Materials Included In Kit

Additional Materials Required

Prelab Preparation

Sodium hexametaphosphate solution, 5%: Add 1 g of sodium hexametaphosphate powder to 20 mL of distilled water in a beaker. Stir to dissolve and mix well. Harvest, or have students harvest, approximately 150 g of soil. Allow samples to air dry on newspaper before the lab.

Safety Precautions

TesTab^® tablets contain small amounts of chemicals that may irritate skin. Please observe all normal laboratory safety guidelines. Have students wear chemical splash goggles and chemical-resistant gloves. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The TesTab solutions may be disposed of down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b. Soil samples may be thrown away in the regular trash.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in two 50-minute class periods. The prelaboratory assignment may be completed before coming to lab, and the Post-Lab Questions can be completed the day after the final lab.
• Encourage students to bring in soil samples from various locations for testing. Air dry the samples by spreading the soil on a piece of newspaper for 1–2 days prior to the lab.|Color comparison charts will need to be shared among student groups.
• Soil samples with high clay contents may not settle well even after 24 hours. The solution may remain brown and cloudy. Adding about 5 mL additional solution to clay soils should help improve settling. However, clay and silt layers may not be easy to differentiate. For accurate results, the amount of clay suspended in the solution may be determined based upon density using a hydrometer.
• TesTab tablets may be used for both soil and water analysis. Other prepackaged soil testing kits are often available at nurseries, home and garden stores, and hardware stores. Typically these kits will come with a unique color comparison chart and cannot be used with the charts included in this kit.
• TesTabs are a vendor product of the LaMotte Company. SDSs are available through the manufacturer website, www.lamotte.com. The TesTabs used in this kit are Nitrate, phosphorus and wide range pH.

Teacher Tips

• Refer to the following websites for more information about soil quality. Interactive maps of soil texture, pH, porosity, density, etc. are available.

  
  Soil Information for Environmental Modeling and Ecosystem Management.
  
  United States Department of Agriculture; Natural Resources Conservation Service http://websoilsurvey.nrcs.usda.gov/app/ (accessed July 2018).

Further Extensions

Alignment with AP^® Environmental Science Topics and Scoring Components

Topic: Earth Systems and Resources. Soil and Soil Dynamics (Rock cycle; formation; composition; physical and chemical properties; main soil types; erosion and other soil problems; soil conservation).
Scoring Component: 2-Earth Resources, Soil and Soil Dynamics.

Answers to Prelab Questions

1. Which soil particle seems to have the largest influence in determining the properties of a class of soil based on the soil texture triangle? Explain.

   Clay appears to have the largest effect on the properties of soil—most of the top of the soil texture triangle is dominated by clay. As a general rule, soil samples that are more than 40% clay are simply classified as clay. The main determining factor is the ability of clay to retain water—clay easily becomes waterlogged. Also, clay easily binds nutrients and other chemicals.

2. A soil sample was analyzed using the method described in Part A of this activity. The heights of the sand, silt and clay layers were 6 mm, 8 mm and 6 mm, respectively. Calculate the percentage of each component and identify the soil textural class using the soil triangle.

   Total soil height = (6 + 8 + 6) mm = 20 mm
   Percent sand = (6/20) x 100% = 30%
   Percent silt = (8/20) x 100% = 40%
   Percent clay = 30%
   Soil containing 30% sand, 40% silt and 30% clay is classified as clay loam.

3. Explain why pH, nitrates and phosphates are important aspects of soil fertility.

   The pH of soil is very important since the wet “soil solution” carries the nutrients required for plant growth. pH affects the solubility of minerals, and therefore their availability to the plants growing in the soil. Soil that is too acidic (pH<5.5) will interfere with the utilization of nitrogen, phosphorus, and potassium by plants. pH also effects the viability of bacteria and other microorganisms. These organisms enrich the quality of the soil. Nitrogen is vital for protein synthesis and photosynthesis it also plays a vital role in leaf growth. It is also important for seed and root development.

