Chapter 3: Soil Science Flashcards
Soil is an ecosystem inhabited by what?
insects, earthworths, nematodes, fungi, bacteria, and other microbes all living together in a delicate balance
nematodes
microscopic roundworm; many are beneficial organisms, but some feed on plant tissue and may cause disease or damage
parent material
soil bedrock or base material from which a soil profile develops
Composition of ideal soil
45% mineral solids (sand, silt, and clay) that originate from the parent material; 50% spore space (filled with air or water), and 5% organic matter and organisms
horizon
visible layer of soil within the soil profile, oriented parallel to the soil surface
Why do soils develop horizontal layers below the surface?
rainfall, leaching, heating/cooling, chemical reactions, biological activities, and accumulation of different elements and materials
soil profile
vertical section through the soil and all of the soil horizons
What are the five major soil horizons?
O - O horizon (Organic layer) A - A horizon E - E horizon (when it exists) B - B horizon C - C horizon
How are soil layers distinguished?
by differences in color, texture, and smell which can indicate variations in drainage, organic and mineral content, and other characteristic changes
organic layer
top (O) horizon of the profile; thin layer of decomposing organic material at the soil surface; has huge impact on the biological characteristics of the soil; contains materials such as leaves, twigs, bark, and organisms known as the litter layer; very active biologically and is gradually broken up and decomposed by this activity
A horizon
contains most of the fine absorbing roots of trees and is very biologically active; composed of inorganic material (sand, silt, and/or clay); normally rich in organic matter which gives this horizon its characteristic dark color
E horizon
found below the A horizon; lacks the organic matter found in the A horizon and lighter in color
B horizon
below the A (and E if present) horizons; zone of accumulation where materials that have leached from the surface mix with soil particles from the lower parent material
topsoil
the uppermost layers (O, A, E, and sometimes the upper part of the B horizon)
C horizon
deepest layer of soil, just above the bedrock; composed of partially weathered parent material; soil here is continually forming through the physical chemical, and biological weathering of the parent material
aggregates (in soil)
close cluster or mix of small particles of soil and/or organic matter of varying sizes that are bonded together; sand, gravel, or small rocks in the soil; improve soil structure
Positive impacts of soil organisms?
their growth and movement through the soil are important for improving aeration, fertility, and the structural components of soils
Soil requirements of roots?
space among soil particles, organic materials and essential mineral elements, and adequate oxygen and water; tree roots grow where soil conditions are favorable
soil texture
relative fineness or coarseness of a soil due to particle size of the the inorganic, mineral soil particles
sand
soil particles with a size between 0.06 and 2.0 mm in diameter; relatively large, resulting in coarser-textured soils; coarse soil dominated by macropores
silt
soil particles with a grain size between 0.004 and 0.062 mm; coarser than clay particles but finer than sand; intermediate in size
clay
soil particles with a typical grain size less than 0.004 mm; soil predominantly composed of such particles; smallest soil particle, resulting in fine textured soils; fine soil with a high percentage of micropores
Rank the soil particles from finest to coarsest.
clay, silt, sand
loam
soil texture classification based on a certain ratio of sand, silt, and clay; mix (but not equal mix) of the three different particle sizes and is often considered an ideal soil texture because of the favorable characteristics for plant growth; contains relatively less clay
What determines a soil’s ability to hold water and provide oxygen to the roots?
soil structure and texture; which have a profound influence on the chemical and biological properties of the soil; thus texture plays an important role in determining which species of trees will do well in a given site
soil structure
arrangement of soil particles into aggregates; formed of shape, size, strength, and arrangement of soil aggregates; root growth, freezing and thawing, and burrowing insects and other animals also contribute to changes
macropore
relatively large space between soil particles (mainly aggregates) that is usually air filled and allows for water movement and root penetration
micropore
relatively small space between soil particles that is likely to be water filled; source of available water to plants between rainfalls
Why are macropores generally filled with air?
Because macropores are too large to hold water against the force of gravity, therefore they become filled with air as water drains from the soil
What determines the amount of macropore and micropore space in soil?
the size and shape of soil aggregates and the soil texture
bulk density
mass of soil per unit volume, often used as a measure of compaction; weight of dried soil per unit of undisturbed volume; used to assess whether adequate pore space exists, if soil texture is known
soil compaction
compression of the soil, often as a result of vehicle or heavy-equipment traffic, that breaks down soil aggregates and reduces soil volume and total pore space, especially macropore space; an increase in bulk density and a decrease in total pore space; difficult to correct, especially around established trees
What are the different soil types susceptibilities to compaction?
Clay: high percentage of fine particles - easily compacted
Sand: high percentage of sand - less prone to compaction
high moisture content: more easily compacted
dry soil: less easily compacted
Effects of compacted soil?
restricted root growth, reduced water infiltration and availability, limited movement of oxygen and carbon dioxide in the root zone, limited biological component of soil by reducing the large pores required by larger organisms to move through the soil, develop surface crusts when aggregates are destroyed and the fine soil particles at the surface orient themselves like shingles on a roof, inhibiting water infiltration and gas exchange with the air above
pH
unit of measure that describes the alkalinity or acidity of a solution, negative log of the hydrogen ion concentration, measured on a scale from 0 to 14, greater than 7 is alkaline, less than 7 is acidic, and 7 is neutral (neither acidic nor alkaline, pure water); logarithmic function
What pH is favorable for most trees?
