Soil is a dynamic natural material composed of fine decomposed mineral and organic matter particles in which plants grow.

GEOG 101 Physical Geography

 

LAB 8: Soils and their Analysis

(modified from Shankman with further additions and major modifications by D. Fairbanks)

 

Name ANSWER KEY Lab Section __________ Date _______

 

 

Materials and sources that will help you

· Soil samples

· Munsell color chart

· Soil texture analysis kit

· Soil dispersion reagent

· Water

· pH meter and distilled water

· 500ml beakers

· Classroom clock or a watch

 

 

Introduction

Soil is a dynamic natural material composed of fine decomposed mineral and organic matter particles in which plants grow. The soil system includes human interactions and supports all human, other animal, and plant life. If you have ever planted a garden, tended a house plant, or been concerned about famine and soil loss, this lab exercise will interest you.

Soil science is interdisciplinary, involving physics, chemistry, biology, mineralogy, hydrology, climatology, and cartography. Physical geographers are interested in the spatial patterns formed by soil types, the environmental factors that interact to produce them, and their effect on plants, animals, human health and the built environment. Pedology concerns the origin, classification, distribution, and description of soil. Edaphology focuses on soil as a medium for sustaining higher plants. Edaphology emphasizes plant growth, fertility, and the differences in productivity among soils. Pedology gives us a general understanding of soils and their classification, whereas edaphology reflects society’s concern for food and fiber production and the management of soils to increase fertility and reduce soil losses.

This lab will give you the opportunity for some hands-on experience with soils, and for using some of the tools and methods that soil scientists use in their work.

 

 

Keywords:
clay edaphology humus loam pH (acidity-alkalinity) pedology permeability polypedon porosity sand	silt soil soil classification soil color soil consistence soil horizon soil profile soil properties soil texture

 

 

 

Objectives

 

· Identify basic components of soil and soil properties.

· Determine main components of soil sample by color.

· Identify major soil texture categories and classify soils by texture.

· Measure pH level in soil samples and determine the soil pH (acidity or alkalinity).

 

Section 1: Soil Texture and Soil Structure

 

Soil texture refers to the mixture of sizes of its individual particles and the proportion of different sizes of soil separates (individual particles of soil). Particles smaller than gravel are considered part of the soil, while larger particles, such as gravel, pebbles, or cobbles are not. If you have been to a beach, you have felt the texture of sand: It has a “gritty” feel. Silt, on the other hand, feels smooth—somewhat soft and silky, like flour used in baking bread. When wet, clay has a sticky feel and requires quite a bit of pressure to squeeze it, like the clay used in making pottery.

Soils nearly always consist of more than one particle size. By determining the relative amounts of sand, silt, and clay in a particular soil sample, it can be placed into one of twelve classes as shown in the soil texture triangle. Each side presents percentages of a particle grade. See the line from each side of the triangle (following the direction indicated by the orientation of the numbers on each axis). You see that a soil consisting of 36% sand, 43% silt, and 21 % clay is classified as loam, a term for soils consisting of mostly sand and silt with a relatively smaller amount of clay. Soils that represent the best particle size mix for plant growth are those that balance the three sizes.

 

 

 

1. Use the soil texture triangle on the last page of this lab to name the following by its correct texture class.   a) 17% sand, 28% silt, 55% clay: CLAY     b) 31% sand, 55% silt, 14% clay: SILT LOAM

 

 

Part I:

The following is a quantitative approach to measure soil texture. Here you will use a soil texture kit consisting of a set of three graduated cylinders, water, a dispersing reagent and a soil sample to be chosen by your lab instructor. This method uses the same principle as standard scientifically more accurate methods (ones you would find a soil analysis lab): the rate of settling of soil particles in water.

 

Step 1: Break up into lab pairs of two and go and get the soil separation tubes and rack, and a graduated cylinder from the back storage cupboards. Go and fill the graduated cylinder to the 50 ml line .

 

Step 2: At the front of the lab your lab instructor will give you your assigned soil sample. Add the soil sample that your lab instructor assigned to your group to soil separation Tube “A” until it is even with line 15 . Note : Gently tap the bottom of the tube on a firm surface to pack the soil and eliminate air spaces.

