Dissolved Oxygen vs Temperature



Dissolved Oxygen vs Temperature

Objective

Dissolved oxygen is an important measure of water quality for aquatic life. In this project you will use a test kit to measure the level of dissolved oxygen in water samples. This project has two goals:

1. to measure dissolved oxygen in water samples at different temperatures, and

2. to determine the saturating oxygen concentration for water samples at different temperatures.

Introduction

Dissolved oxygen is one of many measures of water quality, but an important one for aquatic life. Like land animals, fish and shellfish require oxygen to survive. When oxygen levels fall below 5 mg/l, fish are stressed. At oxygen levels of 1–2 mg/l, fish die.

The amount of oxygen that can dissolve in water (i.e., the saturating concentration of oxygen) depends on water temperature. Colder water can hold more oxygen than warmer water. You'll see for yourself just how much more in this project.

Where does dissolved oxygen come from?

There are two main sources of dissolved oxygen: air and photosynthesis. Consider photosynthesis first. You probably know that photosynthesis is the fundamental biological process that uses light energy to produce sugar from carbon dioxide and water. Oxygen is a by-product of photosynthesis. Both algae (phytoplankton, seaweeds) and plants can be found in natural bodies of water. These organisms are net producers of oxygen in the daytime, but at night become net consumers of oxygen.

Now consider oxygen from the air. At the surface of the water, oxygen from the air equilibrates with oxygen dissolved in the water. This is a dynamic equilibrium: the oxygen molecules are in constant motion. At any given moment, some are leaving the water for the air, and some are leaving the air to dissolve in water. At equilibrium, there is a balance. On average, an equal number of oxygen molecules are leaving and entering the water. If the water temperature increases, the water can't hold as much oxygen as before—the water is oversaturated with oxygen. For a time, there will be more oxygen molecules leaving the water than entering it from the air. Then a new equilibrium will be reached, with less oxygen in the water than before.

Moving water has a rougher surface than still water. With more surface area in contact with air, moving water will equilibrate with air more quickly. (You'll make use of this in your experiment.) In natural situations, water can also become stratified into different layers (see the Science Buddies project Can Water Float on Water?). For example, cold water is denser than warm water, and salt water is denser than fresh water. Can you think of ways that different layers of water might form in a lake or ocean? What do you think happens to the oxygen in a colder layer of water trapped under a warmer layer of water? (Remember that the warmer layer cannot hold as much dissolved oxygen as the colder layer. See the Variations section for a project idea on this topic.)

What causes dissolved oxygen levels to vary?

So far we've seen that dissolved oxygen can come from the air or from photosynthesis, and that when water warms up, there is a net loss of dissolved oxygen. Besides warming, how else can dissolved oxygen become depleted? The answer is another fundamental biological process: respiration. Respiration uses oxygen to break down molecules, in order to produce energy for cells. So the amount of dissolved oxygen will be determined by:

• how much oxygen the water can hold (temperature-dependent),

• how much surface area is available for diffusion from the air,

• how much oxygen is produced by photosynthesis, and

• how much oxygen is consumed by respiration.

Here is a real-world example of variations in dissolved oxygen levels from a continuous monitoring site in the Chesapeake Bay (Maryland DNR, 2006). All of the data were collected at the same location over the same time period. The first graph shows dissolved oxygen, the second graph shows temperature and the third graph shows chlorophyll concentration (a measure of how much algae is present in the water). Notice the daily fluctuations in oxygen level and water temperature. Notice also how the oxygen level and chlorophyll level both declined toward the end of the time period.

|[pic] |

|[pic] |

| |

|Figure 1. The three graphs show (from top to bottom) |

|dissolved oxygen, water temperature, and chlorophyll |

|concentration at a monitoring site in the Chesapeake |

|Bay over a one-week period. (Maryland DNR, 2006) |

Sometimes imbalances occur that lead to skyrocketing concentrations of algae. For a project that investigates water quality measures and algal blooms, see the Science Buddies project Harmful Algal Blooms. You can also check out the references in the Bibliography section.

How is dissolved oxygen measured?

