Understanding Soil Pollution: Testing Methods And Techniques

Soil pollution is a serious environmental concern, as contaminated soil can cause a broad spectrum of negative consequences for plants, animals, humans, and the ecosystem as a whole. Testing soil pollution can be complicated, and there is no way to detect all possible pollutants with a home test kit. The most common pollutants include petroleum products, heavy metals, pesticides, salts, fertilizers, and plant nutrients in high concentrations. Only nutrients, salts, and pH can be tested at home with a soil test kit, but results are often unreliable.

To test for other pollutants, one must contact the local board of health, state department of environmental protection, or a private soil testing company. Testing procedures vary depending on the type of contaminant, but generally involve collecting a small soil sample from 2 to 6 inches below the surface and placing it into a container. The sample is then sent to a lab for analysis, which can include methods such as hierarchical cluster analysis and principal components analysis.

Testing for pH levels

The pH level of the soil is a crucial factor in determining whether your plants will thrive or struggle. The pH level of the soil can be tested in several ways, which we will discuss in this section.

Using a Soil Testing Kit

Soil testing kits are readily available at home centres, nurseries, or online retailers. These kits are generally reliable and provide an easy way to test the pH level of your soil. The process typically involves the following steps:

• Dig a small hole, approximately 2 to 4 inches deep.
• Remove any twigs, stones, or debris from the hole and fill it with distilled water.
• Insert the test probe into the muddy pool created by the water.
• Wait for about a minute to get a reading on the probe.

The pH reading will indicate whether your soil is acidic, alkaline, or neutral. A pH level lower than 7 indicates acidic soil, while a pH level higher than 7 indicates alkaline soil. A pH level of exactly 7 means your soil is neutral, which is ideal for most plants.

Using Red Cabbage

An alternative, natural method to test soil pH involves using red cabbage. Here are the steps to follow:

• Chop a red cabbage into small pieces and boil it in a pot of distilled water for about 10 minutes. The water should turn violet.
• Remove the pot from the stove and strain out the cabbage.
• Pour the violet water into a clear container.
• Add a spoonful of soil to the water and observe the colour change.

If the water turns pink, your soil is acidic, while a blue-green colour indicates alkaline soil. The intensity of the colour change also signifies the level of acidity or alkalinity.

Using Baking Soda and Vinegar

For a quick and simple DIY test, you can use basic kitchen ingredients like baking soda and vinegar. Here's how:

• Dig a small sample of soil, about 4 to 6 inches below the surface.
• Clean the soil by removing any sticks, stones, or debris, and break up any large clumps.
• In a clean glass container, mix the soil with distilled water to create a muddy mixture.
• Add half a cup of vinegar to the soil-water mixture. If the mixture fizzes, foams, or bubbles, your soil is alkaline.
• If there is no reaction, repeat the process with a new soil sample and add half a cup of baking soda. If it fizzes, your soil is acidic.

While this method provides a general indication of acidity or alkalinity, a soil pH testing kit will give you more precise results.

Using pH Test Strips

Another method is to use pH test strips, which can be purchased online or at garden centres. Here are the steps:

• Dig a small sample of soil, about 4 to 6 inches deep, and place it in a clean glass.
• Add distilled water to the glass at the same level as the soil sample.
• Agitate the mixture by stirring or swirling vigorously. Let the solution rest for about 30 minutes.
• Drain the sample by pouring it through a coffee filter into another clean glass, separating the liquid from the solids.
• Dip the pH test strip into the liquid and follow the instructions on the package for the amount of time it should remain in the liquid.
• Compare the colour of the strip to the chart provided by the manufacturer to determine the pH level.

By testing the pH level of your soil, you can ensure that your plants have the optimal environment to grow and thrive. Different plants have specific pH requirements, so it's important to research the needs of your plants and adjust the soil accordingly.

Testing for electrical conductivity

Electrical conductivity (EC) measures the soil's capacity to conduct electrical current, which is influenced by the concentration of dissolved salts and ions (nutrients) in the soil. A high EC value can hinder the growth of crops and the activity of soil microorganisms, while a low EC value indicates that there are insufficient effective nutrients in the plant, resulting in poor growth.

Handheld EC Meter

A handheld EC meter is a simple tool that measures the EC value of a soil extract. It is equipped with a probe that is inserted into the soil, and the reading is displayed on a digital screen. This method is popular due to its high accuracy, ease of use, and ability to provide real-time and remote monitoring.

