Physical and Chemical Properties of Soil(using Rapitest Comparators)

Background: Soil is classified into three categories based on its grain size: sand, silt, and clay. Soil texture describes the relative amounts of these three particles 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. Sandy soils have excellent drainage and lots of air spaces, but they do not bind nutrients or support root growth. Silty 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.

Soil pH influences the solubility and availability of soil nutrients, the viability of essential microorganisms,and the movement of toxic heavy metals into groundwater. A pH range between 6 and 7 is ideal for most plants. When soil is too acidic (<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 C, H, O, N, P, K, Ca, and S 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. 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 fruits. Adequate levels of phosphorus (2-4 ppm) are especially important for root crops.

However, too much nitrogen or phosphorus can cause serious problems due to the runoff of these minerals into water sources. Excess nitrates and phosphates in the ground water may leave it unfit for drinking. It may also lead to algae blooms in lakes, which are detrimental to ecosystems. Algae blooms can lead to a thick blanket of algae on the surface of water, which blocks out sunlight, harming the photosynthetic organisms in the water below. Too much bacteria and algae consume the dissolved oxygen in the water, decreasing the amount available to other life forms that need it.

Part A: Physical Properties of Soil

Procedure:

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 to eliminate air space and pack the soil down in the tube.

2. Carefully add 30 mL of distilled water to the vial.

3. Using a graduated pipet, add 1 mL of sodium hexametaphosphate solution to the vial.

4. If there is room in the container, carefully add another 10 ml of distilled water.

5. Cap the vial and snap securely to prevent leakage. Shake vigorously for two minutes.

6. Place the vial in a secure location and immediately start timing. Do NOT jostle the vial.

7. 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.

8. After 30 minutes, measure and record the combined height in mm of the sand and silt layers.

9. 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: pH of Soil

Procedure:

  1. Remove the cap from the green comparator. Make sure the color chart is in place.
  2. Fill only the test chamber to soil fill line with soil sample.
  3. Holding the green capsule horizontally over the test chamber, carefully separate the two halves and pour powder into the test chamber.
  4. Using the pipette labeled distilled water (), add distilled water to water fill line.
  5. Fit the cap onto comparator, making sure it is seated properly and caps tightly. Shake thoroughly for 30 seconds.
  6. Allow soil to settle and color to develop for about a minute.
  7. Compare color of solution against pH chart. For best results allow daylight (not direct sunlight) to illuminate the solution. Judge colors, if necessary, with other groups, and note your results.
  8. Rinse out the comparator well with tap water.

Part C: Nitrates in Soil

Procedure:

  1. Fill a clean container with 0.25 cup of soil and 1.25 cups of distilled water.
  2. Thoroughly shake or stir the soil and water together for at least one minute, then allow the mixture to stand undisturbed until it settles (30 minutes to 24 hours, dependent on soil). The clarity of the solution will vary, although the clearer, the better. However, cloudiness will not affect the accuracy of the test.
  3. Remove the cap from the purple nitrogen comparator. Make sure the color chart is in place.
  4. Using the pipette labeled “Decant,” fill the test and reference chambers to the fill mark on the chart with solution from your soil sample. Avoid disturbing the sediment; transfer ONLY the liquid.
  5. Holding the purple capsule horizontally over the test chamber, carefully separate the two halves and pour the powder into the test chamber.
  6. Fit the cap on the comparator, making sure it is seated properly and caps tightly. Shake thoroughly for one minute.
  7. Allow color to develop for 10 minutes. Do not wait longer than 10 minutes.
  8. Compare the color of the solution in the test chamber to the color chart. For best results, allow daylight (not direct sunlight) to illuminate the solution in both the test and reference chambers. Judge colors, if necessary, with other groups, and note your results.
  9. Rinse out the comparator well with tap water.

Part D: Phosphates in Soil

Procedure:

  1. Remove the cap from the blue phosphorus comparator. Make sure the color chart is in place.
  2. You will use the same solution from the container with 0.25 cups of soil and 1.25 cups of distilled water that has been undisturbed for 30 minutes or 24 hours and the same pipette labeled “Decant” to fill the blue comparator to the fill mark (both the test and reference chambers).
  3. Holding the blue capsule horizontally over the test chamber, carefully separate the two halves and pour the powder into the test chamber.
  4. Fit the cap on the comparator, making sure it is seated properly and caps tightly. Shake thoroughly for one minute.
  5. Allow color to develop for 10 minutes. Do not wait longer than 10 minutes.
  6. If flakes of blue color appear to have settled to the bottom of the comparator during the 10 minute development period, shake the comparator to suspend them in the solution.
  7. Compare the color of the solution in the test chamber to the color chart. For best results, allow daylight (not direct sunlight) to illuminate the solution in both the test and reference chambers. Judge colors, if necessary, with other groups, and note your results.
  8. Rinse out the comparator well with tap water.

Name ______

Date ______Period ____

Physical and Chemical Properties of Soil

Data Tables and Analysis Questions

Data Table Part A

Sample / Soil Particle Layer / Sand (1 min) / Sand + Silt (30 min) / Sand + Silt + Clay (24 hrs)
1 / Height (in mm)
2 / Height (in mm)
3 / Height (in mm)
4 / Height (in mm)
5 / Height (in mm)
6 / Height (in mm)

Data Table Part B, C, and D

Sample / Test / pH / Nitrate / Phosphate
1 / Level or concentration
2 / Level or concentration
3 / Level or concentration
4 / Level or concentration
5 / Level or concentration
6 / Level or concentration

Analysis Questions:

  1. Calculate the percentages of sand, silt, and clay in the soil samples. (Height of each layer) x 100

Total height

Sample / Sand % / Silt % / Clay % / Soil Texture Class
1
2
3
4
5
6

2. Using the soil texture triangle on the bottom of the next page, identify the soil texture class for each soil sample in the table above.

3. Using the pH data, identify which soil samples are best suited for plants. ______

4. Using the nitrate data, identify which soil samples are best suited for plants. ______

5. Using the phosphate data, identify which soil samples are best suited for plants. ______

6. If you were a farmer planting crops, which soil sample would you choose? You must pick only ONE! Explain why you picked the ONE you did.

______

7. Why are excess nitrates and phosphates in natural bodies of water problematic?

______

8. Explain why few plants grow on beaches or in a sandbox.

______