6.0-6.5; although quite variable for different species
Why does pH effect tree growth?
by effecting mineral nutrient because some essential elements are available for uptake within a relatively narrow pH range
How can you alter the soil pH?
adding sulfur temporarily lowers the pH, adding lime raises the pH; this is easy on the superficial layers of the soil; altering the subsurface pH is more difficult
buffering capacity
ability of a soil to maintain or resist change in its pH; especially true in soils that are high in clay or organic matter
ions
atom or group of atoms with a positive or negative charge
anions
ion that carries a negative charge
cations
ion that carries a positive charge
Most abundant cations in soil?
calcium (Ca), magnesium (Mg), potassium (K), sodium (Na), and aluminum (Al)
cation exchange capacity (CEC)
ability of a soil to adsorb and hold cations, affected by soil pH, measures (gauge of) soil fertility, clay composition, and engineering characteristics; a measure of the soil’s capacity to attract, retain, and exchange positively charged cations
Do soils have a net charge?
Yes, negatively charged sites outnumber positively charged sites, thus soils have a net negative charge; organic matter and clay particles normally have a negative charge density which attracts and holds cations giving soils high in clay and organic matter a high cation exchange capacity
leach
tendency for elements or compounds to wash down through the soil or tendency for elements or compounds to wash into the soil
Possibility of leaching in soil types?
sandy soils have a greater possibility of leaching than clay soils
saline soils
soil with a high concentration (excess levels) of soluble salts; can cause poor plant growth
sodic soils
soil with relatively low levels of soluble salts and a concentration of sodium high enough to adversely affect soil structure (symptoms include waterlogging, erosion, soil surface crusting, and poor plant growth) soils with less than 12 sodium absorption ratio
soil food web
complex network of interconnected food chains in an ecosystem, in the soil in this case
rhizosphere
soil area immediately adjacent to, and affected by, plant roots; typically has a high level of microbial activity; microzone of intense biological activity surrounding actively elongating roots; altered environment (pH may be 1 or 2 units higher or lower compared to bulk soil) within the soil where many organisms flourish
exudates
substance or solution that oozes out of injured plant cells or is secreted through membrane pores and is released into the soil; source of organic matter on which microorganisms feed
mycorrhizae
symbiotic association between certain fungi and the roots of a plant; fungus roots; specialized root structure that is created when fungi infect roots of a suitable host plant
symbiotic
association of two different types of living organisms that is often, but not always, beneficial to each
symbiotic relationship of mycorrhizae
roots provide a place for the fungi to live and provide food (sugar); the fungi increase the capacity of the roots to absorb water and essential elements, especially phosphorus, to protect against certain disease-causing fungi, and help the tree survive stressful conditions
symbiotic relationship of legumes
the bacteria form colonies in nodules on the roots and “fix” or convert nitrogen from the air into forms that can be used by the plants in the bean and pea family
nutrient cycling
movement of mineral elements (sometimes called nutrients) within an ecosystem as organic matter decompresses, releasing bound nutrients back to plants; as a plant grows, roots absorb essential solution and produce new woody material and leaves and as seasons pass, plants or plant parts die and are returned to the soil where they are broken down and eventually decomposed by soil organisms and weathering processes
actinomycetes
group of soil bacteria resembling fungi; play a role in the decomposition of organic matter and the release of mineral elements; symbiotic with some species and play a critical role in the decomposition of organic matter to form humus
humus
dark-colored, stable form of organic matter that remains after most of the plant or animal residues have decomposed
mineralization
process in which an organic substance is converted to or trapped in inorganic substance; organically bound plant nutrients are converted into inorganic plant available forms
water-holding capacity
ability of a soil to hold moisture; large percentage of micropores = high water-holding capacity; large percentage of macropores = low water-holding capacity
gravitational water
water that drains from the larger soil macropores due to the force of gravity
field capacity
maximum soil moisture content following the drainage of water due to the force of gravity; after gravitational water has drained away
capillary water
water held in the capillary pores of the soil; much of this water can move in any direction and is readily available to plant roots; the water that remains after a soil is at field capacity once the gravitational water has drained away
permanent wilting point
point at which a plant cannot pull any more water from the soil and suffers permanent damage; occurs when water stress is prolonged and severe
How does gas exchange between the soil and the atmosphere?
respiration takes place through diffusion at the soil surface; oxygen levels are higher at the soil surface
Problems in urban soil
- ) compaction - damages soil aggregates, reduces macropore (air-filled) space, increases bulk density, limits gas exchange, reduces water infiltration, and alters soil organism populations
- ) lack an organic layer - decreases biological activity, hampers soil structure development and interrupts nutrient cycling
- ) lack microorganisms (mycorrhizae)
- ) subsurface barriers from buildings
- ) altered drainage, elevated pH, chemical contamination
structural soils
pavement substrate that can be compacted to meet engineering specifications yet remains penetrable by tree roots in the urban environment to allow root growth and development and establishment of trees; composed of angular crushed stone, clay loam, and hydrogel mixed in a weight ration of 100:20:0.03; developed at the Urban Horticulture Institute, Cornell University, Ithaca NY
suspended sidewalk
sidewalk that is reinforced and supported with piers or other structures so that it does not rely on compacted subgrade or soil for support
structural cells
modular system consisting of units of soil and integrated support structures that serve as both a foundation for paved surfaces and a hospitable environment for tree root growth
Benefits of amending soil with organic matter?
improved water filtration, increased water-holding capacity, improved aeration, and enhanced cation exchange capacity
Benefits of mulching the soil surface?
reduce surface compaction and crusting, thereby improving water infiltration, reduce soil erosion, decrease weed competition, moderate temperature fluctuations, decrease soil surface pH on excessively alkaline soils
