Study Guide Ch. 4 Abiotic Systems Flashcards
Soil series
a group of soils originating from the same parent material and having similar soil horizons in the soil profile, with the primary difference between them being their soil texture
Each series named for a nearby geographic feature (e.g. town name)
divided into “phases” based upon their difference in texture, and the name of a soil phase indicates a feature that affects management
Through the USDA, the NRCS (National Resources Conservation Service) provides soil maps to the public that can be used to determine the soil series found at a specific site. If more detailed information (including soil chemistry) is required, the landscape architect can commission a soil survey for a site
Soil horizon
a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined by obvious physical features such as color and texture
Soil profile
a vertical section of the soil through all its horizons and extending into the parent material.
Soil Textures
All soils are composed of three components:
Sand
Silt
Clay
Soil textures vary according to the ratio of these three particles
As a general rule, there are equal parts sand and silt in a loam soil, and most sources list loam as being composed of 40% sand, 40% silt and 20% clay
Sand
The largest particle size, with soil particles between 0.05 and 2.0 millimeters in diameter
Silt
Fine soil particles between 0.05 and 0.002 millimeter in diameter that can be picked up by air or water and deposited as sediment
Clay
The smallest particle size, with soil particles smaller than 0.002 millimeters in diameter.
Loam
soil that is primarily composed of sand and silt, with a small amount of clay particles
Soil texture triangle
each side of the triangle represents one particle size, and the position along each axis determines the percentage of that particle in the soil. The intersection of the three sizes on the triangle gives the texture class
Texture is important in determining a soil’s
water-holding capacity
permeability and workability
and it also has a direct influence upon the plant communities found in a soil class
Friable soil
soil with a texture in which large clumps are easily broken apart by hand, but which cannot easily be broken apart into (undesirably) small particles.
As such, friable soils are ideal for agriculture and for the growth of most plants.
Porosity
describes the void size between particles within a soil and can be expressed as the percentage of void space in a soil
has a direct relationship with soil permeability
highly compacted soils generally have poor aeration and experience reduced infiltration because the void space between soil particles has been greatly reduced or eliminated, leaving little room for oxygen and water molecules. Conversely—due to the large particle size of sand—sandy soils are comparatively difficult to compact and therefore maintain their void space, allowing them to drain very quickly
Well-graded soil
a soil with a wide range and even distribution of soil particle sizes, in which the small soil particles fill the voids created by the larger grains
Gap-graded soil
a soil that contains various particle sizes, but in which gradation between sizes is broken by the absence of some particle sizes
Uniformly graded soil
a soil that consists of a single range of particle size
Permeability
is the rate at which water moves through soil
Infiltration rate
the rate at which water flows into soil through small pores
Percolation
the downward movement of water in a soil
highly permeable soils
contain large ratios of sand and/or larger material such as gravel, and—when soils are not draining well—they can be amended with sand and/or gravel to increase their permeability
Low permeable soils
Clay soils are well known for their low permeability, and adding clay to sandy soils can reduce permeability, as can the addition of peat moss or other highly absorbent organic materials
Hydric soils
Soils with low permeability in areas subject to regular moisture
are characterized by being heavily saturated with water for prolonged periods of time
and this prolonged saturation renders the soils anaerobic and generally results in the soils being bluish in color
Soil infiltration rates
are extremely important to the design of “green” stormwater management solutions such as bioswales
Infiltration of stormwater into the soil reduces the amount of water conveyed to “downstream” locations and—in aquifer recharge areas—infiltration can increase the amount of water present in an aquifer
Soil percolation
is particularly important to the design of septic systems in rural areas
Soils with a slow rate of percolation cannot accommodate septic systems, and these areas often preclude the development of housing or other uses that might require a septic system (where a municipal sewer system is not present).
Bearing capacity
can be defined as the measure of a soil to decrease in volume under the pressure of a given weight.
As such, knowing a soil’s bearing capacity can help determine where a foundation or roads can be constructed, given that soils with poor bearing capacity can lead to structural failures and other safety issues
Angle of repose
the maximum slope at which a loose material can be piled while remaining stable.
Soil elasticity
the ability of a soil to return to its original shape after being subjected to a load condition
Soil plasticity
the ability of a soil to be deformed under pressure without breaking apart
Liquid limit
the minimum moisture content at which a soil will flow under its own weight
best base course for roads and foundations
soils with a mix of particle sizes (i.e. a well-graded soil)
they offer greater stability and bearing capacity than a soil with a uniform particle size or those that shrink and expand through wetting and drying cycles (as is typical of clay-heavy soils).
factors that have a significant impact on erosion
precipitation patterns
topography (slope)
soil disturbance and site location (e.g. coastal sites often experience erosion from storm events).
