soils. what is soil? soil – relatively thin surface layer of the earth’s crust consisting of...
TRANSCRIPT
SOILS
What is soil?
• Soil – relatively thin surface layer of the Earth’s crust consisting of mineral and organic matter
Soil Composition
• Soil is composed of 4 distinct parts:– Mineral particles (45% of “typical” soil)– Organic matter (about 5%)– Water (about 25%)– Air (about 25%)
Importance of Soil• Organisms inhabit the soil & depend on it for
shelter, food, & water. • Plants anchor themselves into the soil, and
get their nutrients and water. • Humans need plants and, therefore, need soil.
Soil is a renewable resource
• Soil is a slowly renewed resource • Soil formation begins when bedrock is broken
down by weathering.• Mature soils (soils that have developed over a
long time) are arranged in a series of horizontal layers called soil horizons.
Soil Formation
• Soils form from parent material• Parent material (rock) is slowly
broken down into smaller particles by biological, chemical, and physical weathering.
• It takes a long time to form soil.– Example: To form 2.5 cm (1 in.) it
may take from 200-1000 years.
Physical Weathering
• Physical breakdown by wind, water, ice, etc.
Chemical Weathering
• Chemicals interact with rock and break it down.
• Example: A plant’s roots or animal cells undergo cell respiration and the CO2 produced diffuses into soil, reacts with H2O & forms carbonic acid (H2CO3). This eats parts of the rock away.
Biological Weathering
• Parent material is broken down by tree roots or lichens.
• Lichens play a big role in primary succession.
Soil Properties: Texture
• The percentages (by weight) of different sized particles of sand, silt and clay that it contains.
Soil Properties: Texture, Cont.
• Grain Size– 0.05 to 2mm = sand (the largest soil particles)– 0.002 to 0.05mm = silt (about the size of flour)– <.002mm = clay (only seen under and electronic
microscope)
Soil Properties: Texture, Cont.
• To tell the difference in soil, take the soil, moisten it, and rub it between your fingers and thumb.– Gritty: has a lot of sand– Sticky: high clay content and you should be able to
roll it into a clump – Silt: smooth, like flour.
Soil Structure
• How soil particles are organized and clumped together.– Sand– Silt– Clay
Friability
• How easily the soil can be crumbled.
Porosity
• A measure of the volume of soil and the average distances between the spaces.
Permeability
• The rate at which water and air moves from upper to lower soil layers.
• The Distances between those spaces.
Variability
• Soils (Sand, Silt, & Clay) vary in – the size of the particles
they contain– the amount of space
between these particles– how rapidly water flows
through them.
Big spaces, not a lot of them
LESS surface area
Little spaces but lots of them
GREATER surface area
IMPORTANT TO REMEMBERIMPORTANT TO REMEMBER
The size of the rock particle DOES NOT change the porosity!
EQUAL
POROSITY
1st trial:
look at red
yellow green
only
Demo #1
Which size held the
most water?
Soil Texture (descriptions)• Sand• Silt• Clay
Shrink-Swell Potential
• Some soils, like clays, swell when water gets in them, then they dry and crack. This is bad for house foundations, etc.
Slope• Steep slopes often
have little or no soil on them because of gravity.
• Runoff from precipitation tends to erode the slope also.
• Vegetation?
Depth
• Some soils are very shallow. It can be only two inches of soil and then you hit rock. Other areas can have soil 36 inches deep or more.
Color• Dark soil is rich with lots of organic matter. • Light soil (like sand) is not so rich with very
little organic matter.
Soil Horizons
Organic Layer (O-horizon)
• The uppermost layer; it is rich in organic material.
• Plant litter accumulates in the O-horizon and gradually decays.
• In desert soils the O-horizon is completely absent
• In certain rich soils it may be the dominant layer.
Topsoil (A-horizon)
• It is dark and rich in accumulated organic matter and humus.
• It has a granular texture and is somewhat nutrient-poor due to the loss of many nutrient minerals to deeper layers and by leaching.
Eluvial(E-horizon)
• Mineral horizon in upper part of soil
• Below A-Horizon and above B-Horizon
• Generally forested areas, light color
Subsoil (B-horizon)
• The light-colored subsoil beneath the A-horizon;
• Accumulation of minerals occurs here
• It is typically rich in iron and aluminum compounds and clay.
Parent Material (C-horizon)
• Contains weathered pieces of rock and borders the unweathered solid parent material.
• Most roots do not go down this deep
• often saturated with groundwater.
Fig. 3-23, p. 68
Fern
Mature soil
Honey fungus
Root system
Oak tree
Bacteria
Lords and ladies
Fungus
Actinomycetes
Nematode
Pseudoscorpion
Mite
RegolithYoung soil
Immature soil
Bedrock
Rockfragments
Moss and lichen
Organic debrisbuilds upGrasses and
small shrubs
Mole
Dog violet
Woodsorrel
EarthwormMillipede
O horizonLeaf litter
A horizon
Topsoil
B horizonSubsoil
C horizon
Parent material
Springtail
Red Earth Mite
Fig. 3-24a, p. 69
Mosaic of closely packed pebbles, boulders
Weak humus-mineral mixture
Dry, brown to reddish-brown with variable accumulations of clay, calcium and carbonate, and soluble salts
Alkaline, dark, and rich in humus
Clay, calcium compounds
Desert Soil(hot, dry climate)
Grassland Soil(semiarid climate)
Fig. 3-24b, p. 69
Tropical Rain Forest Soil(humid, tropical climate)
Acidic light-colored humus
Iron and aluminum compounds mixed with clay
Fig. 3-24b, p. 69
Deciduous Forest Soil(humid, mild climate)
Forest litter leaf moldHumus-mineral mixtureLight, grayish-brown, silt loamDark brown firm clay
Fig. 3-24b, p. 69
Coniferous Forest Soil(humid, cold climate)
Light-colored and acidic
Acid litter and humus
Humus and iron and aluminum compounds
Erosion
• Erosion is the movement of soil components, especially surface litter and topsoil, from one place to another.
Importance of Erosion
• In undisturbed ecosystems, the roots of plants help anchor the soil, and usually soil is not lost faster then it forms.
• But, farming, logging, construction, overgrazing by livestock, off-road vehicles, deliberate burning of vegetation etc. destroy plant cover and leave soil vulnerable to erosion. This destroys in a few decades what nature took hundreds to thousands of years to produce.
Soil Erosion & Degradation
Figure 13-9Figure 13-9
Global Outlook: Soil Erosion
• Soil is eroding faster than it is forming on more than one-third of the world’s cropland.
Figure 13-10Figure 13-10