4. Following is a circle 4 cm in diameter representing a large sand particle. using information from the background section, draw two smaller circles to scale within this circle to represent the relative sizes of silt and clay particles.
   {12624_Answers_Figure_4}

Sample Data

Part A. Physical Properties of Soil

Part B. Chemical Properties of Soil

Answers to Questions

1. Calculate the percentages of sand, silt and clay in the soil sample: Divide the height of each respective soil layer by the combined height of all three layers and multiply by 100.

   Combined height of soil layers = 22 mm
   Sand = (3/22) x 100% = 14%
   Silt = (18/22) x 100% = 82%
   Clay = (1/22) x 100% = 5%

2. Using the soil texture triangle (Figure 1) identify the soil texture class.

   Soil containing 14% sand, 82% silt and 5% clay is classified as silt.

3. Describe the quality of the soil using your results from Part B:
   1. Is the pH of the soil acidic or basic?

      The pH of the soil was 6.2, or slightly acidic. This is within the optimal range for soil quality (6–7). Most plants will thrive in soil with this pH.

   2. Are the nitrate and phosphate levels suitable for plant growth?

      The measured nitrate and phosphate levels, 20 ppm and 4 ppm respectively, are adequate nutrient concentrations for plant growth. The phosphate concentration is a little high.

   3. If you were a farmer planning to plant crops in this soil, would fertilizer be necessary? Why or why not?

      Since there is already excess phosphate present, a light application of a nitrogen-only fertilizer may be useful.

4. Why do excess nitrates and phosphates from fertilizers often end up as runoff in natural bodies of water and groundwater? Why is this problematic?

   Nitrates do not bind to the soil, and therefore end up passing down through the soil or being washed away by rain, eventually ending up in bodies of water. Excess phosphate ions added to the soil precipitate in the form of insoluble calcium phosphate, which binds to soil particles and washes away due to erosion or irrigation runoff, ultimately ending up in groundwater or bodies of water. High levels of nitrates and phosphates in groundwater may leave water unfit for drinking. In addition, excess nitrates and phosphates in bodies of water may lead to algae blooms. Groundwater that flows to larger bodies of water, or other runoff contributes to unnaturally high levels of nutrients which cause algae blooms. Algae blooms may be detrimental to ecosystems.

5. Using the information learned in this activity, explain why few plants grow on the beach or in a sandbox.

   Sandy soils do not retain water, bind vital nutrients required for plant growth or support root growth.

References

This activity was adapted from Chemistry in the Environment, Flinn ChemTopic™ Labs, Volume 22, Cesa, I., Ed.; Flinn Scientific: Batavia, IL, 2006.

Introduction

How do the physical and chemical properties of soil affect soil quality? What does soil quality actually mean? These activities will explore both the physical and chemical properties of soil samples obtained locally.

Concepts

• Soil quality
• Acids
• Bases
• pH
• Nitrates and phosphates

Background

Soil texture describes the relative amounts of sand, silt, and clay in a mass of soil—it is one of the most important indicators of soil quality. The texture of soil determines how coarse or fine the soil is, its porosity and permeability, and the capacity to store nutrients and bind waste products. Soil is classified into three categories based on their grain size: sand, silt, and clay (see Figure 1). Sandy soils have excellent drainage and lots of air spaces, but they do not bind nutrients or support root growth. Sandy soils feel dry and gritty, and nutrients leach out quickly. Clay soils, on the other hand, consist of microscopic particles that clump together and retain water. Soils with high clay content are easily waterlogged and have a tendency to exclude air and become anaerobic, killing off the living organisms that are a necessary part of healthy soil. Clay has a large surface area, however, and is chemically very active, binding and storing both mineral and organic nutrients. The most productive soils have a balance of sand, silt and clay and are called loams or loamy soils. (“Rich” soils also contain high concentrations of organic matter.)

