Soils Exam 2

What factors determine flow in saturated soils?

1) Hydraulic force (f) -force driving water through soil (gravity) 2) Hydraulic conductivity (K) -the ease with which soil pores permit the movement of water

How can soil structure be described?

1) Type (shape) 2) Size (fine, medium, coarse) 3) Grade (strong, moderate, weak) 4) Stability

Microbial Activity in Soil

1) burrowing and molding activities of animals 2) production of organic glues by microorganisms (ex. mycorrhizae)

Prismatic

(Soil Structure Type) -B horizons -flat/angular tops -seen in palouse soils (arid/semi-arid regions)

Columnar

(Soil Structure Type) -B horizons -rounded tops -usually have a salt cap -found in arid/semi-arid regions

Granular (spheroidal)

(Soil Structure Type) -Characteristic of A horizons -best for plant growth -high water holding capacity -good root penetration -cemented by "microbial gums"

Blocky

(Soil Structure Type) -Common in B horizons -Could be angular or subangular -organic microbial gums and clays accumulate

Platy

(Soil Structure Type) -Common in E horizons (some A) -could be inherited from parent material -caused by compaction -caused by freeze/thaw cycles

Cementation

(aggregate formation)

Dispersion

(aggregate formation) -monovalent cations (Na+) are prominent (arid/semi-arid regions) the attractive forces are not able to overcome the natural repulsion of one negatively charged clay platelet by another -unable to flocculate therefore, remain dispersed -almost structureless and bad for plant growth

Flocculation

(aggregate formation) -mutual attraction among clay and organic molecules -If two clay platelets come close enough to each other, positively charged ions (Calcium (Ca++)) compressed in a layer between them will attract the negative charges on both platelets and hold them together

Soil Structure

-Determined by aggregates -spatial arrangement of sand, silt and clay (with organic matter and other cementing agents) into secondary particles called aggregates or peds. -Easier to observe when soil is dry (when wet the structural peds can swell, making the individual peds harder to identify) -controls the size and number of pores within and between aggregates -soil conditions and characteristics

Importance of soil structure (Biological)

-root penetration -microbial habitat

Polarity

-V shape angle at 105 degrees (asymmetrical) -charges are not evenly distributed- hydrogen side is more electropositive and oxygen side is more electronegative

Mass Flow

-bulk flow in a fluid -not the primary means of movement -wetting forces air out of soil -drying/drainage draws air into soil (evaporation, plant transpiration)

Importance of Soil Water

-determine the rates of water loss by leaching, surface runoff and evapotranspiration -balance between air and water in soil pores -regulates soil temperature -rate and kind of metabolism of soil organisms -capacity of soils to store and provide water for plant growth -transports toxic chemicals -involved in soil erosion

Effect of polarity

-encourages the dissolution of salts since the ionic components have a greater attraction for water molecules than for each other -hydration of cations -attraction of water to particle surfaces

Importance of Hydrogen Bonds

-high boiling point -high specific heat -high viscosity

What does K determine?

-how well a soil will perform in various uses (irrigate crop land, sanitary landfill cover, waste water storage lining, septic tank drain field)

How is the height of rise determined? (Capillarity)

-inversely proportional to the tube radius -inversely proportional to the density of the liquid -directly proportional to the liquid's surface tension and the degree of its adhesive attraction

Macropore

-larger than 0.08mm -allow for movement of air and water -large enough to accommodate roots and small animals

Particle Density (Dp)

-mass per unit volume of soil solids -(volume of soils includes voids-air and water) -essentially the same as specific gravity -is not affected by pore space (it's not related to particle size or the arrangement of particles)

Bulk Density (Db)

-measures the density of soil including pore space -mass of a unit volume of dry soil -includes both solids and pores -g/cm^3

What does water bond with?

