SOILS 101: Comprehensive Exam 2 Study Aid with Key Terms and Definitions

Sandy soils

One of the 3 common names for soils

-For soils with coarse textures

Loamy soils

One of the 3 common names for soils

-For soils with moderately coarse, medium, or moderately fine textures

Clayey soils

One of the 3 common names for soils

-For soils with fine textures

12

How many basic soil textural class names are there?

Sand

Particles of 2.0 – 0.05 mm diameter

Silt

Particles of 0.05 – 0.002 mm diameter

Clay

Particles of < 0.002 mm diameter

Coarse fraction

Particles greater than 2 millimeters in diameter

-gravel, cobble, stone, boulders

-occupy space but have little to no porosity

-reduce soil's water holding capacity

-may inhibit root penetration

-may inhibit cultivation

Soil sieves

A method to determine particle size distribution

-consists of a stack of sieves

-top sieve has largest pores while bottom sieve has smallest

-after being placed on a shaker, the soil will be sorted into the various sieves according to particle size

small

______ particles have:

-greater surface area per unit mass

-greater capacity to hold water and nutrients

-greater interaction with humus molecules

-greater area for microbial colonization

-greater attraction between particles

greater

Small particle size in soil results in ________ total pore space, even though individual pore sizes are small.

field capacity

The amount of water soil can hold after excess water has left the soil profile

permanent wilting point

The minimum amount of water that must be in the soil profile so that plants don't wilt

water available to plants

The area between the curves of field capacity and permanent wilting point

Infiltration

The movement of water from soil surface into the soil

Permeability

How easily water, air, and roots move through the soil

High

____ permeability results from a mixture of particle sizes, large pores, and connections between pores

Rapid

What is the speed of water movement in single grain and granular structured soils?

Moderate

What is the speed of water movement in blocky and prismatic structured soils? (Water moves by preferential flow)

Slow

What is the speed of water movement in platy and massive structured soils?

Soil structure

The aggregation of individual soil particles into larger units (peds) with planes of weakness between them

-particles are held together by roots, fungi, and weak cements such as organic compounds, clays, carbonates, Fe oxides, and silica

Hydrometer Method

Determination of soil texture based on suspending soil separates in a column of water and measuring density of the liquid with a special instrument

Stokes' Law

An equation describing the velocity of a particle falling through a fluid

-used to determine the time it will take soil particles to settle

Bulk density

The mass of dry soil per unit volume, including the air space

Field rupture test

A soil consistency test used in the field by engineers to see how easily a soil breaks apart

-performed by crushing soil between fingers or under foot

Field penetration test

A soil consistency test used in the field by engineers to see how easily a soil can be penetrated

-performed using a penetrometer or the blunt end of a pencil

Unconfined compression test

A soil consistency test used by engineers to see how easily a soil withstands pressures

-a column of moist soil is placed between two porous stone plates and known pressure is applied.

Standard Proctor Test

A soil consistency test used by engineers to determine the optimum water content for best bulk density (and therefore stability) of a soil

-drop hammer onto cup of compacted soil at known water content

solid (rigid) state

State of soil with low water content

semi-solid state

State in which soil has intermediate water content but may undergo some limited deformation

plastic state

State in which soil has intermediate water content and is very malleable

-as a result, soil can slide (as in mudslides)

liquid state

State in which soil has very high water content and flows like liquid

plow pan

A compacted, impermeable layer 6 inches below the soil surface with high bulk density, caused by tilling

Moldboard plowing

An aggressive form of tilling that disturbs the soil 15-20 cm below the surface.

old root channels

Channels in the soil previously occupied by the roots of previous crops

-Important for allowing air and water movement and spaces for new roots

floatation tires

Wide tires that can be used on farm equipment to distribute weight over a larger area, thus reducing compaction

organic matter

Higher _______ ________ helps remediate compaction because it stabilizes soil structure

subsoiler

A machine for occasional deep ripping tillage to break up compacted subsurface layers

-should only be performed in extreme cases

cover crops

Crops planted to prevent erosion, retain nutrients, and build organic material in the times that main cash crops are not being grown

tillage radish

A cover crop that can be grown to remediate compaction

-the plant has the ability to grow through compaction

-root decays in place, leaving soil broken up

Slake test

A field test to determine the stability of soil aggregates

-when placed in water, stable soil structures will remain intact longer than unstable ones

Macroaggregates

3 mm diameter aggregates held together by roots and fungal hyphae

Microaggregates

0.3 mm diameter aggregates held together by root hairs, hyphae, and polysaccharides exuded by plants and microbes

Submicroaggregates

0.03 mm diameter aggregates consisting of mineral grains encrusted with plant and microbial debris, or plant debris coated with clay

Primary particles

0.003 mm diameter particles of silt, clay, and humus

Glomalin

A substance exuded by some microbes that acts as glue holding aggregates together

-low soil disturbance leads to more being produced, and thus better soil structure

soil tilth

The physical condition of soil in relation of its ease of tillage, fitness as a seedbed, and resistance to root growth and seedling emergence

friability

How easily clods of soil can be crumbled

conventional tillage

Tillage technique that:

-destroys soil structure and old root channels

-depletes soil organic matter

-compacts soil

conservation tillage

Tillage technique that minimizes the effects of disturbance

-soil will still not have as good a structure as a native prairie, for example

water content

What does ϴ symbolize?

gravimetric moisture content

What does %ϴm symbolize?

volumetric moisture content

What does %ϴv symbolize?

bulk density

What do you multiply %ϴm by to get %ϴv?

saturated flow

Relatively rapid water movement in the soil where water flows through macropores

-driven by differences in gravitational potentials

unsaturated flow

Slower water movement, where water moves in thin adhered films on surfaces within the soil

water vapor

Water that moves through the soil in gaseous form

-may be rapid, but does not account for very much mass movement

Darcy's Law

Saturated Hydraulic Conductivity

What does Ksat symbolize?