The United States Department of Agriculture (USDA) has identified 12 main textural classes of soil based on the percentages of clay, sand and silt. The textural class is determined using a three-sided graph called the soil texture triangle (see Figure 2). Each side of the triangle represents one of the soil separates on a scale from 0 to 100%. The graph is read by following the clay percent line parallel to the triangle base, the sand line parallel to the right side of the triangle, and the silt line parallel to the left side of the triangle. For example, follow the arrows in Figure 2: The asterisk marks soil containing 30% clay, 50% sand and 20% silt, which is classified as sandy clay loam. The pH of soil indicates whether the soil is acidic or basic. The pH scale is defined from 0 (very acidic) to 14 (highly basic). pH 7 is neutral, pH >7 is basic, and pH <5.6), the plants cannot utilize the nutrients they need, and excessive amounts of aluminum and iron, which are harmful to plants, dissolve into the soil solution.

The elements carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium and sulfur are considered macronutrients because plants need them in large amounts. Of these, C, H and O come from the atmosphere and Ca, Mg and S come from the mineral content in the Earth. the nutrients that are most likely to be missing are N, P and K—these elements are commonly added to soils in the form of fertilizers. Nitrate ions are the most common source of nitrogen for plants. Before the widespread use of nitrogen fertilizers, soil nitrogen was primarily provided by legumes (soybeans, alfalfa and clover). The root structures of legumes contain bacteria that are capable of converting nitrogen from the air into ammonia and nitrate ions. Nitrogen is an essential component of proteins—plants grown in nitrogen-rich soils provide higher yields and are richer in protein and therefore more nutritious. Nitrogen is also needed to produce healthy leaf growth and green leaves. Nitrate levels in soil of 10–25 ppm are considered optimal for agriculture. Phosphorus, which occurs naturally in soil in the form of phosphate minerals, is important for root growth and also aids in the production of flowers and fruit. Adequate levels of phosphorus (2–4 ppm) are especially important for root crops (e.g., beets, potatoes, carrots, radishes). In addition to fertilizers, other sources of nitrates and phosphates in soil include decaying vegetation, human and animal waste products and industrial waste discharge.

Nitrate and phosphate fertilizer runoff is a serious problem in some areas. Nitrates do not bind to the soil, and therefore end up passing down through the soil or being washed away by rain, eventually ending up in ground water or surrounding bodies of water, respectively. Excess phosphate ions added to the soil precipitate in the form of insoluble calcium phosphate, which binds to soil particles and washes away due to erosion or irrigation run off. High levels of nitrates and phosphates in groundwater may leave water unfit for drinking. In addition, excess nitrates and phosphates in bodies of water may lead to algae blooms. algae blooms may be detrimental to ecosystems. Algae blooms can lead to a thick blanket of algae on the surface of bodies of water, blocking out sunlight needed by other photosynthetic life inhabiting the water below the surface. As plants and microorganisms die off from lack of sunlight, bacteria levels increase. As bacteria and algae consume dissolved oxygen, the oxygen levels decrease. Dissolved oxygen is used by other life forms (e.g., fish, turtles, amphibians) and decreased levels may cause populations to deplete.>

Experiment Overview

Part A. Physical Properties of Soil

The physical properties of soil texture will be determined by measuring the heights of the sand, silt and clay layers at the appropriate time intervals after mixing soil with “softened” water. The rate at which the soil particles settle when mixed depends on their size. Large sand particles settle out quickly, within a minute or so. Silt particles generally settle within 30 minutes, while tiny clay particles may take 24 hours to settle. Dividing the height of the respective soil layer by the combined height of all three layers gives the percentage of each component. The “softening agent” is sodium hexametaphosphate, which causes the colloidal clay particles to clump and settle.

Part B. Chemical Properties of Soil

The chemical properties of soil will be evaluated by measuring the pH and testing for the presence of macronutrients. The levels of these soil quality indicators are determined by mixing the soil sample with water and analyzing the resulting solutions.