-negatively charged particles -positively charged particles -itself

Diffusion

-random movement of molecules from high concentration to low concentration -controls reactions and processes in soils -small discontinued pore spaces (clays) slow diffusion -diffusion is much greater in air than water (10,000 times slower in water)

Micropore

-smaller than 0.08mm -formed from flocking of aggregates through cementing agents -hold water (don't permit much air movement because they're too small)

Adhesion

-water molecules bonding to a charged surface (through hydrogen bonds) -forms a single layer of water molecules on surface

Importance of soil structure (Physical)

-water movement -aeration -heat transfer -porosity

Surface Tension

-water would rather bond to other water molecules than to air -water molecules are highly ordered and resist disruption of this order -allows water to form drops and items with a density<1g/cm3 are able to "float" on the surface

Hydrostatic Potential

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Sub-irrigation

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Factors Affecting Bulk Density

1) soils with a high proportion of pore space to solids have lower bulk densities than those that are more compact and have less pore space. (any factor that affects soil pore space will affect bulk density) -sand has a higher bulk density than clay (clay has more pore space) 2) Depth of the soil profile -the deeper you go the higher the bulk density -due to compaction, less aggregation, fewer roots, less organic matter, etc.

Types of Energy Flow

1)Salts -Difference in concentration (diffusion) 2)Heat -Difference in temperature 3)Water -Difference in pressure and potential

Situations where mass flow occurs

1.Soil temperature changes (results in a change of velocity of all gas molecules-warmer molecules moving faster than colder molecules) 2. Atmospheric pressure changes (movement from high to low pressure) 3. Plant roots extracting water resulting in air flowing into empty pores. 4. Wind blowing over soil surfaces increasing evaporative loss (increase water movement to the atmosphere). 5. Flooding of soils displaces air; drainage draws air into soil.

Pressure Potential

2 factors: 1) positive hydrostatic pressure due to the weight of water in saturated soils and aquifers 2) negative pressure due to the attractive forces between the water and soil solids (soil matrix) -High water potential (value close to zero) indicates that water is loosely held by soil and highly available

Ideal soil composition

45% mineral 25% water 25% air 5% organic matter

Compacted soil composition

75% mineral 10% water 10% air 5% organic matter

How are aggregates described?

According to: -Shape -Size -Stability -Ease with which we can see them in soil

Gravitational Potential

Always pulls water downward -energy level is higher at lower elevations in the soil profile -heavy rains are key in recharging groundwater below the water table -Usually positive since soil water elevation is chosen to be higher than the reference pool (gravitational potential is zero at reference point)

Field Capacity

Amount of water retained in soil after it has been saturated and allowed to freely drain under the force of gravity. WP=-10 to -30kPa

Capillarity

Combined forces of adhesion, cohesion and surface tension -the smaller the radius of the tube, the more the water rises -water molecules are attracted to the side of the tube (adhesion) and to each other (cohesion) which creates surface tension -lower pressure under the meniscus in the glass tube allows the higher pressure on the free water to push the water up the tube

How can soil water content be measured?

Direct measurement -Gravimetric Indirect measurement -neutron probe -TDR (Time Domain Reflectometry) -Capacitance Depth of water in soils

Movement of air and water in soil pores

Even though sandy soils have a relatively low total porosity, the movement of air and water is rapid because of the dominance of macropores

Saturated Flow

Fast All pores full macropores conduct majority of water driven by gravitational and hydrostatic potential (matric potential is equal to zero)

How does water move through soil?

From high soil moisture to low soil moisture -greater matric potential gradient is established between a moist soil and a dry soil resulting in a more rapid water flow

Gravimetric Water Content

Grams of water per gram of soil

How are anaerobic conditions formed?

In water-logged or saturated soils CO2 accumulates relative to O2 because CO2 diffusion out of the soil and O2 replenishment are slow -organisms living in anaerobic conditions produce methane, nitrous oxide and hydrogen sulfide -conditions are generally not goof for plant growth (they require O2) -can lead to a build up of toxins -chemical reduction takes place

What type of pores move water in saturated soils?

Macropores

Soil Air Composition

Mainly nitrogen (same at atmosphere) soil air < oxygen soil air >> carbon dioxide (approximately ten times the amount in the atmosphere) -plants take in O2 from soil air and release CO2 back into soil air

Particle Density

Mass of solids divided by the volume of solids -constant 2.65 g/cm^3 -has nothing to do with size or arrangement of particles -depends on the chemical composition and crystal structure of the mineral particles -not affected by pore space

What forces affect soil water potential?

