Chapter 6: Soil Ecology Flashcards
Give three ways in which rocks are formed.
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
Give four ways parent material may be broken down or added to soil.
- Mechanical or Chemical Weathering
- Climate: temperature and rainfall
- Biotic factors: organisms
- Topography: slopes, runoff, rate of erosion
How does chemical weathering of parent material occur?
when rainwater containing dissolved carbon dioxide becomes carbonic acid and dissolves rock
What is the difference between till and loess?
Till is parent material added by rivers or glaciers
Loess is parent material added by wind
What role do biological growth and decomposition play in soil formation?
it helps break down parent material and affects the accumulation of humus (organic material in soil)
What role does climate play in soil formation?
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
What role do biotic factors play in soil formation?
it is responsible for the organic matter in soil and the color of the surface and it influences the nutrient content in soil
How does topography affect soil ecology?
it determines how much water enters the soil, the rate of erosion and the downhill transport of soil material
How do young and old soils compare in terms of their fertility and why?
Young soil is more fertile because old soil has had long-time leaching of nutrients without replacement from fresh minerals
Characterize each of the soil horizons.
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
What is the difference eluviation and illuviation, and in what soil horizons does each typically occur?
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)
What kind of information can be gained from soil color?
what it is made of, presence of oxides, indicators of good drainage, aeration
How is soil color normally determined?
Standardized color charts; Munsell color charts
Characterize each of the following in terms of soil texture: gravel, sand, silt and clay
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
Give two important properties of the soil controlled by clays.
- Plasticity: the ability to change and retain shape when pressure is applied
- exchange of ions between soil particles and soil solutions used by plants
How does soil depth differ from one place to another?
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
What is meant by the terms “field capacity” and “permanent wilting point”?
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
How do field capacity and permanent wilting point relate to available water capacity?
Available water capacity is the amount of water retained between the field capacity and the permanent wilting point
Give three factors that play a role in determining soil moisture regimes
Texture: intermediates are best
Amount of organic matter
Topographic position: affects movement of water on surface and within soil
What is soil moisture regimes?
the ability to retain moisture against drainage
With respect to soil drainage classes, what type of soil marks the beginning of wetlands?
somewhat poorly drained soils
What are micelles and what role do they play in soil chemistry?
Micelles: particles and complexes of clay and humus that hold ions
They affect the mobility of ions; prevent charged nutrient cations from leeching
Describe the structure of a clay molecule.
aluminum, silica and oxygen arranged in a complex tetrahedral fashion
What are isomorphous substitutions?
the substitution of one element in the arrangement without changing the structure
What type of compounds do clays attract and why?
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
What does the cation exchange capacity of the soil represent and how does it prevent the leaching of soil nutrients?
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.
What does percent base saturation represent?
the percent of sites occupied by ions other than hydrogen
How are hydrogen ions added to the soil?
rain water (carbonic acid), acids from organic matter and metabolic acids produced by microorganisms and plants
why do acidic soils have a low percent base saturation?
because they have a high number of exchangeable hydrogen ions
What is the relationship between soil acidity and soil fertility and aluminum toxicity?
High soil acidity equals low soil fertility and can lead to aluminum toxicity
Low soil acidity equals high soil fertility
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?
high cation exchange and high percent base saturation
Give four characteristics of soil that makes it a good place to live for many organisms.
- 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
What are some of the problems of living within the soil?
- 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
Characterize the importance of water to organisms living within the soil.
- 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
What types of microorganisms are found in the soil?
bacteria, fungi, protozoans, nematodes
What are the most abundant arthropods living within the soil pore spaces?
Springtails - primitive, wingless hexapods
What role do millipedes play in soil ecology?
Mechanical breakdown of litter; easier for microbial decomposers to use
What role to snails and slugs play in soil ecology?
posses enzymes to break down cellulose and other plant polysaccharides
What types of human activities disturb natural patterns of soil ecology?
excavation, surface mining, road construction, agriculture, making of trails and construction
What types of human activities result in soil compaction and what are the effects of such compaction on soil ecology?
- 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
how is topsoil removed and what are the consequences of this removal?
- wind and water remove topsoil
- exposes less fertile soil layers to further erosion
- could cloud water if it gets in it
What soil problem characterized the “Dust Bowl” of the 1930s in the Midwest?
erosion with soil carried by wind as dust clouds
What is the principle role of the Soil Conservation Service?
prevention of soil mismanagement
What kinds of soil mismanagement problems continue to plague the US in spite of the efforts of the Soil Conservation Service?
loss of top soil due to erosion or poisoned by pollutants