Laboratory Analysis

Laboratory analysis provides accurate results but can be time-consuming and expensive. Soil samples are sent to a laboratory, where the EC value is measured using specialised conductivity meters.

Soluble Salt Test Strips

Soluble salt test strips are paper strips that change colour based on the EC value of the soil extract. The colour of the strip is then compared to a colour chart to determine the EC value. While this method is quick and easy, it may not be as accurate or reliable as a meter.

Electrical Resistivity Imaging (ERI)

ERI is a geophysical technique that uses electrodes to measure the electrical conductivity of soil. It involves injecting an electrical current into the ground and measuring voltage differences at other electrode locations. This non-invasive method provides valuable insights into soil properties and heterogeneity.

Soil Moisture Sensor

Soil moisture sensors are commonly used in smart agriculture to help farmers monitor soil EC in real time and ensure optimal plant growth. They can measure soil EC, temperature, and humidity simultaneously.

Direct Soil EC Testing

Direct soil EC testing allows you to skip the soil sampling and preparation steps. You can use a meter or probe specifically designed for this purpose, such as the Hanna HI98331 Soil Conductivity Tester, to directly measure the conductivity of soil in the field. However, this method may not be suitable if the ground is stony or hardened, as it can damage the electrode. It is important to calibrate the probe before measuring and to take multiple measurements at various locations to get a representative overview of the entire field.

Testing for nitrogen, phosphorus and potassium

Nitrogen, phosphorus, and potassium are primary macronutrients that are essential for most plants' survival. Here is a detailed guide on how to test for these nutrients in the soil:

Testing for Nitrogen

To test for nitrogen, start by removing the green cap on the filtering device and taking out the plunger. The plunger has one end with several small holes. Place one of the filter papers into this end of the plunger and use the scoop to press the paper down tightly. Next, fill the barrel of the filtering device with soil up to the 1 mL mark.

Then, get the bottle labelled N1 Nitrate Extractant Solution and fill the barrel with this solution up to the 2.5 mL mark. Place the plunger back into the filtering device and shake gently for 30 seconds. After that, screw the cap back on slowly, pressing the plunger down into the solution. Continue screwing until there is enough solution in the plunger. Be careful not to tilt the device, as the cap has a hole and the solution can spill.

Once done, pour the solution from the plunger into a clean test tube. If done correctly, you should have a test tube with a clear solution. Fill the test tube to the 1 mL mark and add one scoop of N2 Nitrate Reactant Powder. Place the cap on and shake for 10 seconds, then let it sit for 5 minutes. Finally, compare the colour of the solution to the chart in your test kit.

Testing for Phosphorus

Before testing for phosphorus, clean the filtering device and test tubes with soap and water to prevent contamination from the previous test. Place a new filtering disc in the device. For phosphorus, fill the barrel with 0.5 mL of soil and then add the P1 Phosphorus Extractant Solution until it reaches the 2 mL mark. Shake for 30 seconds and then filter out 1 mL of the solution using the same method as the nitrogen test.

After obtaining 1 mL of solution in the test tube, add half a scoop of P2 Phosphorus Reactant Powder. Place the cap on and shake gently for 5 minutes. Immediately remove the cap and compare the colour of the solution to the chart. As with the previous test, clean the filtering device and test tubes before moving on to the next test.

Testing for Potassium

For the potassium test, fill the barrel of the filtering device with 0.5 mL of soil and then add the K1 Potassium Extractant Solution up to the 2 mL mark. Shake for 20 seconds and then filter out 1 mL of the solution, following the same procedure as the nitrogen and phosphorus tests.

Once you have 1 mL of solution in the test tube, add 0.5 mL of the K2 Potassium Reactant Solution. Do not shake the solution. Instead, let it stand for 5 minutes. The solution will become cloudy, and the degree of cloudiness will indicate the amount of potassium present. Place the test tube in the circle next to the potassium reading chart, starting at the surplus reading, and move it down until one of the boxes is just visible. This will be the reading for your sample.

