Chapter 6: Soil Ecology

Question 1

Give three ways in which rocks are formed.

Answer 1

Igneous rocks: rock formed from cooling magma
Sedimentary Rocks: layers of sediments in lakes/oceans put under tremendous pressure which consolidate into rocks
Metamorphic Rock: when either Igneous or Sedimentary are heated and put under pressure so that they transform into new rock

Question 2

Give four ways parent material may be broken down or added to soil.

Answer 2

• Mechanical or Chemical Weathering
• Climate: temperature and rainfall
• Biotic factors: organisms
• Topography: slopes, runoff, rate of erosion

Question 3

How does chemical weathering of parent material occur?

Answer 3

when rainwater containing dissolved carbon dioxide becomes carbonic acid and dissolves rock

Question 4

What is the difference between till and loess?

Answer 4

Till is parent material added by rivers or glaciers

Loess is parent material added by wind

Question 5

What role do biological growth and decomposition play in soil formation?

Answer 5

it helps break down parent material and affects the accumulation of humus (organic material in soil)

Question 6

What role does climate play in soil formation?

Answer 6

it determines the rate of weathering of parent material and decomposition of minerals and organic matter, the leaching and movement of these which develops the soil layers

Question 7

What role do biotic factors play in soil formation?

Answer 7

it is responsible for the organic matter in soil and the color of the surface and it influences the nutrient content in soil

Question 8

How does topography affect soil ecology?

Answer 8

it determines how much water enters the soil, the rate of erosion and the downhill transport of soil material

Question 9

How do young and old soils compare in terms of their fertility and why?

Answer 9

Young soil is more fertile because old soil has had long-time leaching of nutrients without replacement from fresh minerals

Question 10

Characterize each of the soil horizons.

Answer 10

O: surface layer resting on a mineral layer composed of freshly, partially decomposed organic material that has not been mixed into the mineral soil (Oi: litter layer, Oa: humus layer)
A: accumulation of humus, loss of clay, inorganic minerals, and soluble matter by eluviation (leeching), minerals dissolved by water then readily available to plants
B: zone of extensive illuviation (deposit of minerals from A horizon
C: primarily weakly weathered material that resembles parent rock, little affected by soil forming processes
R: unweathered parent material or bedrock

Question 11

What is the difference eluviation and illuviation, and in what soil horizons does each typically occur?

Answer 11

Eluviation is the loss of minerals/organic molecules through leeching (A horizon)
Illuviation is the deposit of the material that has been leeched from higher horizons (B horizon)

Question 12

What kind of information can be gained from soil color?

Answer 12

what it is made of, presence of oxides, indicators of good drainage, aeration

Question 13

How is soil color normally determined?

Answer 13

Standardized color charts; Munsell color charts

Question 14

Characterize each of the following in terms of soil texture: gravel, sand, silt and clay

Answer 14

Gravel: particles greater than 2mm
Sand: 0.05-2mm, feels gritty to the touch
Silt: 0.002-0.05mm, feels like flour
Clay: must be seen with microscope

Question 15

Give two important properties of the soil controlled by clays.

Answer 15

• Plasticity: the ability to change and retain shape when pressure is applied
• exchange of ions between soil particles and soil solutions used by plants

Question 16

How does soil depth differ from one place to another?

Answer 16

Native grasslands: several meters deep
Forests: relatively shallow
Deserts: very shallow
Bottom of slopes, level ground & alluvial (outwash) planes: very deep
Ridge tops and steep slopes: shallow

Question 17

What is meant by the terms “field capacity” and “permanent wilting point”?

Answer 17

Field Capacity: maximum amount of water soil can hold following drainage of gravitational water
Permanent Wilting Point: point where soil dries out to a point at which plants are unable to absorb enough water to replace that lost by transpiration

Question 18

How do field capacity and permanent wilting point relate to available water capacity?

Answer 18

Available water capacity is the amount of water retained between the field capacity and the permanent wilting point

Question 19

Give three factors that play a role in determining soil moisture regimes

Answer 19

Texture: intermediates are best
Amount of organic matter
Topographic position: affects movement of water on surface and within soil

Question 20

What is soil moisture regimes?