Dissolved oxygen can be measured with an electronic metering device or with a chemical test. Dissolved oxygen meters cost hundreds of dollars, so this project will use the chemical testing method. You can buy a dissolved oxygen test kit for around $50. The kit will test 100 water samples. Commercial test kits are based on the "modified Winkler method." You can read more details on this method in the Bales and Gutmann reference in the Bibliography, but here is a basic outline of how the test works:

1. A water sample is collected and the sampling container is sealed under water. This prevents exposure of the sample to the atmosphere.

2. A chemical is added to the water sample to react with all of the dissolved oxygen in the sample. An insoluble precipitate is formed.

3. Additional chemicals are added to drive the first reaction to completion, and to prevent an unwanted reaction from occurring in the final step.

4. A third addition causes the precipitate to change color.

5. The oxygen is now "fixed" and can no longer react with the atmosphere.

6. In the final step, a titration is performed. In this step, a chemical is added in liquid form, one drop at a time. The added compound reacts with the colored precipitate, causing it to lose color. The water sample is mixed after the addition of each drop. When the color change is complete (sample is clear again), it means that the added compound has reacted with all of the fixed oxygen in the sample. By counting the number of drops that were added, the amount of oxygen in the sample can be calculated.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• dissolved oxygen,

• titration,

• oxygen saturation,

• hypoxia,

• harmful algal blooms,

• Gulf of Mexico "dead zone,"

• photosynthesis,

• respiration.

More advanced students will want to study the chemistry used in the test kits (modified Winkler method). The reference by Bales and Gutmann in the Bibliography is a good place to start.

Questions

• What concentration of dissolved oxygen is required to support aquatic life?

• What are some of the processes that increase dissolved oxygen concentration in natural bodies of water?

• What are some of the processes that decrease dissolved oxygen concentration in natural bodies of water?

Bibliography

• For more background information on water quality measures, including dissolved oxygen, see:

Munson, B.H. et al., 2005. "Water on the Web: Understanding: Water Quality: Parameters: Dissolved Oxygen," University of Minnesota Duluth and Lake Superior College [accessed May 15, 2006] .

• The dissolved oxygen test kits can seem rather mysterious. The instructions that come with the kits explain the procedure step-by-step, but they do not explain how the test works. The following webpage is a good resource for understanding what is going on with each step. (For advanced students, there is also a separate page with more detailed information on the chemical reactions involved.):

Bales, R. and C. Gutmann, date unknown. "The Chemistry Section: Dissolved Oxygen," Department of Hydrology and Water Resources, University of Arizona [accessed May 15, 2006] .

• Archived data (including dissolved oxygen) from Maryland DNR continuous monitoring stations in Chesapeake Bay can be found at:

Maryland DNR, 2006. "Eyes on the Bay," Maryland Department of Natural Resources [accessed May 15, 2006] .

• These websites have background information on harmful algal blooms, which can deplete water of dissolved oxygen:

o Anderson, D.A., 2006. "Red Tide and Harmful Algal Blooms," [accessed May 1, 2006] .

o NOAA, 2006a. "Harmful Algal Blooms," NOAA's National Ocean Service [accessed May 1, 2006] .

• These sites have background information on the Gulf of Mexico "Dead Zone," a massive area of hypoxic water that appears every summer near the mouth of the Mississippi River:

o Science Museum of Minnesota, date unknown. "Dead Zone Home," Science Museum of Minnesota [accessed May 15, 2006] .

o Bruckner, M., 2006. "The Gulf of Mexico Dead Zone," Science Education Resource Center, Carleton College [accessed May 15, 2006] .

• Roach, J., 2005. "Gulf of Mexico 'Dead Zone' Is Size of New Jersey," National Geographic News, May 25, 2005 [accessed May 15, 2006] .

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• dissolved oxygen test kit;

Note: kits are available from two manufacturers, Hach and LaMotte. Both use essentially the same chemistry (modified Winkler method, see Bales and Gutmann reference in the Bibliography). Both are available from multiple online suppliers; a web search on the brand name + dissolved oxygen test kit will locate them.

o Hach OX-2P, single-parameter test kit for dissolved oxygen, will test 100 samples,

o LaMotte 7414 or 5860 dissolved oxygen test kit, will test 50 samples.