 

Step 3: At the front of the lab your lab instructor will have chemicals for your use. Use a dropper to add 1 ml of texture dispersing reagent to the sample in soil separation Tube “A”. Fill Tube “A” with your water from graduated cylinder to line 45 .

 

Step 4: Cap and gently shake for 2 minutes , making sure the soil sample and water are thoroughly mixed.

The sample is now ready for separation. The separation is accomplished by allowing a predetermined time for each fraction to settle out of the solution.

 

Step 5: Place soil separation tube “A” in the rack. Allow to stand undisturbed for exactly 30 seconds .

 

Step 6: Carefully pour off all the solution into soil separation tube “B”. Return Tube “A” to the rack. Allow Tube “B” to stand undisturbed for 30 minutes .

 

Step 7: Carefully pour off the solution from soil separation tube “B” into soil separation tube “C”. Return Tube “B” to the rack.

 

While tube “C” would have the suspended clays in a soil, we do not need it to calculate the percentage sand, silt and clay, as having the results of tube “A” (sand) and tube “B” (silt) fractions and subtracting this total from the initial volume of soil used for the separation is sufficient.

 

EXAMPLE:

 

Tube “A” Sand 2 Initial volume 15

+ Tube “B” Silt +8 – Total “A” & “B” –10

Total “A” & “B” 10 Clay 5

 

Step 8: Read soil separation tube “A” at top of soil level. To calculate percentage sand in the soil, divide reading by 15 and then multiply it by 100.

 

Step 9: Read soil separation tube “B” at top of soil level. To calculate percentage silt in the soil, divide reading by 15 and then multiply it by 100.

 

Step 10: Calculate volume of clay as shown above. To calculate percent clay in the soil, divide value by 15 and then multiply it by 100.

 

Sample ID	Percentage	Textural classification
	Sand	Silt	Clay
Answers will vary

 

 

Divide students into pairs and provide one sample from one of the sites. There will be duplication of sites being analyzed. Your lab instructor will record all the class samples on the board. You should record them and calculate the textural averages for each sample.

Sample ID	Sand (%)	Silt (%)	Clay (%)
Answers vary with section

 

Sample ID	Sand (%)	Silt (%)	Clay (%)
3	Answers vary with section
3
3
AVERAGE
4
4
4
AVERAGE
5
5
5
AVERAGE

 

 

 

 

Answer the following questions based on the data analyzed by the entire class.   1) Which of the samples has the largest pore spaces? (The sample with the highest sand content)   2a) Which of the samples has the highest infiltration capacity? (The sample with the highest sand content   2b) Which of the samples has the lowest infiltration capacity? The sample with the highest clay content   2c) Explain why? Clay reduces infiltration capacity.     3a) Which of the samples has the highest water holding capacity?   (The sample with the highest clay content)   3b) Which of the samples has the lowest water holding capacity? (The sample with the highest sand content)   3c) Explain why?   Sand reduces water holding capacity.

 

Part II.

In a less quantitative way, soil texture can be determined in the field by feeling the soil and estimating the percentages of sand, silt, and clay. Try this method using the following procedure with a new soil sample, recording your observations and results through each of these steps.

 

Step 1: Follow the handout that accompanies the last page of this lab. Your lab instructor will fill your palm with a dry soil sample, moistening it with enough water so that it sticks together sufficiently to be worked with your fingers. Add the water gradually. If it becomes too runny or if it sticks to your fingers, add more dry soil. You want a “plastic” mass that you can mold, somewhat like putty.

 

Step 2: Follow the remainder of the handout to determine soil texture.

 

Record your observations in the space provided. Once you have a simplified named textural classification review the soil texture triangle and work out the percentage ranges for sand, silt and clay.

 

Name according to handout Results will vary

 

Sand _________Depends on your answer above Silt _________ Depends on your answer above Clay _________ Depends on your answer above

 

 

Section 2: Soil Color

Soil properties are their characteristics, some of which include soil color, texture, structure, consistence, porosity, moisture, and chemistry. We examine a few of these properties, beginning with color.