Answer 20

the ability to retain moisture against drainage

Question 21

With respect to soil drainage classes, what type of soil marks the beginning of wetlands?

Answer 21

somewhat poorly drained soils

Question 22

What are micelles and what role do they play in soil chemistry?

Answer 22

Micelles: particles and complexes of clay and humus that hold ions
They affect the mobility of ions; prevent charged nutrient cations from leeching

Question 23

Describe the structure of a clay molecule.

Answer 23

aluminum, silica and oxygen arranged in a complex tetrahedral fashion

Question 24

What are isomorphous substitutions?

Answer 24

the substitution of one element in the arrangement without changing the structure

Question 25

What type of compounds do clays attract and why?

Answer 25

cations, water molecules and organic substances
This is because when aluminum is isomorphously substituted for silica, a negative charge results which must be balanced by a positive charge

Question 26

What does the cation exchange capacity of the soil represent and how does it prevent the leaching of soil nutrients?

Answer 26

It is the total number of negatively charged exchange sites on clay and humus particles that attract cations.
Because nutrients are cations, they are pulled to these negatively charges particles and will not leach.

Question 27

What does percent base saturation represent?

Answer 27

the percent of sites occupied by ions other than hydrogen

Question 28

How are hydrogen ions added to the soil?

Answer 28

rain water (carbonic acid), acids from organic matter and metabolic acids produced by microorganisms and plants

Question 29

why do acidic soils have a low percent base saturation?

Answer 29

because they have a high number of exchangeable hydrogen ions

Question 30

What is the relationship between soil acidity and soil fertility and aluminum toxicity?

Answer 30

High soil acidity equals low soil fertility and can lead to aluminum toxicity
Low soil acidity equals high soil fertility

Question 31

Assuming they are not saline or do not contain toxic heavy metals, what types of soils are most fertile with respect to their cation exchange capacity and percent base saturation?

Answer 31

high cation exchange and high percent base saturation

Question 32

Give four characteristics of soil that makes it a good place to live for many organisms.

Answer 32

• relatively stable structurally and chemically
• climate much less variable than above ground
• remains pretty much saturated with water
• refuge from extremes of temperature, wind, evaporation, light and dryness

Question 33

What are some of the problems of living within the soil?

Answer 33

• soil particles hamper movement
• organisms depend on size of pore space; determines availability of living space, humidity and oxygen
• only part of the upper layer of soil is suitable as a living space

Question 34

Characterize the importance of water to organisms living within the soil.

Answer 34

• most organisms live in water or require high humidity
• too much and they drown or go above ground and risk exposure
• too little and they dessiccate

Question 35

What types of microorganisms are found in the soil?

Answer 35

bacteria, fungi, protozoans, nematodes

Question 36

What are the most abundant arthropods living within the soil pore spaces?

Answer 36

Springtails - primitive, wingless hexapods

Question 37

What role do millipedes play in soil ecology?

Answer 37

Mechanical breakdown of litter; easier for microbial decomposers to use

Question 38

What role to snails and slugs play in soil ecology?

Answer 38

posses enzymes to break down cellulose and other plant polysaccharides

Question 39

What types of human activities disturb natural patterns of soil ecology?

Answer 39

excavation, surface mining, road construction, agriculture, making of trails and construction

Question 40

What types of human activities result in soil compaction and what are the effects of such compaction on soil ecology?

Answer 40

• use of heavy machinery, making of trails and playing fields
• reduces the size of pores, cannot absorb water, so it flows across the surface resulting in erosion

Question 41

how is topsoil removed and what are the consequences of this removal?

Answer 41

• wind and water remove topsoil
• exposes less fertile soil layers to further erosion
• could cloud water if it gets in it

Question 42

What soil problem characterized the “Dust Bowl” of the 1930s in the Midwest?

Answer 42

erosion with soil carried by wind as dust clouds

Question 43

What is the principle role of the Soil Conservation Service?

Answer 43

prevention of soil mismanagement

Question 44

What kinds of soil mismanagement problems continue to plague the US in spite of the efforts of the Soil Conservation Service?

Answer 44

loss of top soil due to erosion or poisoned by pollutants