• thermometer (range 0°C–100°C),

• gloves,

• safety glasses/goggles,

• at least 3 containers (1 L or more) for water, one to be heated on stove,

• spray bottle (for rinsing test container in the field),

• sealable container (at least 500 ml, for test waste in the field).

• Optional equipment for aerating the water samples (if not readily available, see the Experimental Procedure section for an alternative aeration method):

o aquarium aerator pump,

o 2.5 cm or larger fine-mist air stone,

o plastic aquarium tubing.

Experimental Procedure

|Safety notes: |

|Read and follow all of the instructions in your test kit, including all safety precautions. |

|Wear safety goggles and gloves when using test kit reagents. |

|Avoid skin contact with test kit reagents. |

In this experiment, you will measure how dissolved oxygen changes in water samples at different temperatures. You will test both aerated and non-aerated water samples at each temperature.

1. Do your background research and make sure that you are knowledgeable about the terms, concepts, and questions, above.

2. Read the instructions that came with your dissolved oxygen test kit so that you know how to perform the test. The Bales and Gutmann reference in the Bibliography has an explanation of what is happening with each of the steps, plus a separate page with a more detailed explanation of the chemistry involved (for more advanced students).

3. Collect your water sample (4 l minimum). The sample can be from a natural body of water, such as an estuary, ocean, lake, pond, or stream. You can also use plain old tap water.

4. Take a baseline dissolved oxygen measurement.

a. When you collect your water sample, bring along your dissolved oxygen test kit, thermometer, spray bottle and sealable waste container.

b. Measure the temperature of the water at the collection site.

c. Test the dissolved oxygen content of the water at the collection site. This is your baseline measurement of dissolved oxygen.

d. When your measurement is complete, discard the test sample down a drain; do not throw it back in the body of water you sampled. Do the same with the rinse water when you clean the sampling container. If need be, bring the test waste back home in a sealable container and flush it down the drain at home.

e. To be sure that your results are consistent, you should repeat the test at least three times, using a fresh sample each time. Use the spray bottle to rinse your test container. Discard rinse water down a drain or into your waste container for disposal at home.

f. Be sure to record the temperature of the water.

5. At home, divide your water sample equally into three separate containers:

a. container 1 will be cooled with ice,

b. container 2 will be allowed to equilibrate to room temperature, and

c. container 3 will be heated slightly.

6. Add enough ice to container 1 to bring the water to about 4–8°C. When the water has cooled, record the temperature and measure the dissolved oxygen concentration. As before, you should run the test at least three times, to be sure that your results are consistent.

7. Next, aerate the sample and re-test. The point of aeration is to saturate the water with oxygen (i.e., dissolve as much oxygen as the water can hold). You can aerate the water with an aquarium aeration pump and airstone. Lots of small bubbles work best. Allow 5–10 minutes for equilibration. Alternatively, you can pour the water back and forth between two large buckets for 5–10 minutes to aerate the water. In either case, check the temperature periodically and add more ice if needed to maintain the temperature.

8. When the water has been aerated, repeat the dissolved oxygen test. Make sure to record the temperature. As before, you should run the test at least three times to be sure that your results are consistent.

9. Run similar tests (aerated and non-aerated) for container 2, the water sample at room temperature (it may take a few hours to equilibrate, depending on how cold the sample was to start).

10. Run similar tests (aerated and non-aerated) for container 3, the water sample that you heat. You can warm it on the stove, or in the microwave. Mix the sample gently and check the temperature frequently. Aim for a temperature from 35–40°C. You don't want to scald yourself when testing the dissolved oxygen concentration.

