NRES 251 Exam 2

Plant Available Water

Difference between field capacity and permanent wilting point

Field Capacity

Maximum amount of water the soil can hold

Permanent Wilting Point

Point where plant can no longer extract water from soil

Active Acidity

the quantity of hydrogen ions that are present in the soil water solution

Exchangeable Acidity

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

Soil Structure

the way individual particles of sand, silt, and clay are assembled

Reserve/Residual Acidity

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

pH

the log10 hydrogen ions (H+) in the soil solution

Water Potential

the tendency of water to move from one area to another

Infiltration

process by which water on the ground surface enters the soil

Surface Seals (Crusts)

the orientation and packing of dispersed soil particles that result from the physical breakup of soil aggregates due to raindrop impact

Compaction

when soil particles are pressed together, reducing pore space between them

Plastic Limit

the moisture content at which a fine-grained soil can no longer be remolded without cracking

Liquid Limit

the water content where the soil starts to behave as a liquid

COLE

Quantifies the expansiveness of a soil

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

Soil Color

Hue/Value/Chroma

What soil color tells you: Comp of parent material, fertility, temp, drainage, soil classification

Hue: Dominant wavelength

Value: Brilliance of color/ light

Chroma: Relative purity of dominant wavelength

Thermal Conductivity

a measure of the soil's ability to conduct heat

Heat Capacity

characterizing the amount of heat that can be stored in soil

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

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

CEC

the total capacity of a soil to hold exchangeable cations

Capillary Rise –
Pore size, Wetting angle
etting angle

the ability to draw water upward, against the force of gravity through tiny channels or capillaries within the soil

Infiltration –
Particle size, Structure, Crusting, Runoff and Flooding

humus, clay, silt, sand, gravel

Structure-the grouping of soil particles (sand, silt, clay, organic matter, and fertilizers) into porous compounds.

Crusting- when rain separates the soil into very small aggregates and individual particles that cement into hard layers at the soil surface when drying occurs rapidly

Runoff- the likelihood for surface runoff to occur during rainfall or snowmelt

Flooding- those in which water covers the soil,

Plant Available Water –
Capillarity and matric potential
Soil texture, Pore size, Evaporation/Transpiration
Wetting angle and water retention
Water repellency and fire

Capillarity- the ability to draw water upward, against the force of gravity through tiny channels or capillaries within the soil

Matric potential-the pressure potential that arises from the interaction between water and the matrix of solid particles in which the water is embedded

Soil Texture-the proportion of sand, silt and clay sized particles that make up the mineral fraction of the soil

Transpiration-Plants put down roots into the soil to draw water and nutrients up into the stems and leaves

Wetting angle-angle between a water droplet and a solid surface

Water retention-strongly related to particle size

Water repellency- reduces the affinity of soils to water such that they resist wetting for periods ranging from a few seconds to hours, days or weeks

Fire- can make soil water repellent

Soil Acidity and pH
Components

Connection to CEC

Acidity- caused by organism metabolic processes, leaching of bases, redox rxns, weathering of minerals, rain

pH- Effective CEC changes with pH

○ Higher pH higher effective CEC (selectivity) ○ Higher pH leads to more negative charge associated with base cations

○ It is the percent saturation of acids or bases not the total number present that relates to pH ○ Fertility

Buffering Capacity

Resistance to a change in pH

pH 5-7 buffering from equilibrium of active, salt- replaceable and residual acidity

 Addition of H - movement closer to surfaces

 Removal of H - movement from surface to solution

Permanent charge

Variable charge

The negative charges associated with isomorphous substitution

pH-dependent charge

Soil Acidification

Carbonic acid

Acids from biological metabolism

Accumulation of OM

pH and nutrient availablility

pH determines nutrient availability

Building/Losing Soil Organic Matter

Adding OM decreases pH

Binds tightly to Al

Soil Consistence (strength)

The behavior of soil particles to applied pressure

Forces that resist applies pressure include:

Cohesion (Like substances)

Adhesion (Unlike substances)

Interlocking forces

Related to:

Particle size, clay content, type of clay content, water content

Classified based on moisture content

Atterberg limits: Water contents of fine-grained soils at different states of consistency (plastic and liquid limit)

Plasticity Index: PI = LL-PL

PI > 25 Is poor for construction

Hydrology and Water (Chapter 5)

a. Hydrologic cycle basics

b. Physical properties of water i. Why does it behave that way?

c. Adhesion, cohesion, and capillarity

d. Pore size and water potential

e. Components of total water potential

f. Plant available water i. Texture and organic matter effects

a.

b. Cohesion, adhesion, surface tension

Density: water has a low density

c. Cohesion: attraction between like molecules

Adhesion: attraction between unlike substances

Capillarity: Water rises in a small tube against gravity

d.the amount of work needed to move water from a reference pool to another point.

e. gravitational, submergence, matric, osmotic potential energy

f. Type of plant, soil moisture tension, plant nutrients, soil layering

Soil Air and Temperature (Chapter 7)

a. Composition i. What is it and what affects it

b. What causes it to move i. Factors affecting air movement

c. Soil air and effects on oxidation and reduction

d. Soil Temperature i. Importance ii. Thermal properties iii. What influences temperature and thermal properties iv. Management to control soil temperature

a. Oxygen, nitrogen, carbon dioxide, humidity

b. Excess water rate of respiration, soil heterogeneity, subsoil vs surface soil

c. Redox and impact on soil color and classification

d. i. Biological activities, physical and chemical rxns.

ii. Heat capacity: Amt of energy needed to raise temp of a volume of substance by 1C

Specific Heat: # calories needed to raise temp of 1 gram of substance by 1C

iii. Location, face and angle of slope, large water bodies, vegetation, surface soil conditions

iv. Drainage, manage surface conditions

4. Colloids (Chapter 8)

a. Mineral structures i. 2:1 ii. 1:1 iii. Examples of each

b. Functions of colloids

c. Surface Area

d. Charge

e. CEC and AEC

a. i. Mica, illite, vermiculite, montmorillonite; chlorite (ex. smectite)

ii. kaolinite (ex. silicate clays)

b. Holds plant nutrients, retains pollutants, controls buffering, equilibrium between soil and solution

c. Very large surface area

d. Silicates- negative charge

non-silicates- positive charge due to covalent bonds

e. Cations able to exchange with other cations in the soil solution

Exchangeable anions held on positive charges – anion exchange capacity