Methods for analyzing soil as evidence

General Description

1. Write a description of the soil sample. Note the color, texture, and general appearance. Record all observations on the data sheet.

2. Then, use a magnifying glass, and note the presence of any vegetation or any unusual materials. Record your observations on the data sheet.

Ultraviolet Light

1. Observe the soil sample with an ultraviolet light. It is best to have the soil in a dark area when doing this.

pH

1. Mix 2 grams of the soil sample in 50 mL of distilled water.

2. Stir the mixture for 1 minute then use a pH probe to read the pH of the solution.

Soil Density Profile

1. Before making the soil profile, dry the soil, then put through a 30-45 mesh seive.

2. Make a density gradient tube by carefully putting 1-2 mL of each of the following liquids in a small test tube: rubbing alcohol, corn oil, water, glycerin, and corn syrup. Allow to settle.

3. Drop a small amount of well-mixed soil into the tube and allowed to sit for 12 to 24 hours. Then sketch the sample profile and estimate the percentage of the total sample at each density.

Reaction with Acid

1. In a well-plate place a small sample of soil (about the size of an aspirin). Add 10 drops of HCl to the sample. Carefully look for the presence of any gas bubbles and record your observations in the data sheet.

2. If gas bubbles are produced it indicates presence of a carbonate or the presence of the metals, zinc, iron, or magnesium. In soil the most likely would be carbonate but at a crime scene you might find the metal fragments mixed in with the soil.

Settling Rate

1. Use a spectrophotometer to determine the settling rate of the soil particles. Make sure the Spec20 has been warmed up and calibrated.

2. Obtain 0.5 g of a well-mixed soil sample. Fill a clean cuvette about two-thirds full of water. Add the soil to the water and shake vigorously for about 1 minute.

3. Immediately insert the tube into the sample compartment, close the cover, and record the percent transmittance from the scale.

4. Continue to take readings every 30 seconds for the first few minutes, then at one-minute intervals until the transmittance reaches a stable value or until 10 minutes of recording has been completed.

5. Use a piece of graph paper to plot time versus percent transmittance. This graph can then be compared to graphs from other soil samples.

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