11. Summarize your results in a table. For example:

|Sample |

|pH Value |H+ Concentration |Example |

| |Relative to Pure Water| |

|0 |10 000 000 |battery acid |

|1 |1 000 000 |sulfuric acid |

|2 |100 000 |lemon juice, vinegar |

|3 |10 000 |orange juice, soda |

|4 |1 000 |tomato juice, acid rain |

|5 |100 |black coffee, bananas |

|6 |10 |urine, milk |

|7 |1 |pure water |

|8 |0.1 |sea water, eggs |

|9 |0.01 |baking soda |

|10 |0.001 |Great Salt Lake, milk of |

| | |magnesia |

|11 |0.000 1 |ammonia solution |

|12 |0.000 01 |soapy water |

|13 |0.000 001 |bleach, oven cleaner |

|14 |0.000 000 1 |liquid drain cleaner |

|[pic] |

|Figure 3. Map of U.S. annual average precipitation pH for 1992. |

|(USGS, 1997). |

Figure 3 shows a map of the average pH of precipitation in the continental U.S. for the year 1992. "The areas of greatest acidity (lowest pH values) are located in the Northeastern United States. This pattern of high acidity is caused by the large number of cities, the dense population, and the concentration of power and industrial plants in the Northeast. In addition, the prevailing wind direction brings storms and pollution to the Northeast from the Midwest, and dust from the soil and rocks in the Northeastern United States is less likely to neutralize acidity in the rain." (USGS, 1997)

Plant Growth

Most plants prefer soil that is near neutral pH. There are particular varieties (strawberries, azaleas and rhododendrons, for example) that prefer acidic soil. Soil pH also influences how readily available many soil nutrients are to plants.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• titration,

• water hardness,

• pH.

More advanced students will also want to understand the following terms and concepts:

• molarity,

• stoichiometry.

Bibliography

• Wikipedia contributors, 2006. "Titration," Wikipedia, The Free Encyclopedia [accessed May 9, 2006] .

• These USGS webpages have information on patterns of water hardness of rivers and acidity in rainwater across the United States.

o USGS, date unknown. "Explanation of Hardness," United States Geological Survey [accessed May 9, 2006] .

o USGS, 1997. "What Is Acid Rain?" United States Geological Survey [accessed May 9, 2006] .

• These sites explain the pH scale:

o Environment Canada, 2002. "Kids' Corner pH Scale," The Green Lane: Acid Rain, Environment Canada website [accessed March 14, 2006] .

o Decelles, P., 2002. "The pH Scale," Virtually Biology Course, Basic Chemistry Concepts, Johnson County Community College [accessed March 14, 2006] .

• Review pages on moles:

Park, J.L., 2004. "The Mole Table of Contents," The ChemTeam, A Tutorial for High School Chemistry [accessed May 9, 2006] .

• Allison, J.R., 2003. "It's Raining, It's Pouring, the Radishes Are Growing: Chemical Analysis of Rainwater for the Nation's Food Production," California State Science Fair Project Abstract [accessed May 9, 2006] .

Materials and Equipment

• This project requires planning ahead. Remember that it will take some time for your volunteers to collect samples and send them to you. You also need to allow time (at least one week) for the plant growth experiment once you have received all of the samples. Start early and make sure your volunteers send their samples in a timely manner!

o Where to get samples? You will need to obtain rainwater samples from a wide geographical area. Consult the maps in the Introduction for historical patterns of variation. Ask friends and relatives to collect samples for you.

o How much water do I need? Check your test kit instructions (see below) to see how much water is required for each test (usually about 5 ml). You will want to repeat your tests for each sample at least 3 times to assure that your results are consistent. So you'll need a minimum of 30 ml just for testing (best to plan on more). You will also need water for the plant growth experiment. Calculate how much water you will need for plant growth, and add 50 ml for testing purposes. This is how much rainwater each of your volunteers will have to send to you.

o How should my volunteers collect rainwater samples? Simply putting a jar out on the lawn during a rainstorm is not going to be very efficient. In order to get enough water, your volunteers need a large catchment area. Probably the most straightforward solution is to collect water from the roof, by placing a collection jar underneath a downspout.

o Make sure your volunteers label the water sample with the date and location from which it was collected.

• For performing the water quality tests, the simplest method is to use a pre-packaged kit designed for testing aquarium water. There are several different brands available. You should be able to find a choice at a local pet store that sells fish. The kit will say how many water samples it will test. You should be able to find kits to test 50 samples for under $10. The kits you need for this project are:

o general hardness (GH) test kit,

o pH test kit.

