Lab 6: Soil Properties

ENVIRONMENTAL SCIENCE

Properties of Soil: Agricultural and Water Availability Impacts

Investigation Manual

PROPERTIES OF SOIL: AGRICULTURAL AND WATER AVAILABILITY IMPACTS

Overview Earth’s soil plays a major role in the world’s agriculture and has a substantial effect on water availability in a given area. In this inves- tigation, the student will analyze the natural porosity and particle size of soil samples along with the chemical composition and profile of different soil types.

Outcomes • Examine the properties of soil and their effects on agriculture

and water availability. • Describe and identify soil horizons based on their chemical and

physical composition. • Distinguish between the particle sizes of three different types of

soil: sand, silt, and clay. • Determine the porosity of different soil types. • Analyze soil samples for a variety of nutrients to determine soil

fertility.

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Key Personal protective equipment (PPE)

goggles gloves apron follow link to video

photograph results and

submit

stopwatch required

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Made ADA compliant by NetCentric Technologies using the CommonLook® software

Key Personal protective equipment (PPE)

goggles gloves apron follow link to video

photograph results and

submit

stopwatch required

warning corrosion flammable toxic environment health hazard

Table of Contents

2 Overview 2 Outcomes 3 Time Requirements 3 Background 8 Materials 9 Safety 9 Preparation 10 Activity 1 11 Activity 2 12 Activity 3 12 Activity 4 13 Disposal and Cleanup 14 Observations

Background Soil Horizons and Chemical Composition The type of dirt that makes up the dry surfaces of the earth has numerous effects on humans and the environment, and vice versa. Humans can modify the suitability of some areas for agriculture based on prior land use. The properties of soil also determine water availability in a given area. Areas that contain the most suitable soil for farming are often limited.

Certain properties of soil determine whether an area is suitable for human activity. When consid- ering the properties of soil, texture, shape, particle aggregation, and suitability for growth come to mind. These properties all play a major role in determining the capability of an area to retain water and air, which are necessary for several agricultural processes that are vital to human life.

It is important to understand the profile and chemical composition of soil to understand how they affect agriculture and water availability. For instance, some farmlands have been plowed for hundreds of years yet the soil remains very

fertile. However, in other areas with a similar history, much of the soil has been adversely affected (over one-third of the soil in the United States is now deemed as destroyed). With years of continuously turning over the soil to cultivate crops, the damage accumulates and many areas are left vulnerable to erosion, weathering, and deterioration of nutrient and organic material. Why then is there such great disparity in the way certain soils flourish? It is because of the various layers, or horizons, of the soil.

Soils differ greatly depending on the proportion of each of these horizons (see Figure 1). If you were to dig a hole or to drive a corer (a special drill that uses a hollow steel tube to remove a cylindrical dirt sample) deep into the ground to extract a sample of soil, a visible color difference would be evident in the soil profile, or horizon composition. The colors of various horizons differ based on the organic content and mineral composition of each soil. Each horizon can also vary in texture, which is determined by the makeup of sand, silt, or clay.

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Time Requirements Preparation ............................................................................................................................. 5 minutes Activity 1: Particle Size Distribution and Determination of Soil Texture

Day 1 .................................................................................................................... 20 minutes Day 2 .................................................................................................................... 30 minutes

Activity 2: Porosity of Different Soil Types ............................................................................ 60 minutes Activity 3: pH Test Comparison of Soil Samples .................................................................. 30 minutes Activity 4: Nitrogen, Phosphorus, and Potash Test Comparisons of Soil Samples

Day 1 .................................................................................................................... 20 minutes Day 2 .................................................................................................................... 60 minutes

PROPERTIES OF SOIL: AGRICULTURAL AND WATER AVAILABILITY IMPACTS

Background continued As shown in Figure 1:

• The O-horizon is the most nutrient-rich part of the soil profile mainly because of its abun- dance in organic matter. This layer is often referred to as the humus layer and is usually dark brown or black in color.

• The A-horizon, also known as topsoil, is the next layer down. This layer contains some organic matter in addition to a mixture of minerals. This horizon tends to be lighter black to brown in color.

• Further down is the B-horizon, or the subsoil. Much of the soil in this region has undergone some degree of weathering and is composed almost entirely of mineral material. Its high iron and clay content usually imparts a reddish color.

• The C-horizon is generally composed of weathered rock fragments and material from the layers above.