Soil color is one of the most obvious traits, suggesting composition and chemical makeup in mineral soils. If you look at exposed soil, color may be the most obvious trait. Among the many possible hues are:

 

· the reds and yellows (high in iron oxides, its rusting);

· the dark browns to blacks (richly organic);

· white-to-pale hues (silicates and aluminum oxides);

· Gray and greenish-bluish (reduced iron from being inundated in water) and;

· White color (calcium carbonate or other water-soluble salts).

 

However, color can be deceptive. Soils of high humus content, organic materials from decomposed plant and animal litter, are often dark, yet clays of warm-temperate and tropical regions with less than 3% organic content are some of the world’s blackest soils.

To standardize color descriptions, soil scientists describe a soil’s color by comparing it with a Munsell Color Chart. These charts display colors arranged by:

 

· Hue (H, the dominant spectral color, such as red),

· Value (V, degree of darkness or lightness), and

· Chroma (C, purity or saturation of the color, which increase with decreasing grayness).

 

The complete Munsell notation for a chromatic color is written symbolically like this: H V/C. As an example, for a strong red having a hue of 5R (R denoting red), a value of 6, and a chroma of 14, the complete Munsell notation is 5R 6/14. Another example, a pale brown is 10YR 6/3 (YR denoting yellow-red). A dark brown is noted as 10YR 2/2. More refined divisions of any of the attributes, use decimals.

The light you use when you view the sample is important and can affect your assessment of the color notation. It is best to view the chart and the sample with the Sun over your shoulder shining on the sample, with you facing away from the Sun. Under artificial classroom light you will find low values and low chromas—the most difficult to match against the color chips.

 

Using soil samples assigned to your group note the predominant soil color and indicate the likely soil component responsible for the color. Be sure and note whether the sample is wet, moist, or dry.

 

Sample ID	Munsell color	Soil component creating the color	Moisture level
Answers will vary.	Very dry

 

 

Note: When doing actual fieldwork with a soil (the complete soil profile and basic sampling unit in soil surveys), you will find different colors in each horizon, and maybe more than one color in a single horizon. These details, in an assessment, would be noted.

 

Section 3: Soil Acidity and Alkalinity

 

Soil fertility is strongly affected by soil acidity or alkalinity as expressed on the pH scale. Nutrient availability is low in soils that are either very acidic or very alkaline. A soil solution may contain significant hydrogen ions (H+), the cations that stimulate acid formation. The result is a soil rich in hydrogen ions, or an acid soil. On the other hand, a soil high in base cations (calcium, magnesium, potassium, sodium) is a basic or alkaline soil.

Pure water is nearly neutral, with a pH of 7.0. Readings below 7.0 represent increasing acidity. Readings above 7.0 indicate increasing alkalinity. Acidity usually is regarded as strong at 5.0 or lower, whereas 10.0 or above is considered strongly alkaline. Several factors influence soil acidity. The chemistry of soil parent materials, as well as any added fertilization or removal of plants can increase soil acidity. However, the major contributor to soil acidity in this modern era is acid precipitation (rain, snow, fog, or dry deposition). Acid rain actually has been measured below pH 2.0 – an incredibly low value for natural precipitation, as acid as lemon juice. Increased acidity in the soil solution accelerates the chemical weathering and depletion rates of some mineral nutrients, yet it can also decrease the availability of other nutrients. Because most crops are sensitive to specific pH levels, acid soils below pH 6.0 require treatment to raise the pH. This soil treatment is accomplished by the addition of bases in the form of minerals that are rich in base cations, usually lime (calcium carbonate, CaCO3).

 

1) Your lab instructor will have five beakers representing the five soil samples with the addition of distilled water to them on his/her desk. Using the pH meter provided dip it into each sample and record the pH level. Make sure to clean off the meter each time in the clean water beaker before dipping into a new soil pH test beaker.

 

Sample ID	pH
1	Answers will vary by section
2
3
4
5

 

 

2) Are any of the class samples strongly acidic? If any were, what remedial actions could be taken to make them more pH neutral and under what circumstances might you want to do this? Hint: what does one take for heartburn?

 

pH numbers vary with section. An acidic soil could be made more neutral by adding a base, such as crushed limestone.

 

Soil Texture Triangle