• For the plant growth experiment, you'll need:

o radish seeds, (or other suitable, fast-growing seeds),

o small containers (peat pots or seedling trays),

o potting soil, and

o a measuring device for dispensing water.

Experimental Procedure

1. For the water hardness and pH tests, follow the instructions that come with the water test kit. When titrating samples, it is important to mix the solution well after each drop of test solution is added.

2. For the plant growth portion of the experiment, it is important to keep all of the other growth conditions (sun exposure, soil, temperature, etc.) constant, and to vary only the source of water used for the plants. Be sure to use the same amount of water. Consult the Science Buddies resource, Measuring Plant Growth for methods you can use to quantify differences in growth.

Variations

• Does rainwater chemistry in your area vary with weather patterns? Collect samples over several weeks or months, and test the water quality. Keep track of the weather systems that produced the precipitation. Were there variations in the ultimate source of the moisture? Can you correlate these variations with changes in rainwater chemistry?

• If you live in an urban area, is rainwater chemistry affected by smog? Check the air quality reported in the newspaper for the days that samples were collected. Do you see differences in rainwater chemistry after days with high smog compared to days with cleaner air?

• For the samples in your study, how does rainwater hardness compare with groundwater hardness? (See Figure 2 in the Introduction, above.) How does the acidity compare to the 1992 U.S. data? (See Figure 3 in the Introduction, above.)

• Here are two related Science Buddies projects you might want to check out:

o How Does Soil Affect the pH of Water?, and

o Conductance as a Water Quality Measurement.

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

This project was based on:

• Allison, J.R., 2003. "It's Raining, It's Pouring, the Radishes Are Growing: Chemical Analysis of Rainwater for the Nation's Food Production," California State Science Fair Project Abstract [accessed May 9, 2006] .

How does soil change with depth?

Objective

This project has two goals:

1. to observe and measure core samples of soil to see how soil properties changes with depth, and

2. to compare soil core profiles collected from multiple sites.

Introduction

"Soils are one of Earth's essential natural resources, yet they are often taken for granted. Most people do not realize that soils are a living, breathing world supporting nearly all terrestrial life." (GLOBE, 2005d)

We depend on soil for food production from crops, but our dependence on the soil goes much deeper than that. Soil is such a vital part of every ecosystem on Earth that it is often called "the great integrator" (GLOBE, 2005d). Here are some examples that help to explain the nickname: "Soils hold nutrients and water for plants and animals. They filter and clean water that passes through them. They can change the chemistry of water and the amount that recharges the groundwater or returns to the atmosphere to form rain. The foods we eat and most of the materials we use for paper, buildings, and clothing are dependent on soils. Soils play an important role in the amount and types of gases in the atmosphere. They store and transfer heat, affect the temperature of the atmosphere, and control the activities of plants and other organisms living in the soil." (GLOBE, 2005d)

Yet arable soil covers only a small fraction of the Earth's surface—about 10% (Levine, 2001a, see Bibliography for an interesting Web-based demonstration of this fact). This thin layer of soil is called the pedosphere. Soil formation is a grindingly slow process. To produce one inch of soil can take 500 years (NRCS, 2001, see Bibliography).

The study of soil is called pedology. Learn more about the world beneath our feet! In this project, you'll become an amateur pedologist and learn how to make the following soil characterization measurements:

• site description,

• horizon depths,

• soil structure,

• soil color,

• soil consistence,

• soil texture,

• roots,

• rocks,

• carbonates.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• soil formation,

• soil types,

• humus,

• silt,

• sand,

• clay,

• soil horizon,

• pedosphere,

• ped.

Questions

• What is a soil horizon?

• What are the five factors that contribute to soil formation?

Bibliography

• Levine, E., 2005a. "Soil Science Education Home Page," Goddard Space Flight Center, NASA [accessed March 16, 2006] .

• Levine, E., 2002. "Soil Science Basics," Goddard Space Flight Center, NASA [accessed March 16, 2006] .

