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LAB 12: Soils

Soil is a mixture of weathered rock & organic matter that usually covers bedrock (solid rock that underlies all soil). Both chemical & mechanical processes are involved in the development of soils.

• Chemical weathering turns hard minerals into soft ones • Mechanical weathering breaks solid rock into smaller pieces • Plant & animals add organic materials in the form of waste products & dead organisms • The decay of organic matter produces acids which accelerate chemical weathering • Burrowing Animals, such as earthworms, insects, & rodents, help circulate air and water

through the soil & mix mineral & organic remains The material from which soil forms is called its parent material. Soil that has weathered directly from the bedrock beneath it and therefore matches its parent material is called residual soil. Soil that does not match the bedrock it is over is called transported soil. It did not weather from the bedrock beneath it but was brought there by agents of erosion such as winds, rivers, or glaciers. Much of New England & the Midwest are covered by soil that was deposited by the movement of glaciers after the last Ice Age.

A cross section of soil exposed by digging is called the soil profile. The weathering of soil produces layers known as soil horizons. The topsoil or A horizon is usually rich in dark-colored organic remains called humus (labeled O horizon below). The subsoil or B horizon contains minerals that have been transported deeper by groundwater. Most of the clay in soil has also been washed down to this layer. The partially weathered bedrock or C horizon is composed of broken up bedrock on top of the solid bedrock (parent material). Soil erosion is the removal of topsoil by the action of running water or wind. It takes between 100 & 400 years for one centimeter of topsoil to form.

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1. For the soil profiles below, label the horizons (A, B, or C) and the parent material in each of the soil profiles using the spaces provided next to each image.

______ ______ ______ 2. At the base of each profile above, number the profiles according to the proper sequence of development. 3. Match each soil profile above to the graph below that would most likely represent that profile. Write the letter of the matching profile in the space provided below each graph.

4. Approximately how many years does on centimeter of topsoil take to form? 5. What causes soil erosion? 6. Which layer of a soil profile forms first from the bedrock? 7. Which layer in a soil profile contains the most organic material… and why?

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Soil Texture Soil classification is typically made based on the relative proportions of silt, sand and clay. Follow any two component percentages to find the nominal name for the soil type. For example, 30% sand, 30% clay and 40% silt:

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Soil Textural Triangle Practice Exercises

% Sand % Silt % Clay Texture Name

a) 75 10 15 sandy loam b) 10 83 7 _______________________ c) 20 10 70 _______________________

Soil Texture Worksheet

Directions: Using your soil texture chart and example, determine the following soil textures using the percentages given.

% sand % silt % clay Soil Texture example 75 10 15 sandy loam

a) 42 _____ 37 __________________ b) ______ 52 21 __________________

c) ______ 35 50 __________________

d) 64 30 ______ __________________

e) 50 _____ 40 __________________

Now for a challenge!:

f) 36 ______ ______ Clay Loam

g) _____ ______ 42 Silty Clay

h) _____ ______ ______ Loamy sand

i) _____ ______ ______ Silt Loam

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Soil Color: Munsell Color Chart Soil color is an important soil property that is reported in all soil profile descriptions because it constitutes a useful first approximation of soil conditions and properties. Color can be estimated with a spectrophotometer or other mechanical device; but it is frequently done by visual inspection. The practice of describing soil color first began in Russia, where attempts were made to form a cohesive system of soil color identification. In America, soil colors were occasionally mentioned in reports of the early 1900’s, but no formal system was agreed upon until the 1940’s, when the work of Dorothy Nickerson and Albert H. Munsell led to the use of the color chip system now employed. The system has led to a uniform and systematic description of soil color employed in all current scientific literature. Soil color is used for both soil classification and evaluation. From color, inferences regarding such things as reduction status (i.e., whether or not a soil remains waterlogged for long periods of time), organic matter content, and mineralogy are possible. For example, red, yellow, or reddish brown colors suggest the presence of oxidized iron and are indicative of good aeration and adequate drainage. Poor aeration and imperfect drainage are indicated by blue and gray soil colors, denoting reduced iron. Similarly, a dark brown soil color is usually attributed to organic matter. Minerals can be distinguished by inspection from the differing values of redness; acid sulfate soils are frequently in the gray-green-black spectrum; and types of clays present have also been characterized by color. To classify soil color, a moist representative soil sample is compared to the color chips in a Munsell color book. The Munsell color system describes color in three parts: hue, value, and chroma. For example, a complete color description reads 10YR 4/3. Such a notation translates to: a hue of 10YR, a value of 4, and a chroma of 3. Hue is the spectral or rainbow color and is described by such notations as 10YR (yellowred), 7.5YR (more red, less yellow), 2.5Y (yellow), etc. Each page in the Munsell color book is a different hue. Value is defined as the relative blackness or whiteness, the amount of reflected light, of the color. The value designation is found on the left side of the color book, and increases from the bottom (0 = pure black), to the top (10 = pure white). The chroma notation is the purity of the color or the amount of a particular hue added to gray. The chroma designation is located at the bottom of each page of the color book and increases from left (grayest) to right (least gray or brightest)

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Soil Texture & Color Activity Soil Texture by Feel Follow the flow chart titled “Thien: Texture-By-Feel Analysis”. Then record the texture in the Summary chart under “Texture: Feel”. Soil Color Place a pinch of soil in the white spot plate and determine the color using the Munsell color book. Then moisten the sample and determine the color of the moistened sample. Repeat for each soil and record in the Summary chart.

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Soil Texture & Color Activity

Sample % Sand % Silt % Clay Name1234

Sample Hue Value Chroma Code1234

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Soil Profiles Directions: Below are three soil profiles. The first being a traditional soil profile, the second being a tropical rainforest profile, and the last being a post-deforestation profile. Draw and identify the horizons of a post-deforested soil profile. Consider that deforestation is the removal of vegetation, so what would you expect this profile to look like, and what layers will remain present versus what layers do you expect to change? Once you have labeled the diagram, write an explanation on the bottom of this handout about the profile you have drawn.

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