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