• The lowest region is known as the R-layer (sometimes referred to as the D-horizon), which mostly consists of unaltered bedrock material.

It is important to note that all these layers are not necessarily present in every soil profile and the proportions of each layer can vary drastically among various soil samples. Thus, with the farmland example earlier, much of the fertile soil may have had thicker O- and A-horizons, making it more suitable for agriculture even after many years, whereas the damaged soil may have had much thinner top layers. The ages of these soils may make a considerable difference as well. Older soils tend to have almost all

horizons present, and younger soils tend to have far fewer horizons.

Identifying Soil Types: Texture and Structure In addition to distinguishing the chemical and biological makeup of soil, it is also important to understand the impact of soil on water avail- ability in a given area. Particle size is one of the most important aspects of soil type descriptions that helps to determine the holding capacity of the soil as well as its ability to filter water. The size of soil particles determines their texture, which can be classified into smaller subcate- gories (primary units) depending on the mineral components of the soil. Texture is determined by the ratio of sand, silt, and clay in the soil sample (see Table 1).

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Figure 1.

Every soil sample will have different proportions of these primary units. Based on these proportions, each soil sample can be further

categorized and identified. Figure 2 shows a soil analysis chart that illustrates the different classes of soil based on their combinations of sand, silt, and clay. Loam is a close-to-equal mixture of these three primary units and can be used to identify multiple soil types.

Determining particle size (based on these subcategories of soil texture) is the first step in soil characterization. The next step is deter- mining the structure of the soil, which defines how the individual particles aggregate. The soil structure affects how easily air, water, and the roots of plants are able to move within the soil. The arrangement of soil particle aggregates can

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Figure 2.

Soil Type Particle Size

Sand Particles with a diameter greater than 0.05 mm

Silt Particles with a diameter between 0.002 and 0.05 mm

Clay Particles with a diameter less than 0.002 mm

Table 1.

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PROPERTIES OF SOIL: AGRICULTURAL AND WATER AVAILABILITY IMPACTS

Background continued be broken down into peds, or secondary units of the primary soil particles—sand, silt, and clay (see Figure 3).

Knowing the type of soil present by first identifying the primary and secondary particle types can help determine whether the soil is suitable for agriculture. Soil permeability (ability of water to flow through a soil) is directly related to the particle size (texture) and the aggregation of those particles (structure). Rounded, granular peds are particularly suitable for plant growth because their structure easily permits penetration by air, water, and roots. Clay and loamy soils often have blocky peds that are angular and somewhat irregular in shape

and permit the flow of air and water. In platy peds, however, some passageways are blocked because the soil particles are tightly packed. A platy soil usually has high clay content and often occurs in areas that are frequently flooded. These classifications are not applicable to sand because of its inability to form aggregates; thus, instead of clumping, it falls apart.

The roots of plants require air and water. Just as it can die from lack of water, a plant can also die in waterlogged soil owing to a lack of air. Soil must retain water and permit root penetration to support plant life. However, certain particle sizes and arrangements of particles allow for perme- ability conditions that could be detrimental to agriculture. Therefore, particle size and arrange- ment need to be identified to determine proper soil usage. Porosity is the amount of void space between individual soil particles. With high porosity between particles, a greater volume of water can permeate but might also waterlog certain systems. Porosity is another major prop- erty of soil that has a significant effect on water infiltration and soil fertility, or the ability of soil to hold sufficient nutrients for plant growth. In the following section, nutrient availability and chemical composition—and their effects on different soil types—are discussed.

Processes Affecting the Fertility of Soil Determining the productivity of the soil in a given area is crucial. The pH value, or the number of hydrogen ions present in the soil, is a major indicator of soil fertility. The pH scale ranges from 0 to 14, with a value of 7 being neutral. Anything less than 7 is acidic, and anything greater than 7 is considered basic. This scale

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Figure 3.

Platy

Blocky

Granular

Columnar

is logarithmic, so if a solution has a pH of 6, it has ten times the number of hydrogen ions (or is ten times more acidic) than a solution with a pH of 7 possesses. There are wide ranges of pH for a variety of soils; for example, quartz-rich sandstone is rather acidic, whereas limestone tends to be basic.

