EVERYTHING YOU’VE EVER WANTED TO KNOW ABOUT SOIL.*

* But, were afraid to ask.

External Processes

Weathering – disintegration and decomposition of rock at or near Earth’s surface.

Mass Wasting – transfer of rock material downslope under the influence of gravity.

Erosion – Incorporation and transportation of material by a mobile agent, usually water, wind, or ice.

Weathering

All materials are susceptible to weathering. Consider the difference between fresh concrete and concrete exposed to the elements for a period of time. (root heaves, cracks from temperature changes, wearing away from traffic, effects of chemicals)

There are two types of weathering – mechanical and chemical.

Mechanical Weathering Rocks are broken into smaller and smaller pieces,

each retaining the characteristics of the original material.

When the rocks break into smaller pieces, the rate at which they weather increases. Why?

Surface area/Volume.

Frost Wedging

Any cracks in rocks can be made bigger when water infiltrates and expands upon freezing.

When this occurs and rocks fall to the bottom of a rise, we called that a talus slope.

Unloading

Rocks like granite will break off in large pieces in a process called sheeting.

This process can be likened to removing the layers of an onion.

This end result of this process is an exfoliation dome.

Thermal Expansion

Different materials have different expansion rates due to heat gain and loss.

Rocks which contain different materials will fracture with drastic temperature changes.

This is also present when different materials are used in construction. (Expansion joints)

The relationship between rebar and concrete.

Biological Activity

Roots penetrate cracks and exert hydraulic pressure on surrounding rock, expanding the crack.

Burrowing animals can increase the rate at which rock disintegrate by gnawing and clawing.

Decomposition of organic material also releases acids.

Chemical Weathering

Primarily involves water. If oxygen is dissolved in water, oxidation will

occur. Carbon oxides, nitrogen oxides, and sulfur oxides

(released from the combustion of carbonaceous material) will react with water vapor to form acids.

Spheroidal weathering refers to the physical wearing away of stone edges and forms rounded stones.

Weathering has accelerated since the 1800s. Why?

Weathering

Weathering

Factors that affect the rate of Weathering.

The chemical composition of the rock material. Granite, composed primarily of silicates, wears away far more quickly than marble, which is composed primarily of carbonates.

Climate (not weather) greatly affects the rate of weathering. Cleopatra’s Needle was relocated to England and the rate of weathering was much faster than similar obelisks from the same era that remained in Egypt.

The optimum climate for weathering would be a warm and moisture-rich environment.

Soil

Soil is the interface between the biotic and abiotic.

Soil is composed primarily of regolith (layer of rocks and mineral fragments produced by weathering), organic material, water, and air.

Pore spaces hold air and water, allowing plants and microorganisms to survive.

If soil becomes compacted, pore space is limited and fertility of the soil suffers.

Soil (continued)

Productive surface soil is composed of 50% inorganic minerals and 50% humus (decayed remains of animals and plants).

Soil texture is descriptive of the percent composition of different particle sizes.

Loam is optimal soil.

Soil Profile

Soil Horizons – distinct, identifiable, and measureable layers. O Horizon – primarily organic material (leaf litter,

twigs, branches) A Horizon – mixture of organic and minerals. E Horizon – zone of eluviation (“washing out”).

Leaching describes nutrients and minerals being carried deep in soil by water.

B Horizon – sub-soil. High in minerals and nutrients. C Horizon – partially altered parent material Parent material – bedrock (residual soils) or

unconsolidated deposits (transported soils).

Soil Types

Pedalfer – combination of “ped” (Pedon, Greek, meaning soil), “Al” (aluminum) and “Fe” (iron). Typical of the clays in mid-latitude regions

where iron oxides and aluminum laden clays are found.

A-Horizon is leached of minerals that accumulate in the B-Horizon.

This soil is very common in the mid-Atlantic region.

This soil tends to be acidic.

Soil Types (continued)

Pedocal – combination of “ped” (Pedon, Greek, meaning soil) and “cal” (from calcium carbonate). Typical of the drier soils in the Western

United States. Weather is less intense, so clay build-up is

not typically seen.

Soil Types (continued)

Laterite – the result of intense weathering in hot, moist environments. Because of the rapid weathering and

bacterial action, laterite soil is devoid of humus and contains virtually no nutrients or minerals.

This soil is typical of the tropical region of the Earth, particularly, the rain-forests.

This soil is the poorest soil for farming.

Soil Erosion

Water, wind, and ice are eroding agents. Water droplets from rain strike the ground

with surprising force and loosen the top layer.

Loose soil is transported downhill in rills and gullies.

Soil Erosion (continued)

Wind erosion occurs when soil becomes desicated and can be carried aloft.

Typically occurs when areas that receive minimal rainfall are intensively farming – removing all moisture from pore spaces.

Soil Erosion

Ice erosion occurs as glaciers advance and retreat over land masses, grinding away the soil beneath the ice.