Testing for heavy metals

Heavy metals in the soil can be toxic to humans and plants if ingested in high enough quantities. These metals include lead, copper, arsenic, and cadmium, and they can be present in the soil due to industrial and agricultural pollutants. If you suspect your soil might be contaminated, it is important to get it tested, especially if you plan to use the area for a vegetable garden, farm, or children's play area. Here are some options for testing heavy metals in your soil:

Local Extension Services

Your local extension services may offer soil testing for heavy metals. For example, the UNH Cooperative Extension Soil Testing Program offers an "Environmental Package" that includes testing for cadmium, chromium, copper, nickel, lead, and zinc. They also offer individual tests for arsenic, mercury, molybdenum, and selenium. However, there may be a long wait for these services due to the pandemic.

Private Companies

There are also private companies that offer soil testing for heavy metals. For example, RxSoil provides testing kits for lead and the full RCRA 8 list of metals, which includes silver. The test kit is sent to your home, and you mail your soil sample to their lab. Results are typically available within two weeks. Another company, TPSLab, offers customizable heavy metals testing packages for soil or solids using their Inductively Coupled Argon Plasma Instrument (ICP). They can test for various heavy metals, including lead, mercury, cadmium, and arsenic, with results available in 7-14 business days.

Local Companies

You can also engage local companies that specialize in soil testing and remediation. These companies typically come to your property, take soil samples, and send them to a lab for testing. If the samples come back positive, they can advise on remediation measures, such as replacing the top few inches of soil.

University Services

If you live near a university with agricultural or environmental sciences programs, they may offer soil testing services. For example, The Ohio State University offers soil heavy metal screening for arsenic, cadmium, and lead, which is designed for gardens and urban farms.

When interpreting your test results, it is important to consider the natural variability of soils and heavy metals, as well as the intended use of the area. Different regulatory agencies, such as the U.S. Environmental Protection Agency, have established guidelines and screening levels to help assess the safety of your soil. If your soil is contaminated, there are measures you can take to minimize the risk of exposure, such as adjusting the pH of the soil, adding organic matter, or importing new soil for raised garden beds.

Testing for other pollutants

If you are concerned about other pollutants in your soil, you should contact your local board of health, state department of environmental protection, or search for an appropriate lab to test your soil. You can also contact your county's Cooperative Agriculture Extension Service, the nearest state university, or a private soil-testing company.

Before testing, it is important to identify which contaminants are likely to be in your soil, as it would be very expensive to test for all contaminants. First, learn about the history of the site and the neighboring area. Consider what kinds of industry or land use activities may be contributing to soil pollution. You can do this by talking to your neighbors, visiting your town or city hall to look through land use records, and searching records via the Bureau of Land Management's online record search. Once you have identified possible sources of pollutants, you can identify which contaminants are likely to be present.

Some contaminants are so common and known to be toxic that they are good to test for in most scenarios, such as heavy metals and polycyclic aromatic hydrocarbons (PAH's). Others are associated with proximity to certain industries and past and present land uses. For example, if your home was built before 1978, it is likely that lead paint was used on the outside, which may have chipped or peeled off into the soil. If your home was built before the late 1970s, your land also has a higher risk for arsenic contamination. If your property is near a road or highway with frequent traffic, your soil likely contains lead and PAH's. If your property is near a landfill or dump, many different contaminants may be present, including solvents, pesticides, petroleum products, heavy metals, and PCB's.

After identifying the likely contaminants, you can make a soil testing plan. Consider how you will use the information. If you plan to use the information in a legal case or for a scientific or professional project, you should use an accredited/professional lab and follow EPA guidelines for sampling procedure, chain of custody, and analysis. If you are using the information for personal use, you may opt for cheaper options, such as taking a composite sample or using a community soil lab or DIY soil-testing methods.

When creating your soil testing plan, you should also consider your budget, your sampling strategy, and which lab you will use. Soil testing can be expensive if done through a professional lab. For example, to test one soil sample for PAH's, key heavy metals, and PCB's can cost between $200-300 USD, and one soil test for dioxins is $450. To keep prices down, you can take composite samples, see if your local community or university soil lab can perform the tests, or explore DIY soil testing methods.

When taking soil samples, use a spade or trowel to take small samples from three to ten random spots in your garden. Mix the soil in a container, removing any pebbles, leaves, or roots. Then, transfer at least one cup of the soil mixture into a plastic bag and seal it. Make sure the sample is dry; do not dry the soil on a stove or radiator, as this may affect the readings. Mail the bag to your preferred testing site. Many colleges and universities offer testing services at a low cost (most tests cost between $10-20).

Frequently asked questions