• To see an online, step-by-step guide to soil characterization, see:

Levine, E., 2001a. "Soil Characterization Protocols: A Step-by-Step Guide," GLOBE Soil Characterization Investigation [accessed March 16, 2006] .

• This brief field guide shows the soil characterization protocol in condensed format:

Potter, R. and I. Trakhtenberg, 2002. "Soil Characterization Field Guide," Goddard Space Flight Center, NASA [accessed March 16, 2006] .

• For a detailed soil characterization protocol, download and print out this pdf document:

GLOBE, 2005b. "GLOBE Soil Science Protocols: Soil Characterization," Global Learning and Observations to Benefit the Environment [accessed March 16, 2006] .

• For suggestions on finding a local expert who might be able to serve as a mentor for your project, see:

Levine, 2001b. "Resources," Goddard Space Flight Center, NASA [accessed March 16, 2006] .

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• soil characterization field guide,

• homemade soil auger, consists of:

o length of 1" PVC pipe,

o 1" wood dowel (for pushing soil core from PVC pipe),

o sturdy wood block (to cover end of pipe when driving it in with hammer),

o hammer or mallet.

• spray bottle with water,

• golf tees, nails or other horizon markers,

• soil color book,

• trowel or shovel,

• paper towels,

• meter stick or tape measure,

• magnifying glass,

• camera,

• latex gloves,

• vinegar,

• eyedropper or disposable squeeze pipette,

• sheets of paper or paper plates,

• #10 sieve (2 mm mesh openings, should be able to find this at a gardening store),

• soil characterization data sheet,

• helper,

• lab notebook,

• pencils,

• Munsell Soil Color chart (optional—$150 new; perhaps you can find this at your local library),

• sealable bags or containers (optional),

• marking pen (optional).

Experimental Procedure

1. Select your sites.

2. Describe each site in your lab notebook. Include in your description:

a. slope,

b. landscape position (e.g., hillside, summit, large flat area),

c. cover type (e.g., trees, grass),

d. land use (e.g., forest, lawn, beach),

e. parent material (underlying bedrock, if you know it—a county soil survey may be available and would be helpful here).

3. Take soil core samples at each site.

a. Have your helper hold the PVC pipe vertically, with one end firmly on the ground where you want to sample the soil.

b. Hold the wood block in place on top of the PVC pipe, to distribute the force of the blows from the hammer or mallet. (Keep your fingers out of the way!)

c. Pound the PVC pipe into the soil, until there is about 6 in remaining above the surface. (Alternatively, you can take the soil cores out in smaller chunks, carefully assembling them in order.)

d. Holding the pipe straight up and down, rotate it back and forth to loosen the pipe in the hole.

e. Pull up the pipe with your soil core sample inside.

f. Use the 1 in dowel to carefully push the soil core sample out of the pipe. Use paper plates or sheets of plain paper as a background for examining the soil core samples.

4. Identify soil horizons. Use all of the information at your disposal:

a. soil color and texture,

b. root depth,

c. evidence of worms and other soil organisms, etc.

d. This is a good time to use your magnifying glass!

5. Using golf tees or nails (or markers on the background paper), mark the soil horizon depths.

6. Measure the horizon depths (top and bottom of each horizon) and record them in your data table (see below for example).

7. For each soil horizon, use the table below to identify the soil structure and record it in your data table (GLOBE, 2005b; information also available in the printable Soil Characterization Field Guide).

a. Use a trowel or other digging device to remove a sample of soil from the horizon being studied.

b. Hold the sample gently in your hand and look closely at the soil to examine its structure. This is a good time to use your magnifying glass!

