effective surface is exterior
— Low net negative charge

2:1 Qualities?

— 1 octahedral and 2 tetrahedral

Smectite (Expanding)

(2:1)

— Interlayer expansion
— Mostly negative charge
— High cation adsorption capacity
— Top and bottom planes form
— Oxygen – water / cation bonds
• Strongly attracted to interlayer space
• High shrink swell

Vermiculite (Expanding)

(2:1)

– Dioctahedral dominated
– Very high negative charge
compared to other clays
– Highest cation adsorption
capacity of all clays
– Limited expansion

Micas (non-expanding)

— 2:1

— Fairly High negative charge
— Charge attracts Potassium -
tightly fits in interlayer space

Chlorite (non-expanding)

– Fe or Mg in octahedral
sheets
– Mg hydroxide sheet between
• Bonded to tetrahedral sheets

Nonsilicate Minerals

• Iron/Aluminum oxides and
Hydrous oxides
• Neither tetrahedral sheets nor Si in
their structure
• Covalent bonds —> positive charge
• Hydroxylated surface —> anion
adsorption

CEC

The amount of positive charge that can be exchanged per mass of soil, usually measured in cmol_c/kg.

Wisconsin CEC?

We usually have relatively large CEC and 2:1 clays.

Capillary Rise

A specific effect of capillarity where water rises in small pores or tubes due to capillary action.

How does capillary rise relate to pore size?

It is greater in smaller pores. So, the smaller the pore, the tighter the water is held by capillary forces. The opposite is true for larger pores.

Wetting Angle/Contact Angle

The angle formed between a liquid (usually water) and a solid soil particle at the point where the liquid, solid, and air meet. It is a measure of how well water spreads over or adheres to the soil surface.

How does capillary rise relate to wetting angle?

Soils that are hydrophilic have smaller wetting angles, meaning water spreads easily on the soil particle surfaces. Soils that are hydrophobic have larger wetting angles, meaning water does not spread easily on the particle surfaces.

How does infiltration relate to particle size?

• Large Particles (Sandy Soils):

  • — High infiltration rate due to large pores (macropores).

  • — Water quickly moves downward with little surface accumulation.

  • — Lower water retention, leading to faster drainage.

• Small Particles (Clayey Soils):

  • — Low infiltration rate due to small pores (micropores).

  • — Water moves slowly and remains near the surface longer.

  • — High water retention, increasing runoff when saturated.

• Loamy Soils (Medium-Sized Particles):

  • — Moderate infiltration rate, balancing water retention and drainage.

  • — Preferred for agriculture due to optimal water availability.

How does infiltration relate to soil structure?

• Well-Structured Soils (Granular, Aggregated):

  • — High infiltration due to interconnected pores.

  • — Promotes good drainage and reduces surface runoff.

• Poorly Structured Soils (Compacted, Platy, Massive):

  • — Low infiltration due to restricted pore space.

  • — Leads to waterlogging and increased runoff.

How does infiltration affect crusting?

Low infiltration rates for raindrops can lead to droplets hitting the soil surface and forming a “crust” over the rest of the soil.

How does infiltration affect runoff?

Runoff occurs when infiltration is lower than rainfall intensity.

How does infiltration affect flooding?

Flooding results when runoff accumulates and exceeds drainage capacity.

What is the relationship between plant available water, capillarity and matric potential?

Capillarity

• — Water moves through soil pores due to adhesion (water sticking to soil particles) and cohesion (water molecules sticking together).

• — Fine-textured soils (e.g., clay and silt) have stronger capillary forces, retaining water longer.

• — Coarse-textured soils (e.g., sand) have weak capillary action, leading to rapid drainage and lower PAW.

Matric Potential

• — The force by which soil holds water against gravity.

• — High matric potential means water is tightly bound to soil particles, making it harder for plants to extract.

• — Low matric potential means water is loosely held and more available to plant roots.

What is the relationship between plant available water, soil texture, pore size, and evaporation/transpiration

• Soil Texture & Pore Size:

  • — Coarse-textured soils (sandy): Large pores, fast drainage, low water retention → Low PAW.

  • — Fine-textured soils (clay): Small pores, high water retention, but water held too tightly → Moderate PAW.

  • — Loamy soils: A mix of particle sizes, optimal balance of retention and drainage → High PAW.

• Evaporation & Transpiration:

  • — Evaporation removes water from the soil surface, reducing PAW.

  • — Transpiration pulls water through plant roots and releases it through leaves.

  • — Sandy soils dry out quickly due to high evaporation.

  • — Clay soils resist evaporation but lose water slowly over time.

  • — Organic matter in soil reduces evaporation and improves PAW by enhancing water retention.

What is the relationship between plant available water, wetting angles and water retention?

Hydrophilic soils (low wetting angle)

• — Higher PAW due to better water absorption.

Hydrophobic soils (high wetting angle)

• — Lower PAW due to poor infiltration and increased runoff.

What is the relationship between plant available water, water repellency and retention?

Fire can cause soil to become hydrophobic, creating a water-repellent layer. This leads to lower PAW.

What is the relationship between soil acidity, pH, and CEC?

Soil Acidity and pH

— Soil acidity refers to the concentration of hydrogen ions in the soil. More H⁺ ions = more acidic soil. pH is how we measure soil acidity.

pH and CEC

• High CEC soils (clay, organic matter):

  • — More sites to hold cations like calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), and hydrogen (H⁺).