Even natural disasters can impact erosion. For example, wildfires often result in hydrophobic soils, in which the soil in burn areas exhibits water repellence. Infiltration is decreased, thereby increasing erosion.
Soil erosion
removes fertile topsoil
it introduces high concentrations of sediment into watersheds, thereby reducing water quality and causing aggradation (the filling in of stream channels with sediment), among other issues
best practices to reduce soil erosion
- Preserve existing vegetation
- Reduce the total area of land disturbance
- Stabilize excavated areas with seeding, sodding, matting or mulching and divert runoff away from these areas
- Minimizing disturbance to steep slopes
- Schedule clearing and grading activities during the dry season and suspend them prior to and during precipitation events
- Locate non-point pollution sources (e.g. construction access roads) in areas that do not drain directly into water bodies
- Introduce erosion control fencing, blankets and stabilize drainage channels with erosion-resistant materials (e.g. riprap)
more common types of erosion
Gully
Rill
Sheet
Gully erosion
the widening, deepening, and headcutting of small channels and waterways due to erosion
Rill erosion
the removal of soil by running water with formation of shallow channels that can be smoothed out completely by normal cultivation
Sheet erosion
the removal of a fairly uniform layer of soil or materials from the land surface by the action of rainfall and runoff water
soil fertility
generally framed as an issue of NPK values
NPK values are always listed on a fertilizer and can be used to establish soil fertility. NPK values describe the ratio of:
Nitrogen (N)
Phosphorous (P)
Potassium (K)
Therefore, a fertilizer that is, for example, listed as 20-10-20 contains 20 parts N (nitrogen), 10 parts P (phosphorous) and 20 parts K (potassium)
Each of these elements plays a critical role in the development of healthy plants, and although each plant species has specific requirements for successful growth, we can make the following generalizations
Nitrogen supports plants' rapid growth and encourages the healthy development of foliage and fruit.
Phosphorous helps a plant convert other nutrients into usable building blocks with which to grow.
Potassium helps strengthen plants' abilities to resist disease and plays an important role in increasing crop yields.
Prime soils
a special USDA classification for highly fertile soils
these soils require the fewest inputs for productive agriculture.
Locations with prime soils require comparatively less irrigation and fewer fertilizers and pesticides than areas without prime soils.
soil pH
plays a critical role in plant health and in determining the presence of specific plant communities
pH is measured on a scale from 0-14. A pH of 7 is considered neutral, a pH below 7 is considered acidic (becoming more acidic as it approaches 0) and a pH above 7 is considered alkaline (becoming more alkaline as it approaches 14).
pH affects the solubility of soil minerals and nutrient availability to plants. Just as they prefer nutrient-rich, loam soils, most plants do best when the soil pH is between 5.5 and 7.5.
Note that alkaline soils and alkaline-tolerant plants are more common in the Western United States
bogs—a type of wetland—are characterized by highly acidic soils. Many plants in the Ericaceae family (e.g. rhododendrons, azaleas, blueberries) are acid-loving.
In situations where soil pH is not conducive to plants (e.g. weathered concrete releasing calcium carbonate into the soil, making it overly alkaline), amendments can be added.
When soils are overly acidic, lime should be added.
When soils are overly alkaline, sulfur should be added.
Note that soils high in salt can be amended through the addition of gypsum.
iron chlorosis
A condition occurring in plants where the pH is overly alkaline (and exacerbated by overwatering and a lack of aeration).
Chlorotic plants typically have yellowed leaves with green veins and browning along the leaf margin.
Topography
The physical features of a surface area, including relative elevations and the position of natural and artificial features
Comprising elevation, slope and aspect
topography is generally considered the most important variable when siting a new project
influences a wide variety of critical issues, including grading, development density, construction costs and the location of roads and other infrastructure
site planning and design should follow or otherwise relate to existing landforms not only to respect context, but also because grading causes significant site disturbance and is costly
Site elevations
impact drainage patterns and visibility
elevation both on a site and in the surrounding landscape determines the size and spatial configuration of local viewsheds.
Elevation data is generally shown in two different ways on topographic maps: spot elevations and contour lines.
Spot elevations
are highly accurate readings shown for specific points.
Often these are areas of importance to the designer (e.g. the finished floor elevation of a structure, the top or bottom of a wall) or to the understanding of a landform (e.g. the high point of a hill, the summit of a mountain).
Spot elevations are more accurate than the information provided by contour lines.