|Step 7. Identifying Soil Structure |

|Granular: Resembles cookie crumbs and is usually less than 0.5 cm in diameter. Commonly found |[pic] |

|in surface horizons where roots have been growing. | |

|Blocky: Irregular blocks that are usually 1.5–5.0 cm in diameter. |[pic] |

|Prismatic: Vertical columns of soil that might be a number of cm long. Usually found in lower |[pic] |

|horizons. | |

|Columnar: Vertical columns of soil that have a white, rounded salt "cap" at the top. Found in |[pic] |

|soils of arid climates. | |

|Platy: Thin, flat plates of soil that lie horizontally. Usually found in compacted soil. |[pic] |

|In certain cases, soil samples may have no structure. These would be classified as either "Single-Grained" or "Massive." |

|Single-Grained: Soil is broken into individual particles that do not stick together. Always |[pic] |

|accompanies a loose consistence. Commonly found in sandy soils. | |

|Massive: Soil has no visible structure, is hard to break apart and appears in very large |[pic] |

|clods. | |

8. For each soil horizon, identify the soil color by matching to the swatches in a soil color book (optional, but preferable), or by using descriptive color terms. Photographs would also be useful here for your display board.

a. Take a ped (a ped is a small, naturally-occuring aggregate of soil—in other words, a small "lump" of soil) from the horizon being studied and note whether it is moist, dry or wet. If it is dry, moisten it slightly with water from your spray bottle.

b. Break the ped and hold it next to the color chart.

c. Stand with the sun over your shoulder so that sunlight shines on the color chart and the soil sample you are examining.

d. Find the color on the color chart that most closely matches the color of the inside surface of the ped.

e. Record the color of the ped in your data table, in the row for the appropriate soil horizon. Sometimes, a soil sample may have more than one color. Record a maximum of two colors if necessary, and indicate (1) the dominant (main) color, and (2) the sub-dominant (secondary) color.

9. For each soil horizon, use the table below to identify the soil consistence (GLOBE, 2005b; also available in the printable Soil Characterization Field Guide).

a. Take a ped from the soil horizon being studied. If the soil is very dry, moisten the face of the core sample by squirting water on it, and then remove a ped for determining consistence.

b. Holding the ped between your thumb and forefinger, gently squeeze it until it pops or falls apart.

c. Record your findings for each soil horizon in your data table.

|Step 9. Identifying Soil Consistence |

|Loose: You have trouble picking out a single ped and the structure falls apart before you |[pic] |

|handle it. Note: Soils with Single-Grained structure always have Loose consistence. | |

|Friable: The ped breaks with a small amount of pressure. |[pic] |

|Firm: The ped breaks when you apply a larger amount of pressure and the ped dents your fingers|[pic] |

|before it breaks. | |

|Extremely Firm: The ped can't be crushed with your fingers (you need a hammer!) |[pic] |

10. For each soil horizon, identify the soil texture using the instructions and the soil textural triangle, below (GLOBE, 2005b, also available in the printable Soil Characterization Field Guide).

Soil Texture, Step 1

a. Place some soil from a horizon (about the size of a small egg) in your hand and use the spray bottle to moisten the soil. Let the water soak into the soil and then work it between your fingers until it is thoroughly moist. Once the soil is moist, try to form a ball.

b. If the soil forms a ball, go on to Soil Texture, Step 2.

c. If the soil does not form a ball, call it a sand. Soil texture is complete. Record the texture for this horizon in your data table.

Soil Texture, Step 2

d. Place the ball of soil between your thumb and forefinger and gently push and squeeze it into a ribbon.

e. If you can make a ribbon that is longer than 2.5 cm, go to Soil Texture, Step 3.

f. If the ribbon breaks apart before it reaches 2.5 cm, call it a loamy sand. Soil texture is complete. Record the texture for this horizon in your data table.

Soil Texture, Step 3

g. If the soil:

▪ is very sticky,

▪ is hard to squeeze,

▪ stains your hands,

▪ has a shine when rubbed,

▪ forms a long ribbon (> 5 cm) without breaking,

then call it a clay and go to Soil Texture, Step 4.

h. Otherwise, if the soil:

▪ is somewhat sticky,

▪ is somewhat hard to squeeze,

▪ is at most slightly sticky,

▪ forms a medium ribbon (between 2 and 5 cm) before breaking,

then call it a clay loam and go to Soil Texture, Step 4.

i. Otherwise, if the soil:

▪ is smooth,

▪ is easy to squeeze,

▪ is at most slightly sticky,

▪ forms a short ribbon ( ................
................

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