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