Soils Exam 2

Weakest forms of bonding

Hydrogen bonds.

As water gets colder

It gets denser.

Coheasion

water molecules attracted to each other

Adhesion

water molecules attracted to solid surfaces

Surface tension

Cohesion between water molecules allows for resistance against an applied force. It can help give structure to soils.

Unsaturated zone

zone of soil, sediment or rock above the water table in which pore spaces are partially or completely filled with air.

Saturated zone

Rock openings are filled with water.

Soil water potential (Ψ) [kPa]

Energy state of the soil water relative to pure water.

Soil water content (Θ)

How much water is associated with a given volume of soil.

Percent saturation (S)

Vwater/Vpores

Θvolumetric

n*S or Vw/Vt

Gravitational potential (Ψg)

water density*gravity*height of soil from reference elevation.

Pressure potential (Ψm and Ψh)

m is matric potential (unsaturated) and h is hydrostatic potential (saturated)

Matric potential (Ψm)

negative pressure or tension.

Osmotic potential (Ψo)

Decreases with increasing solute content.

Θgravometric

Mw/Ms

Infiltration

Describes water entering the soil pore space from the surface and becoming soil water.

Percolation

describes the transport of water downward in the soil profile once it has entered the soil.

Diffusion

vapor water gradients

Saturated flow

Soil pores are full

Unsaturated flow

water is only held in smaller soil pores

Darcy’s Law (quantity of water per time)

Q(length3/time)=AK_sat[(h1-h2)/L]

Ksat

Saturated hydraulic conductivity.

How to find Ksat

Ksat=(Q/A)*(L/(h1-h2))

Infiltration Rate

i=Q/(A*t)

Qualitative soil water terms……

gives a general idea of soil wetness and plant-available water.

Saturated soil

water that percolates downward mainly due to gravity

Field Capacity

water that remains after gravity-driven water is removed

Wilting coefficient

Water content below which plants will wilt.

Hygrosicopic coefficent

water that remains because the soil is exposed to the atmosphere.

Gravitational Water

Water content at saturation (s) - water held at FC (FC)

Plant available water holding capacity (AWHC) or Plant available water (PAW)

water held at FC (FC) - water held at WP (WP)

Unavailable water

Water held at WP (WP) - oven dry.

Present wetness (PW)

Current soil moisture condition

Available water depleted (AWD)

percent of plant available water that has already been used/removed.

Field capacity depleted (FCD) or Soil water depletion (SWD)

percent of field capacity depleted at present wetness.

Finer texture………

has a higher wilting coefficient and field capacity.

Strenghts of ag drainage

Increase drainage and promote root growth

Limitations of ag drainage

Expensive and impacts the integrity of the soil.

Precipitation (P) =

Soil storage (SS) +Evapotranspiration (ET) + Discharge (D)

Evapotranspiration

Evaporation from land and water surfaces or plant leaves.

Where is the most available water found?

The ground.

Keys for plant water uptake

Surface area must be large, water potential must be strongly negative, and plants must maintain contact.

Soil Colloids

extremely small soil particle, usually clay or organic matter (humus), that carries an electrical charge and has a very large surface area. Soil colloids are important because they hold water and nutrients and help make them available to plants.

Specific Surface Area

total surface area/mass

Silicon tetrahedral

Clay type 1

Aluminum Octahedral

Clay type 2

Phyllosillicates

Layered sheet silicates.

Non silicate colloids

Octahedral sheets of Iron and aluminum oxides.

Humus

A complex soil colloid that is derived from organic material.

How is a mineral type determined in a colloid

X-ray diffraction.

How does X-Ray diffraction work?

Certain minerals have certain peaks in readings that can be matched with your colloid.

Colloid Surface Charge

Positive and Negative. Depends on pH.

Basic pH means

More negative.

Acidic pH means

More positive.

Isomorphic substitution

Permanent charge induced by fewer positive cations substituting into the crystal structure, resulting in a net negative charge on the clay surface.

Cation Exchange Capacity (CEC)

total amount of negative exchange sites on soil.

Acid cations

H⁺ and Al³⁺

Nonacid cations

remaining cations

Base saturation

percent of non-acid cations

Strength of cation absorption depends on

The cation charge density.

Smectite clays

Structures that can swell.

Is CEC pH dependent?

Yes. Low pH means lower CEC and vice versa.

Outer-sphere complexes

hydrated ions that have an electrostatic interaction

Inner-sphere complexes

Form covalent bonds with the colloid surface.

How to calculate CEC

[NH₄₊]*V_Wash*(1/MW)*(cmol/mmol)*(1/msolid)

How to calculate pH

-log(H+)

Inorganic Soil range

pH between 4 and 10.

What process produces H+ ions

Acidifying processes.

What process consumes H+ ions

Alkalinizing processes.

Soil pH “Location”

Soil Solution (active acidity), near colloidal surface (exchangeable acidity), and absorbed to colloids (residual acidity).

High leaching effect on pH

Decreased Soil pH

Low leaching effect on soil pH

pH decrease is less extreme.

pH buffering

Resistance to pH change in soils.

low pH buffers

Al-compunds

neutral buffering

cation exchange

Sting Ray Robb

Goat

High buffering

carbonates

Acidification

top of soil and acid rain is the input.

Alkalinizing

bottom of the soil from mineral weathering.