Materials Included In Kit

Additional Materials Required

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash 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. Universal indicator solution may be flushed down the drain according to Flinn Suggested Disposal Method #26b. The soil samples may be treated according to Flinn Suggested Disposal Method #26a.

Teacher Tips

• Enough materials are provided in this Super Value Kit for 5 classes of 30 students each, working in pairs (75 total student groups).
• Fifteen reusable sample containers, 15 reusable color indicator charts, 75 pipets and 500 mL of universal indicator solution.
• If various areas to take soil samples are available in your area, soil samples may be taken ahead of time or simulated soil samples may be created. Add dilute hydrochloric acid (1.0 M) to soil samples for low pH readings and dilute sodium hydroxide (1.0 M) to other samples for higher pH readings. If the samples are going to be simulated, you may want to give students a brief description of each sampling site for their pH of Soil Worksheet. Be sure to match the pH to the sampling areas description (i.e., an alfalfa field should have a low pH).
• Students may be divided into groups to perform individual tests or they may take multiple tests from various areas.
• Have the students compare their results and fill in the pH of Soil worksheet with the rest of their classmates’ results.
• You may wish to place all of the tested soil samples into one central container for disposal.
• If multiple tests are being performed, be sure to thoroughly rinse the sample tube with water between soil samples.
• Flinn Scientific sells a soil sampling tube, Catalog No. AB1148, that works well for taking soil core samples.

Answers to Questions

1. What are some possible causes of basic soil?
   Soil with basic pH readings are caused by the reactions of water with magnesium, calcium, and sodium carbonates and oxides in the soil. Basic soils can result from excess limestone in the soil, etc.
2. What are some possible causes between acidic soil?
   Acidic soils result from acidic water in the soil, respiration of organisims in the soil, and from crop production.
3. What are the similarities and differences of calcareous and sodic soils?
   Calcareous and sodic soils both have high pH readings. Calcareous soils are caused by high levels of calcium carbonate and sodic soils result from high levels of sodium.
4. Did your soil sample(s) have a low or high pH?
   Answers will vary.
5. Were your classmate’s soil samples acidic or basic?
   Answers will vary.
6. What effect did the location of the soil samples have on the pH of the soil? Look at the descriptions of each area above to help identify trends and formulate an answer.
   Answers will vary.

References

Plaster, E. J. Soil Science and Management; Delmar: Albany, NY, 1992; Chapter Ten.

Introduction

What are the pH levels of the soils in your area? What causes soil to have a low or high pH level? In this activity, soil samples will be obtained and the pH levels of the samples analyzed.

Concepts

• pH
• Soil
• Soil sampling

Background

The pH test is a standard soil test. pH is a measure of the relative abundance of hydrogen ions in a water sample. In pure water, the hydrogen ion concentration [H⁺] is equal to 1.0 x 10^–7 moles per liter. Equation 1 shows how the pH value of a sample is calculated from the hydrogen ion concentration.

As soil becomes more acidic, the pH values decrease from 7 to 6 to 5 and so on. As the soil becomes more basic, the pH values increase from 7 to 8 to 9, etc. See Figure 1 for examples of everyday substances with different pH levels.

Soil pH ranges are typically between 4 and 8. The pH of soil is dependent upon the interactions between minerals, ions in solution and exchange of cations in the soil. Basic pH readings are found in soil due to the reaction of water, magnesium, calcium and sodium. When water and high amounts of calcium, magnesium or sodium carbonates and oxides are present in soil, hydroxide ions are formed. See Equation 2.

The reaction illustrated in Equation 2 is known as a hydrolysis reaction. The hydrolysis of calcium carbonate in soil generally results in soil with a pH of 8 to 8.5. Very basic soils, generally soils with a pH of 8 or higher, are more than 100% base saturated. Soil saturation of bases occurs when all exchange sites in the soil are fully filled with bases. Soils with very basic pH readings are generally composed of mineral carbonates (i.e., calcium carbonate). Soils which are more than 100% base saturated and contain an excess amount of calcium carbonate are known as calcareous soils. Calcareous soils generally result from excess limestone in the soil.

If the amount of sodium saturation in soil exceeds 15%, a large amount of sodium hydroxide or lye is formed. Excess amounts of lye can result in soil pH levels over 10. Soils with high sodium contents are known as being sodic.

Soils with a low pH (acidic soils) on the other hand are caused by the presence of slightly acidic water in soil (acid rain), respiration of organisms in soil, and also by crop production. Acidic water percolates through soil and exchanges the basic ions in soil with hydrogen ions and aluminum ions. This replacement of bases in soil is especially prevalent in humid areas where the amount of rainfall exceeds the natural amount of evaporation. The large amount of precipitation allows for a large amount of leeching of soil to occur.

Soil also becomes acidic during respiration of plant roots and other organisms that are present in soil. When respiration occurs, an excess of carbon dioxide is found in soil. When carbon dioxide (CO₂) reacts with water, carbonic acid (H₂CO₃) is formed. Carbonic acid then decomposes, releasing hydrogen ions into the soil and lowering the soil pH. See Equation 3.

As crop plants grow, potassium ions (and other ions) are taken out of soil and are “exchanged” with an equal amount of hydrogen ions. When the crops are harvested, calcium and magnesium ions contained in the crops are depleted from the soil. For example, it has been estimated that for every ton of alfalfa that is harvested 30 pounds of calcium and 8 pounds of magnesium are taken away from the soil.

The pH of a soil affects both the soil, plants and organisms in the soil. Soil that is unusually acidic or basic may be toxic to the roots, but these conditions do not normally directly affect plants nearly as much as they affect nutrient availability. For example, in basic soils, minerals, such as copper, iron and manganese, become less available to plants while acidic soils may inhibit the growth of nitrogen-fixing bacteria.

It is common agricultural practice to counteract soil acidity by adding compounds of calcium or magnesium in a process known as liming. Alkaline soils can be made more acidic by the addition of sulfur which is converted by bacteria to sulfuric acid. The addition of some nitrogen-based fertilizers may have the same effect.

Materials

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

1. Obtain a pH of Soil Worksheet and an area number from your instructor.
2. Locate an area for your soil sample. Record the description of the area chosen in the pH of Soil Worksheet.
3. Obtain a soil sample from your test area. Do this by clearing the surface litter from the area. Take a sample of the soil at least six inches deep in the ground using a soil sampling tube or other similar device. Make sure that the soil in the area you are testing is relatively dry.
4. Place approximately 5- to 6-mL of the soil into the sample container.
5. Repeat Steps 3 and 4 to obtain a total of three 5- to 6-mL samples from random spots from the area you are testing. Three random samples will allow a better “sample” of the test area.
6. Place all three of the soil samples into the sample container. The total amount of soil in the tube should be approximately 15–18 mL.
7. Keep the cap off of the sample container and allow the soil in the tube to air dry.
8. Once the soil has dried, pour distilled water into the sample container until the soil is slightly oversaturated (water is at the top of the soil).
9. Place the cap on the sample container and swirl the water and soil mixture.
10. Allow the soil to settle for approximately five minutes.
11. Once the soil has settled, use a pipet to obtain five drops of the soil water suspension. Place this into the cap of the sample container.
12. Now place five drops of universal indicator into the cap.
13. Use the universal indicator color chart to find the corresponding pH of your soil. Record the pH of your sample in the pH of Soil Worksheet.
14. Repeat steps 1–13 for as many areas as are determined by your instructor.
15. Share your results with the rest of the class. Record the results from the other groups in the pH of Soil Worksheet.
16. See your instructor for appropriate treatment of the soil water mixture in between each test.

Student Worksheet PDF

11986_Student1.pdf