Materials

Prelab Questions

1. Which soil particle seems to have the largest influence in determining the properties of a class of soil based on the soil texture triangle? Explain.
2. A soil sample was analyzed using the method described in Part A of this activity. The heights of the sand, silt and clay layers were 6 mm, 8 mm and 6 mm, respectively. Calculate the percentage of each component and identify the soil textural class using the soil triangle.
3. Explain why pH, nitrates and phosphates are important aspects of soil fertility.
4. Following is a circle 4 cm in diameter representing a large sand particle. Using information from the Background section, draw two smaller circles to scale within this circle to represent the relative sizes of silt and clay particles.
   {12624_PreLab_Figure_3}

Safety Precautions

TesTab tablets contain small amounts of chemicals that may irritate skin. Please observe all normal laboratory safety guidelines. Wear chemical splash goggles and chemical-resistant gloves. Do not handle soil samples with bare hands. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part A. Physical Properties of Soil

1. Using a plastic spoon, add about 10 cm3 of air-dried soil to a plastic Snap-Seal vial. Gently tap the vial on the table or countertop to eliminate air space and pack the soil down in the tube.
2. Carefully add 40 mL of distilled water to the vial.
3. Using a graduated Beral-type pipet, add 1 mL of sodium hexametaphosphate solution to the vial.
4. Cap the vial and snap securely to prevent leakage. Shake vigorously for two minutes to thoroughly mix contents of the vial.
5. Place the vial on the lab bench and immediately start timing. Set the vial down in a convenient place for making observations and to measure the soil layers but where the vial will not be disturbed for 24 hours. Do not jostle the vial.
6. After exactly one minute, measure the height in mm of the sand layer that has settled to the bottom of the vial. Record the measurement in the Data Table for Part A.
7. After 30 minutes, measure and record the combined height in mm of the sand and silt layers.
8. After 24 hours, measure and record the total height of the clay, sand and silt layers. Record the color and appearance of the water solution on top of the soil. Note: The clay will probably look “congealed” and is usually lighter in color than the other layers.

Part B. Chemical Properties of Soil

1. Obtain TesTab tablets for pH, nitrate and phosphate testing.
2. Mark the 1-mL level in each test tube: measure 1 mL of water in a graduated cylinder and add the water to one of the test tubes. Using a permanent marker, draw a line on the test tube to mark the 1-mL level. Measure and add 9 mL of water to the test tube and draw a second line to mark the 10-mL level. Hold two test tubes side-by-side with the marked test tube and draw lines for the 1-mL and 10-mL levels on each. Discard the water.
3. Using a clean spatula or plastic spoon, add soil to the 1-mL level in each test tube and label the test tubes pH, N and P.
4. Add distilled water to the 10-mL mark in the pH test tube.
5. Add a pH TesTab tablet to the water in the water in the pH test tube.
6. Stopper the test tube and shake vigorously for 30 seconds. Place the test tube in a test tube rack and allow 2–3 minutes for the soil to settle.
7. Compare the color of the liquid in the pH test tube to the colors on the pH Color Comparison Chart. Record the approximate pH value in the Data Table for Part B. Rinse the stopper with water.
8. Using a graduated Beral pipet, add 1 mL of vinegar to both the N and P tubes.
9. Add distilled water to the N and P test tubes until the liquid level in each is at the 10-mL mark. Stopper the test tubes and shake vigorously for one minute. Rinse the stopper with water and repeat for the P test tube.
10. Place the test tubes in a test tube rack and allow 3–5 minutes for the soil to settle.
11. Decant 5 mL of liquid from the N test tube into a clean test tube and add a nitrate TesTab tablet to the clear liquid. Stopper the test tube and shake vigorously for at least one minute or until the tablet dissolves completely.
12. Place the test tube in a test tube rack and let it sit undisturbed for 5 minutes. Compare the color of the liquid to the colors of the Nitrate Color Comparison Chart. Record the nitrate concentration in the Data Table.
13. Repeat steps 11 and 12 with the P test tube, using a phosphate TesTab tablet and the Phosphate Color Comparison Chart. Record the phosphate concentration in the Data Table.

Student Worksheet PDF

12624_Student1.pdf