Day Two Activity (Students)

Erosion:

Have you ever seen warning signs that say, "Caution, falling rocks" while riding in the car? This is an example of erosion. There are many forms of erosion. Maintaining the top soil layer is crucial in preventing erosion, and Wind erosion is caused when it is not maintained. Wind erosion is often caused by a lack of water, causes the soil to be picked up by the wind and transferred to another location. During the 1930's the Great Plains in the central United States was called the "Dust Bowl" when massive wind storms picked up the dry and cracked soil.

Photo courtesy of the NRCS.

Wind erosion in the Great Plains during the 1930s.

Rill erosion, on the other hand, is often caused by too much water. This type of erosion is characterized
by streamlets carved out on the side of rivers and streams. If these rills (streamlets) become deeper than
10 to 14 inches, gullies form which widen and deepen the erosion of the rill.

Photo by Tim McCabe, courtesy of the NRCS.

A large gully formed on the edge of the field as a result of water erosion.

Sheet erosion is a removal of soil particles in thin layers from an area of gently sloping land. Sheet
erosion is commonly unnoticed by many.

Soil Color:

Color is a good indicator of a soils organic matter and iron content. The organic matter is made up
mainly of the element carbon, which is rich in nutrients and good for plants. As the organic material
increases the color of the soil darkens. Iron is a common element found in soil; when it is oxidized
(that is, when it's electrons are transferred to another element of compound) the color of the soil turns
a reddish or yellowish color. These soils are mostly found in tropical areas. Bright red soils are found
where there is good water drainage and air exposure. If a soil lacks these qualities the soil will resemble
a yellow color. In arid climates, the third soil layer (also known as the B horizon) can be white, which
indicates calcium carbonate and other salts. Because arid climates receive little rainfall these salts
remain in a crystallized form and are not dissolved by water like they would be in a more humid climate.

Photo taken from http://www.silsoe.cranfield.ac.uk/sslrc/soilinfo/level1.htm

Which layer of soil contains the most carbon?

Soil Texture:

A soil's texture can be classified by observing the individual grains of the sample. The following
chart can be used to correlate grain size with soil type.

Mm Inch Classification
2.0 0.08 Very Coarse    Sand
1.0 0.04 Coarse               Sand
0.500 .002 Medium             Sand
0.100 0.004 Fine                     Sand
0.050 0.002 Very Fine          Sand
0.002 0.008                              Silt
0.002 0.009                             Clay

 

Soil Horizons

Soil is divided up onto five layers. The following chart describes each layer.

Soil Horizon

Description

Horizon Depth

(Measurements are approximate)

O Horizon

Mainly twigs, leaves and other organic matter. Not all soil has an O horizon.

0.5 in. - 1 in.

A Horizon

Also known as the topsoil, essential for plant growth. The A Horizon is composed of mainly nitrogen, phosphorus and potassium. Has a dark brown color and a light texture.

1 in. - 2 ft.

B Horizon

Also known as the subsoil. Has few plant nutrients and a higher clay content. Usually the subsoil has a brighter color and a high salt content.

2 ft. - 4 ft.

C Horizon

Also known as the substrata. The C horizon (also known as the weathered parent material) is often a reddish-tan color and unaltered by rainfall or other natural conditions.

5 ft. - varies greatly

R Horizon

(Bedrock)

Also known as the fresh parent material. Bedrock is usually a light tan color.

In the Central Valley, bedrock can be found as deep as 5,000 ft.

Soil profile chart with horizons:

Chart and diagram taken from the following link:: http://www.silsoe.cranfield.ac.uk/sslrc/soilinfo/level3.htm

Photo courtesy of the NRCS

This soil profile is from the San Joaquin Valley. Can you differentiate between the various horizons?

pH: pH is the concentration of hydrogen ions [H+] in a solution. pH is relevent to all substances, including soil and food. More acidic foods, such as orange juice, have a high hydrogen ion concentration and thus a low pH. On the other hand, foods like cottage cheese have a lower hydrogen ion concentration and a higher pH. Water has a neutral pH of 7. Measuring the pH of soil can help a scientist measure the rainfall or water that passes through the soil. The following diagram explains how pH is directly correlated with water.

<------------------------------------------------------------------------------------------------------------------->

Acid pH                                             Neutral pH                                         Basic pH

(4-5.5)                                                    (6.6-7.3)                                              (7.5-10)

High rainfall                                                                                              Low rainfall
washes hydroxide                                                                                   allows hydroxide
content from soil                                                                               to accumulate in soil

 

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