Rainwater, which has a pH of approximately 5.5, has a significant impact on soil pH. The low pH of rainwater is caused by the mixing of water with carbon dioxide in the atmosphere. Carbonic acid is formed when the precipitation makes contact with the earth’s surface. As this rainwater flows into and along soil surfaces, it transmits many nutrients into the depths of the soil. Iron, along with other minerals such as calcium and magnesium, tends to flow through the higher horizons, which are usually flourishing with life, and continues into the lower horizons that tend to be lacking in nutrients.

Besides pH, decomposition is another important factor in determining the health of soil. Organisms such as bacteria and fungi decompose (break down) organic matter that plants and animals produce at the surface and deposit into the soil, allowing for the survival of organisms deeper in the soil. The rate of decomposition can be increased by worms and other organisms living near the surface. These organisms break down detritus (debris and waste) into smaller bits that sink farther into the soil, feeding the bacteria and fungi.

Nutrients are essential for plant growth. One major issue regarding the health of certain soil types in some areas is the inability of the soil to retain sufficient nutrients. Adding fertilizers and pesticides helps replenish crops and soil, but an

excess of these nutrients can also exert negative effects. Macronutrients are nutrients needed at high concentrations by all living things. Nitrogen, potassium, phosphate, and magnesium are examples of these nutrients necessary for plant growth. Micronutrients—such as the metals zinc, iron, and copper—are those needed in smaller quantities.

Fertilizers are useful to replace macronutri- ents lost from the soil. Many fertilizers, such as manure, are either purely or mostly organic matter; however, numerous synthetic fertilizers have also been developed. Nitrogen, phospho- rous, and potassium are commonly regarded as the limiting nutrients in crop production and are therefore typically added to soils in specific amounts via fertilizers.

Nitrogen is a special case; plants require nitrogen in the form of ammonium or nitrate, and atmospheric nitrogen can only be converted synthetically, or by soil-dwelling microbes, as part of the nitrogen cycle. Additional ways in which nitrogen can enter the soil include via the death and decomposition of plants and by the production of nitrogenous waste from plants being eaten by animals. Nitrogen is vital for plant growth; it is a very important structural component of chlorophyll, the compound used by plants in photosynthesis to produce oxygen and sugars. Nitrogen is also a major component of amino acids, which are the building blocks of proteins. Since it is not retained by soil, excess nitrogen can leach onto the surface or into groundwater, causing algal blooms and a loss of oxygen. Although nitrogen is an essential element, because it is not retained by the soil,

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PROPERTIES OF SOIL: AGRICULTURAL AND WATER AVAILABILITY IMPACTS

Background continued farmers frequently need to reapply fertilizers containing nitrates, which in turn can lead to environmental challenges.

Phosphorus is usually present in the form of phosphates in the soil. Phosphate is an important component of adenosine triphos- phate/adenosine diphosphate (ATP/ADP) in plants and is therefore necessary for energy transport and storage. Phosphates, along with pentose sugars, make up the backbone of DNA and RNA. Phosphorus availability frequently limits plant growth and flowering; therefore, the introduction of phosphates into soil can lead to rapid plant growth. This element also plays a major role in keeping the root systems of plants strong and thriving. However, like excess nitrates, too much phosphorus can lead to algal blooms, which can harm aquatic ecosystems.

The last main nutrient in fertilizer is known as potash. It is a mixture of various salts that contain soluble potassium, which is vital for the development of many flowers and fruits. Potas- sium is required for the activation of numerous enzymes and the regulation of pH as well. It also controls the opening and closing of the stomata (pores in the leaves), which affects photosyn- thesis, water and gas transport, and tempera- ture regulation.

In this investigation, you will conduct experi- ments to determine the texture and structure of soil samples as well as the ability of water to permeate each sample. You’ll also perform chemical tests to determine the fertility of two soil samples.

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Materials Included in the materials kit:

Bag of sand, ¼ c

rapitest® Soil Tester kit

Bag of clay, ½ c

Cheesecloth

3 Plastic tubes

6 Twist ties

Needed but not supplied: • Sheet of white paper • 2 Soil samples • Distilled water • Tap water

• Liquid hand soap • Scissors • Stopwatch/timer

3 Rubber bands

Graduated cylinder, 10 mL

Safety Wear your safety goggles, gloves, and lab apron for the duration of this investigation.

Read all the instructions for this laboratory activity before beginning. Follow the instructions closely, and observe established laboratory safety practices, including the use of appropriate personal protective equipment (PPE).

Avoid contact with your skin, eyes, and mouth when working with rapitest® Soil Tester capsules. Wear

PPE at all times when using these capsules. Wash your hands immediately following the use of these capsules.