  • — pH changes more slowly due to buffering capacity.

  • — Can hold acidic cations (H⁺, Al³⁺), increasing soil acidity over time.

• Low CEC soils (sandy soils):

  • — Fewer cation exchange sites, meaning fewer nutrients are retained.

  • — pH can fluctuate more easily with acid rain or fertilizers.

  • — Prone to nutrient leaching and acidity buildup.

Buffering Capacity

Active hydrogens floating around in the water stick to a CEC site and don’t float around anymore, and limit pH levels. The higher the CEC, the more buffering you have, because it is harder to acidify the soil. It would also be harder to undo the full acidification of a soil.

What can color tell us about a soil?

• Composition of parent material

• Fertility

• Temperature (Climate)

  • — Can get a vague idea of the past climate. Red colors are generally indicative of a warm climate.

• Redox/drainage

• Soil classification

Adhesion

Sticking to something else.

Cohesion

Attraction between like molecules.

Capillarity

A general term describing the ability of a liquid to move through narrow spaces (such as soil pores) against gravity due to adhesion, cohesion, and surface tension.

Water Potential

The difference in energy levels between pure water in the reference state and that of the soil water.

Components of Water Potential

Ѱ_g — gravitational potential energy

Ѱ_s — submergence potential energy

Ѱ_m — matric potential energy

Ѱ_o — osmotic potential energy

How does organic matter affect PAW?

Direct Impact

— Organic material has a high capacity for water, so soils with more OM can hold more water

Indirect Impact

— OM can help stabilize soil, so the more OM a soil has, the better infiltration and water-holding capacity a soil will have.

Soil Aeration Composition

— 20.6% Oxygen
— 79.2% Nitrogen
— .25% Carbon dioxide

Factors Affecting Soil Air Composition

— Excess water
— Rate of respiration
— Subsoil vs. surface soil
— Soil Heterogeneity
— Profile
— Tillage
— Size of pores
— Plant roots

Soil Aeration — Diffusion

Most of the gaseous interchange between the soil and the atmosphere takes place by diffusion. Diffusion is the process by which each gas tends to move in the space occupied by another as determined by the partial pressure of each gas.

Soil Aeration — Mass Flow

Occurs when a pressure gradient exists. This involves the bulk flow of gas in a particular direction.

How does soil air affect oxidation and reduction?

The amount of oxygen in soil air determines whether oxidation or reduction processes dominate.

What is the importance of soil temperature?

— Biological Activities

— Chemical Reactions

— Physical reactions

Thermal Properties — Heat Capacity

The amount of heat energy needed to raise the temperature of a volume of substance 1 degree Celcius.

Thermal Properties — Specific Heat

The number of calories needed to raise the temperature of 1 gram of a substance 1 degree Celsius (cal/gm).

Thermal Properties — Thermal Conductivity

The ease of heat movement through a material. NOT how much heat. (cal/cm² s)

Factors Affecting Soil Temperature

• Location relative to the equator

  • — Direct sunlight

• Face of slope

  • — How much direct sunlight one slope gets

• Angle of slope

  • — The more an angle aligns with the sun, the more heat you’ll get

• Larger water bodies

  • — High heat capacity = better temperature regulation

• Vegetation

• Surface soil condition

Managing Soil Temperature

• Water in soil

  • — Moisture is a darker color and absorbs radiation better, but too much moisture will make it hard to increase temperature.

• Roughness

• Mulch

  • — Usually keeps things cooler

• Slope

  • — Terraces

• Vegetation

Surface of Colloids

All soil colloids have a large external area per unit mass, more than 1000 times the surface area of the same mass of sand particles. Some silicate clays also possess extensive internal surface area between the layers of their platelike crystal units.

Colloid Charges

Both internal and external colloid surfaces have positive and negative charges.

Importance of pH in Soils

• Corrosion of structures

  • — Concrete is part calcium carbonate, and can be dissolved by acid. Caves also form from carbonic acid. 

  • Metal is affected the same way.

• pH dependent charge: CEC, nutrient availability

  • — Toxicities and deficiencies

  • — You can have too much iron in acidic conditions.

  • —The master variable. It is the most important chemical measure we will ever do.

  • — pH is important for plant and microbial activity. 

Importance of Phosphorous?

An incredibly important nutrient. It’s greatly affected by pH. H₃PO₄ will only be present if it’s very acidic. Unfortunately, if it’s too acidic, it locks up with aluminum, and if it gets too basic, it locks up with calcium.

Nutrient Availability

• — Neutral pH – N, P, K, Ca, S, Mg

• — Acid pH - Fe, Cu, Zn, B, Mn

• — Alkaline pH - Mo, Se

Ionic Radius

The charge to size ratio of a particular ion.

Lyotropic Series (Order of Preference)

Tells us about the selectivity for certain cations. It doesn’t necessarily follow charge exclusively, but it is a part of it.

1. Aluminum (Acidic cations). Can be good if the aluminum is on the surface, so organisms aren’t coming into contact with it.

2. Hydrogen (Acidic cations). If you have hydrogen stuck on an exchange site, it affects pH.

3. Calcium (Base cation).

4. Magnesium (Base cation).

5. Potassium (Base cation) + Ammonium. They’re on equal footing, but there’s not a whole lot of ammonium out in the natural soils of the world. It is, however, the same size and charge as potassium.

6. Sodium (Base cation).