Matric osmotic gravitational

What type of pores move water in unsaturated soils?

Micropores (macropores are filled with air)

What are the types of water flow in soil?

Saturated Unsaturated Water Vapor

What are the factors that affect K?

Size and configuration of soil pores Sand=1x10^-2 cm/s Loam=1x10^-4 cm/s Silt Loam=1x10^-5 cm/s Clay=1x10^-8 cm/s

Hydraulic Gradient

The amount of force driving the water (in saturated soils)

Matric Potential

The attraction of water to solid surfaces (capillarity and adhesive forces) -Adhesive forces are greater than cohesive -Always negative because water attracted by the soil matrix has an energy state lower than that of pure water -sometimes called suction or tension -decrease in moisture content increases matric potential

Porosity

The volume occupied by pores divided by the total soil volume Generally range from 30% to 60%

Capillary Water

The water between field capacity and the hygroscopic point

Permanent Wilting Point (PWP)

The water content of soils at which plants will not regain turgor (they wilt and will not recover). WP=-1500kPa

Darcy's Law

Used to calculate quantity of water flowing through a column of saturated soil per unit of time

Richard's Equation

Used to calculate quantity of water flowing through a column of unsaturated soil per unit of time

How are soil pores distributed?

Varies with: -texture -aggregate size and type -bulk density

Volumetric Water Content

Volume of water per volume of soil (use decimal value of the ratio)

Hygroscopic water

Water held very tightly in soil, mostly by adsorption on soil colloids. WP=-3100kPa

Hydrogen Bond

a hydrogen atom of one water molecule is attracted to the oxygen end of a neighboring water molecule forming a low-energy bond between water molecules

Saturation

all soil pores are filled with water WP=0kPa

Aggregate

congregation of sand, silt and clay particles into one mass that are held together by binding materials like organic matter and clays -when soil is aggregated it is said to have structure

Water Potential

difference in energy levels between pure water (at reference state) and the soil water -water moves from an area of high water potential to low water potential

What influences water movement in soils?

differences in energy levels between water and adjacent points in the soil profile -Important because it reflects how easily plants can extract water (always a negative value)

How do stratified soils affect water movement?

differences in texture layers in the soil profile results in impeded water flow -each layer has a different conductivity ex. landslides, quicksand

How are particles brought together?

flocculation, water (freeze/thaw, shrink/swell), dispersion (animals), and roots

Aerobic

has oxygen unsaturated

Electronegative

having a negative electric charge

Reducing Conditions (anaerobic)

iron- reducing (gley-grey/blue) iron- oxidizing (rust-orange)

Anaerobic

lack of oxygen compaction saturated

Movement of Soil Air

moves from low oxygen concentration to high oxygen concentration

Unsaturated Flow

much more complex (compared to saturated soils) very slow not all pores full micropore flow driven by gravitational and matric potentials (matric potential is less than -30kPa)

What are the cementing agents?

organic matter, root exudates, carbonates and fine roots

Osmotic Potential

presence of solutes in the soil water relative to pure water -always negative -as water molecules cluster around the solutes, potential energy is lost (less freedom of movement) -water moves from low salt concentration to high salt concentration -can only happen with the presence of a semi-permiable membrane

Covalent Bond

sharing of valence electrons (water is covalently bonded)

What determines capillarity in soils?

size and distribution of pores -large pores allow for rapid initial capillary rise but limit the hight of rise

Oven-dry

soil dried at 105 degrees C for twenty-four hours. The only water that remains after oven-drying is very tightly bound to clay and organic matter.

Pure Water (water potential)

standard pressure and temperature unaffected by the soil and located at some reference elevation

Single-Grain

structural condition in which particles are not aggregated

Massive

structural condition in which soils occur as large, cohesive masses of material

Water potential values

terms of pressure or as the hight of the column bars or kPa (1bar=100kPa) measured with a tensiometer

Compaction

the process in which the volume and porosity of a sediment is decreased by the weight of overlying sediments as a result of burial beneath other sediments -decreases total pore space -results in higher bulk densities -lower water holding capacity -lower aeration -lower plant growth

Plant Available Water

the water between field capacity and the permanent wilting point

Cohesion

water molecules bonding to each other