Forensics Lab 5.3: Examine the Chemical Characteristics of Soil

This article incorporates, in modified form, material from the not-yet-published Illustrated Guide to Forensics Investigations: Uncover Evidence in Your Home, Lab, or Basement.

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Even if two soil specimens have very similar physical characteristics, their chemical characteristics may differ sufficiently to allow them to be discriminated from each other. Professional forensics labs use standard quantitative analysis laboratory techniques to test soil specimens for the presence and concentration of various inorganic and organic compounds. We were about to do the same–in fact, the original title of this laboratory session was Determine Soil Phosphate Concentration by Quantitative Analysis–when we were struck by a cunning plan.

At your local lawn and garden supply store, you’ll find soil testing kits that are used by gardeners to determine the levels of various important plant nutrients present in their soil. In other words, these kits contain everything needed to perform a quantitative analysis of several aspects of soil chemistry. We visited Ace Hardware, where we found the $16 Rapitest Soil Test Kit shown in Figure 5-11, which included everything necessary to perform ten quantitative tests each for pH, nitrogen content, phosphorus content, and potassium content, using visual colorimetry. As it happens, all of those analytes are also useful for our forensic soil analysis. Determining their values for our various soil specimens adds more data to the “fingerprint” we’re establishing for each specimen.

Figure 5-11. The Rapitest 1601 Soil Test Kit

The kit contains four color-coded plastic test chambers, each with 10 color-coded capsules. To prepare the soil specimen, you soak it in five times its volume of water, allow it to settle, and then fill up a test chamber with the clear solution. You break one test capsule into the test chamber, allow it to react for 10 minutes and then compare the color to a transparent color chart right next to the reaction chamber. That’s a lot faster, easier (and cheaper) than using standard laboratory quantitative analysis methods, and the results are accurate enough to be useful. In this lab session, we’ll use the Rapitest Soil Test Kit to analyze four of the chemical characteristics of our soil specimens: pH, nitrogen content, phosphorous content, and potassium content.

Required Equipment and Supplies

• goggles, gloves, and protective clothing
• foam cups (6, but see Substitutions and Modifications)
• graduated cylinder, 100 mL (see Substitutions and Modifications)
• wash bottle
• stirring rod
• Rapitest Soil Test Kit 1601 (see Substitutions and Modifications)
• pH meter or pH test paper (optional; see Substitutions and Modifications)
• watch, clock or other timing device
• distilled or deionized water
• dry soil specimens, known and questioned

All of the specialty lab equipment and chemicals needed for this and other lab sessions are available individually from the Maker Shed or other laboratory supplies vendors. Maker Shed also offers customized laboratory kits at special prices, and a wide selection of microscopes and microscope accessories.

Substitutions and Modifications

• You may substitute any similar size containers, such as beakers or drinking glasses, for the foam cups. The Rapitest 1601 Soil Test Kit requires soaking soil specimens in five times their volume of water, so choose your specimen and container sizes accordingly. Most similar kits have similar requirements.
• You may substitute a graduated beaker, small measuring cup, or similar graduated container for the graduated cylinder. Exact volumes are not critical.
• You may substitute any packaged soil test kit for the Rapitest 1601. Ideally, you want a kit that tests for at least pH, nitrogen, phosphorus, and potassium, although it’s acceptable to substitute other standalone or consolidated test kits. If the test kit you buy does not include pH, or if you prefer to obtain a more accurate pH value, you can use standard pH test paper or a pH meter to obtain pH values.
• If you prefer not to spend $17 on the Rapitest 1601 kit, you can substitute the $6 Rapitest 1609cs kit, although it provides only two tests for each analyte, sufficient for analyzing the questioned specimen and one known specimen.

Procedure

This lab has two parts. In Part I, we’ll determine the concentration of nitrogen, phosphorus, and potassium ions for each of our known and questioned soil specimens. In Part II, we’ll determine pH for each of our soil specimens.

Part I – Determine concentration of nitrogen, phosphorus, and potassium ions

The Rapitest 1601 Soil Testing Kit uses visual colorimetry to determine the concentrations of nitrogen, phosphorus, and potassium in soil specimens. The soil specimen is soaked in water to solubilize the nitrate, phosphate, and potassium ions. That solution is treated with solid reagents, supplied in capsule form, and allowed to react for 10 minutes, after which the color of each solution is compared against a standard color chart to determine the concentration of the ion being tested. If you use a different brand of soil test kit, modify these instructions accordingly as required.