Do not eat, drink, or chew gum while performing these activities. Wash your hands with soap and water before and after performing each activity. Clean the work area with soap and water after completing the investigation. Keep pets and children away from lab materials and equipment.

Preparation 1. Read through the activities. 2. Obtain all materials. 3. Clean and sanitize the work area. 4. Using a plastic cup from the equipment kit,

collect a few handfuls of soil from about 8 cm (3 in) below the surface (enough to fill the cup approximately halfway). This sample is the “Collected Soil” sample and will be used in all the activities. Before using the sample, remove excess grasses, roots, and other plant materials and break up any large clumps.

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Graduated cylinder, 100 mL

Plastic cup

Permanent marker

Ruler

Test tube rack5 Test tubes

2 Pipettes

Needed from the equipment kit:

Reorder Information: Replacement supplies for the Properties of Soil: Agricultural and Water Availability Impacts investigation can be ordered from Carolina Biological Supply Company, kit number 580821.

Call: 800-334-5551 to order.

ACTIVITY

ACTIVITY 1

A Particle Size Distribution and Determination of Soil Texture

Day 1 1. Take three test tubes from the equipment kit

and label them “Sand,” “Clay,” and “Collected Soil” (one for each sample).

2. Fill each test tube halfway with its designated soil sample. For each test tube, add tap water to about 1 cm below the top and, with gloved hands, place your thumb over the opening.

3. Shake each test tube for 30 seconds. Add one drop of liquid hand soap to each sample to help settle the particles. Allow the samples to settle overnight in the test tube rack. Place a piece of paper over the top of the three test tubes to prevent contamination of the samples.

Day 2 1. Place a sheet of white paper behind the test

tubes containing the settled soil samples, and observe the layers that have formed in each.

2. Using the “Sand” and “Clay” samples as controls, identify the layer types in the “Collected Soil” sample. You may use a marker to label each layer. See Figure 4 for guidance.

3. Use a ruler to measure (in centimeters) the depth of the sand, silt, and clay layers in the “Collected Soil” sample. The silt layer is the layer formed between the clay and sand layers, owing to its particle size that is larger than that of clay but smaller than that of sand (see Figure 4). Record these values in Data Table 1 (see page 14).

All layers may not be visible or may vary substantially in depth in the “Collected Soil” sample.

4. Measure the total depth of the soil. Be careful to exclude the humus layer containing floating dirt and grasses at the top of the sample (see Figure 4).

5. Calculate the particle size distribution of the “Collected Soil” sample by dividing the depth of each layer by the total depth of the soil, then multiplying that value by 100. Record the percentages of the primary particles clay, silt, and sand in the “Collected Soil” sample in Data Table 1.

6. Use the soil analysis chart (see Figure 2) to determine the soil texture of the collected sample, and record this in Data Table 1.

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Figure 4.

Note: In Figure 2, each corner of the triangle represents 100% of one of the three types of soil: silt, sand, and clay. Locate these points. Loam is found in the center of the triangle. Following the arrows on the sides of the triangle, start by pointing to the percentage of your soil sample that is composed of silt (from Table 1) on the right edge of the triangle. With your other hand, point to the percentage composed of sand along the bottom edge. Following the corre- sponding diagonal lines, move your fingers toward each other until the lines intersect. (If you have found the correct intersecting lines, the percentage of clay of your sample should line up horizontally on the left edge from the point at which your fingers meet). This point of intersection represents the class of soil texture; if the point falls on a boundary between two types, choose the one that occupies a larger area. Also, if the percentage of either silt or sand is less than 10% for your specific “Collected Soil” sample, start your fingers on clay and the existing primary particle and follow the same process.

ACTIVITY 2

A Porosity of Different Soil Types 1. Ensure the remaining “Sand,” “Clay,” and

“Collected Soil” samples are ready for use. 2. Cut two 3-cm squares from the cheesecloth. 3. Secure these two pieces of cheesecloth over

one end of a plastic tube with a rubber band (see Figure 5).

4. Make a mark on the tube 4 cm up from the end covered by cheesecloth.

5. Pour one of the three soil samples onto a paper towel, and make sure any lumps or rocks are crushed or removed. Fill the tube to the 4-cm mark with the particular soil sample. Use twist ties to suspend the tube in the 100-mL graduated cylinder (see Figure 5).

