Ch 5 - SoilsImportant effect of weathering for people!

Important area of study for geologists.

Objectives • What is soil. • Know soil horizons

and characteristics. • Know 2 types of

soils • Causes &

preventions of soil erosion

What is Soil?Soil consists of: 1. Regolith - minerals & rock

fragments

2. Humus - organic matter

3. Gases

4. Water

5. Living organisms

How does soil form?

Five factors influencing soil formation:

1. Climate

2. Parent rock

3. Organisms• Plants• animals

4. Topography

5. Time

What is a soil profile? A soil profile consists of several soil

horizons.

O horizon – humus on the ground surface.

A horizon – Top soil. – Rich in organic matter. Typically

dark color. – Also called zone of leaching.

B horizon – Subsoil. – Also called zone of accumulation. – May contain soluble minerals such

as calcite in arid climates (caliche). C horizon

– Weathered bedrock (rotten rock). – Bedrock lies below the soil profile.

Soil Horizons – Field

Figure 6. 17

Major Soil Types

• Pedalfer – rich in Al and Fe. – form in humid climates

• Pedocal – rich in Ca. – form in arid climates– commonly contain caliche

(or hardpan), a calcium carbonate deposit which accumulates in the soil.

US Soil mapSoft & organic rich

Strongly weathered & clay rich

Gray-brown & moist

Soil Orders

Have you ever had your soil analyzed? This can be done by county extension

office. Gives you information on: • pH of your soil.

– Is your soil acidic or basic?

• Elements - Plants require – P, K, Ca, Mg – B, Mn, Zn

• Nitrates (fertilizer) – – N, P, K (in that order) – listed as weight percents– Ex. 5-10-5 or 10-10-10.

• Organic matter • Soluble salts

Particle Sizes

Loam

Loam – Combination of Sand, Silt, Clay

Soil's productivity is mainly due to

the clays in the soils

Names for Loams

Based on % of sand, silt, & clay

Soil erosion - (weathering & erosion are different).

• Due to water (runoff) or wind. – transported by streams– deposited on floodplain or

at mouth of river. – trapped behind dams

• reduces beach sand• accelerate beach erosion.

• Dust Bowl of the 1930's - due to wind erosion of soil following extended drought and over-tilling of the soil.

Slopes Make it Worse• 1 % slope = 3 tons soil lost per hectare/yr• 5 % slope = 87 tons lost per hectare/yr

At this rate a 15 cm thickness of topsoil would disappear in about 20 years,

• 15 % slope = 221 tons lost per hectare/yr hectare = 10,000 square meters, or 2.471 acres

• Contour plowing • Terraces • Crop rotation• Planting groundcovers• Windbreaks • Reduce overgrazing

How can we slow soil erosion?