1. If you have not already done so, put on your splash goggles, gloves, and protective clothing.
2. Label six foam cups Q1 and K1 through K5 for your questioned soil and five known soil specimens.
3. Based on the capacity of your cups, decide how large a soil specimen to use. We used 175 mL (6 ounce) foam cups, and decided to use 20 mL of soil per cup.
4. Use the graduated cylinder to measure the decided volume of the questioned soil specimen, and transfer that specimen to cup Q1.
5. Use the graduated cylinder to measure fives times as much distilled water as the volume of soil you used and transfer the water to cup Q1. (In our case, we added 100 mL of water to our 20 mL soil specimen.)
6. Use the stirring rod to mix the soil and water thoroughly for at least one minute, and then set the cup aside to settle.
7. Repeat steps 4 through 6 for each of the five known soil specimens, placing each in the corresponding labeled cup.
8. Allow all six soil specimens to settle thoroughly. Depending on the characteristics of the soil specimens, this may take anything from a few minutes to 24 hours or more.
9. Use the Beral pipette provided with the test kit to fill each of the three color-coded reaction chambers–pink for nitrogen, blue for phosphorus, and orange for potassium–provided with the kit to the fill line with the clear solution from cup Q1.
10. Carefully open one of the pink capsules and add the contents to the nitrogen reaction chamber. Cap the chamber and shake it until the solid reagent is completely dissolved. Note the time.
11. Repeat step 10 using the blue (phosphorus) chamber and capsule and the orange (potassium) chamber and capsule.
12. After allowing the reaction in each chamber to proceed for 10 minutes, compare the color of the solution against the standard color scale on the reaction chamber, as shown in Figure 5-12. Use open shade (daylight, but not direct sunlight) to do the comparison. Interpolate the values as closely as possible. For example, if the color in the potassium chamber appears to be almost-but-not-quite halfway between the values on the comparison chart for 2 and 3, you might record the observed value for potassium as 2.4. Record the values you observe for nitrogen, phosphorus, and potassium concentrations for specimen Q1 in the corresponding column of Table 5-4.
13. Empty each reaction chamber and rinse it thoroughly.
14. Repeat steps 9 through 13 for each of the known soil specimens, K1 through K5.

Figure 5-12. Reading the results of a phosphorus test

The soil specimen test shown in Figure 5-12 showed a phosphorus concentration between P1 (Deficient) and P2 (Adequate). We judged the intensity of the color to be about midway between those reference points, and so recorded the phosphorus test results for this specimen as 1.5.

Table 5-4. Chemical characteristics of soil – observed data

Item	Q1	K1	K2	K3	K4	K5
A. Nitrogen concentration	_______	_______	_______	_______	_______	_______
B. Phosphorus concentration	_______	_______	_______	_______	_______	_______
C. Potassium concentration	_______	_______	_______	_______	_______	_______
D. pH	_______	_______	_______	_______	_______	_______

Part II – Determine pH

The Rapitest 1601 Soil Testing Kit also uses visual colorimetry to determine the pH of soil specimens, but the procedure is slightly different from what we used for testing the nitrogen, phosphorus, and potassium concentrations. If you use a different brand of soil test kit, modify these instructions accordingly as required. Alternatively, if you have a pH meter (or pH test paper) you can use it to get a more accurate value for pH of your soil specimens, using some of the extract solution you prepared for each specimen in Part I.

1. If you have not already done so, put on your splash goggles, gloves, and protective clothing.
2. Remove the cap from the pH (green) reaction chamber, and fill that chamber to the soil fill line with soil from specimen Q1.
3. Carefully open one of the green capsules and add the contents to the pH reaction chamber.
4. Use the Beral pipette to fill the reaction chamber to the water fill line with distilled water.
5. Cap the chamber and shake it to mix the water and solids. Note the time.
6. After one minute, compare the color of the solution in the chamber against the standard color chart on the reaction chamber using daylight. Again, interpolate the value as closely as possible. Record the values you observe for pH for specimen Q1 in the first data column of Line D in Table 5-4.
7. Empty the pH reaction chamber and rinse it thoroughly.
8. Repeat steps 2 through 7 for each of the known soil specimens, K1 through K5.

Disposal

All of the material used in this lab can be disposed of with household waste.

Review Questions

Q1: Based on the chemical characteristics you observed for your soil specimens, how well can you discriminate between the specimens?

Q2: Which of the chemical tests were most and least useful in discriminating between specimens? Why?

Q3: Why is it useful to perform several different chemical tests on each soil specimen?