6. Start a timer as you pour 10 mL of tap water from the 10-mL graduated cylinder into the plastic tube in the setup.

7. Observe the time (in seconds) taken for the first drop of water to emerge from the column, and record this data in Data Table 2. For some samples this could happen very quickly (even as you are pouring), so keep a close eye on the sample as you are pouring.

8. Repeat Steps 2–7 with the two remaining soil samples (using new materials for each sample).

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Figure 5.

ACTIVITY

ACTIVITY 3

A pH Test Comparison of Soil Samples

For Activities 3 and 4, use the soil from the initial collection in the “Preparation” section (the “Collected Soil” sample). For the other soil sample, collect a sample of your choice—for example, this can be commercial potting soil, soil from a different outdoor location, or any remaining clay from this lab. Avoid using any remaining sand since it carries little nutritional value due to its large pore space.

Ideally, the best option is to find another soil sample from a different outdoor location with contrasting properties. For example, if your collected soil sample was from a very dry area without plants, try to find a very moist soil sample that is covered with grass. You can also select a deeper soil sample as a comparison of the surface- based collected soil sample.

1. Open the rapitest® Soil Tester kit. 2. Wearing PPE, remove the capsules from

one of the green-capped vials (the pH test capsules).

3. Fill the vial with the “Collected Soil” sample (collected in “Preparation”) up to the first line.

4. Carefully open one of the green capsules and pour the powder into the vial. (Do this over the vial to prevent any spillage.)

5. Add distilled water to the fourth line on the vial.

6. Cap the vial, and shake it thoroughly (about 20 seconds).

7. Allow the soil to settle for approximately one minute.

8. Compare the color of your sample to those on the pH color chart on the back of the rapitest® Soil Tester kit instructions to determine the pH of the soil sample. Record this data in Data Table 3.

9. Discard the soil mixture. 10. Rinse the vial, and repeat Steps 2–9 with the

other soil sample you chose.

ACTIVITY 4

A Nitrogen, Phosphorus, and Potash Test Comparisons of Soil Samples

Day 1 1. Use a ruler to measure 2 cm up from the

bottom of a clean test tube. Fill the tube to that point with the “Collected Soil” sample. Fill the tube with tap water up to 1 cm below the brim.

2. Repeat Step 1 with the other soil sample from Activity 3.

3. For each sample, place your gloved thumb over the mouth of the test tube and shake the sample thoroughly for approximately one minute. Allow each mixture to settle (for approximately 24 hours) in the test tube rack.

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Day 2 1. Wearing PPE, remove the caps from the

purple, blue, and orange vials from the rapitest® Soil Tester kit and remove their capsules.

2. Using a pipette, fill each vial up to the fourth line with the supernatant (liquid above the soil) from one of the soil mixtures made on Day 1.

3. Carefully separate the capsules, and pour the powder into the corresponding-colored vials. Make sure the colors of the capsules match the respective vials. (Perform this step over the vial to prevent spillage of the powder).

4. Cap the vial, and shake thoroughly. 5. Allow the color to develop for

10 minutes. 6. Compare the color of your sample to those

on the Plant Food Color Chart on the back of the rapitest® Soil Tester kit instructions to determine the nutrient content of the soil sample. Record this data in Data Table 4.

7. Rinse the vials, and repeat Steps 2–6 with the other soil mixture.

Disposal and Cleanup 1. Rinse and dry the graduated cylinder, and

return it to your equipment kit. 2. Discard the soil samples and other materials

in the household trash. 3. Sanitize the work space, and wash your

hands thoroughly.

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ACTIVITY

Observations Data Table 1.

Depth of Clay Layer

(cm)

Depth of Silt Layer

(cm)

Depth of Sand Layer

(cm)

Total Depth (cm)

% Clay

% Silt

% Sand

Soil Texture

Collected Soil Sample

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Data Table 2.

Time Taken for First Drop to Emerge from Column(s)

Sand Sample

Clay Sample

Collected Soil Sample

Data Table 3.

Collected Soil Sample

Other Soil Sample (Location Description: _____________________________)

pH

Data Table 4.

Nitrogen Phosphorus Potash

Collected Soil Sample

Other Soil Sample

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NOTES

Environmental Science Properties of Soil: Agricultural and Water Availability Impacts

Investigation Manual

www.carolina.com/distancelearning 866.332.4478

Carolina Biological Supply Company www.carolina.com • 800.334.5551 ©2018 Carolina Biological Supply Company

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