Faster

Will water move faster or slower in soils with larger pores?

perc test

Test used to determine if a soil is suitable for installing a septic system by observing how quickly water is absorbed by the ground.

Moderate

When doing a perc test, would you want a low, moderate, or high Ksat value?

matric forces

Forces within the soil matrix that drive unsaturated flow

Perching effect

What is it called when an abrupt texture change in a profile results in water being unable to drain into the soil below?

Total water potential

What does Ψt symbolize?

-the sum of water potentials

positive

When Ψt is ________, the soil will have ponded water on the surface, creating a hydraulic head.

0 kpa

When Ψt is ________, the soil will be saturated.

-33 kpa

When Ψt is ________, the soil will be at field capacity.

-1500 kpa

When Ψt is ________, the soil will be at wilting point.

Matric Potential

What does Ψm symbolize?

-a negative potential that is between -33 and -1500 kpa

-results from the cohesive and adhesive properties of water, which allows capillarity and therefore matric forces.

Osmotic Potential

What does Ψo symbolize?

-a negative potential that is driven by excess salt (or fertilizer)

-usually negligible unless in a desert or greenhouse

Gravitational Potential

What does Ψg symbolize?

-a positive potential resulting from the force of gravity pulling water downwards through the soil

-negligible in unsaturated soil, but important in saturated soil

Colloid

Soil particles less than 2 micrometers in size with very high surface area

-may be clay minerals or humus

Phyllosilicate clays

A group of minerals that are very important in soils

-name derived from Greek for "leaf"

-includes 1:1, 2:1, and 2:1:1 types

Tetrahedron

An arrangement of 1 silicon cation surrounded by 4 oxygen anions

Octahedron

An arrangement of 1 aluminum or magnesium cation surrounded by 6 oxygens or hydroxyls

1:1 clay

A clay made of layers consisting of one tetrahedral sheet and one octahedral sheet

-Hydrogen bonding between layers limits expansion when wet

-External surface area only

-Low isomorphic substitution -> no permanent negative charge -> low CEC

Kaolinite

An example of a 1:1 clay

-used for fine china and porcelain, glossy paper, filler for paint and plastics

2:1 clay

A clay made of layers consisting of 1 octahedral sheet sandwiched between 2 tetrahedral sheets

Isomorphous substitution

The replacement of one atom by another of similar size in a mineral

-usually the replacing cations are lower valence (have less charge)

-occurs during mineral formation in both tetrahedral and octahedral sheets

-example: Al3+ can substitute for Si4+ in a tetrahedral layer

Negative

What is the charge of most clay particles in soil?

-caused by reducing the positive charge of cations in the layers via isomorphous substitution

Cation Exchange Capacity (CEC)

-Sum total of exchangeable cations that a soil can adsorb

-Expressed as cmol of charge / kg of soil

Internal surface area

Area of surface exposed between individual crystal layers

External surface area

The area of surface exposed on the top, bottom, and sides of a clay crystal

increased

Increased surface area will lead to ________ CEC

1:1 clays

Less weathered minerals, most likely found in cold or hot deserts, or young volcanic soils

2:1 clays

Moderately weathered minerals, most likely found in temperate climates

Micas

A group of 2:1 clays

-non-expanding

-higher rate of isomorphous substitution because there is space between the two tetrahedral sheets

-excess negative charge satisfied by the interlayer K+ cations

-low CEC

-K+ acts as a binding agent preventing expansion

Vermiculites

A group of 2:1 clays

-semi-expanding

-more surface area and higher CEC than micas

-interlayer space is filled with cations and water

-isomorphous substitution occurs in tetrahedral sheets

Smectites

A group of 2:1 clays

-fully expanding - "shrink-swell clays"

-can adsorb huge amounts of water

-high surface area and high CEC

-limits construction on sites where it is present

-isomorphous substitution occurs in octahedral sheets

Smectite uses

cat litter, landfill or pond liners

2:1:1 clays

Clays with an additional Mg hydroxide sheet between 2:1 layers

-binds the 2:1 layers together very tightly

-nonexpansive, no interior surface area

-high net negative charge

-negative charge compensated by a positively charged Mg hydroxide sheet

-low CEC

chlorite

Example of a 2:1:1 clay

Noncrystalline silicate clays

Colloids associated with fresh volcanic materials - relatively "young"

-poorly crystalline

-very reactive

-high CEC

-high water holding capacity

Allophane and Imogolite

Examples of noncrystalline silicate clays

Fe and Al oxides

Most severely weathered soil colloids, most likely found in warm, wet climates like rainforests

-weathered from Fe- and Al-bearing primary minerals

-very reactive surfaces

-net charge is slightly negative to moderately positive

-low CEC or have anion exchange capacity (AEC)

goethite and hematite

Examples of iron oxides

gibbsite

An example of an aluminum oxide

humus

Colloids made of highly decomposed biomass

-dark in color

-contain common organic functional groups (carboxyl and phenolic)

-Deprotonation of these functional groups leads to pH-dependent CEC

-High CEC

-Very high water holding capacity

Aridisols

-CaCO3 - containing soils of arid regions that exhibit at least some subsurface development

-Ochric epipedon with light color and low organic matter

-Often have an accumulation of calcium carbonate, salts, gypsum, etc.

-Widely distributed worldwide in desert regions

-Where irrigation is available, they can be highly productive