NYC requires that all school gardens be grown in raised beds. Please follow Healthy Soils, Healthy Communities' Best Practices for Healthy Gardening. We recommend that schools test the soil for heavy metal and nutrient levels at least every two years. Certain chemical elements occur naturally in soils as components of minerals, yet may be toxic at some concentrations. Other potentially harmful substances may end up in soils through human activities. This could happen if former industrial or agricultural lands are later used for residential properties, and contaminants remain in the soil. Spills, runoff, or aerial deposition of chemicals used for agriculture or industry can also cause soil contamination in residential areas. At times, the amounts of some soil elements and other substances may exceed levels recommended for the health of humans, animals, or plants.
How to Collect Soil
People are generally exposed to soil contaminants through skin contact, breathing in dust, accidentally eating small amounts of soil, or eating fruits and vegetables with contaminants on or in them. Depending on the layout of your garden and how your students will use it, pick the most logical soil collection method.
To measure contaminant levels in specific areas, such as children’s play areas, collect separate samples of the top one to two inches of soil. This could help identify a contamination source, such as a chemical spill.
To measure the average levels of contaminants in surface soil, collect several composite (combined) samples of the top one to two inches of soil from across the property. To find out if the concentrations are different in different areas, collect a separate sample from each area.
To measure contaminant levels in garden soil, collect deeper samples (the top six inches) from several locations and mix them together as a composite sample. To find out if some parts of the garden have higher concentrations than others, collect separate samples from the areas of interest.
- Collect the top six inches of soil from garden areas, or the top one to two inches from other areas.
- Take a composite sample by combining a number of samples from different locations usually five to ten spots) and mixing them together.
- Collect at least three separate composite samples for each area of interest because the levels of a particular contaminant can vary throughout a site.
- Consider dividing larger areas (larger than 100 feet by 100 feet) into smaller parts for planning purposes.
- Record where samples are collected and how they were collected (including the depth of soil collected), and label the samples accordingly. This information will be useful for interpreting the test results from the laboratory.
- For individual samples, use a different trowel, scoop or spoon for each sample or wash with soap and water between samples. It is fine to use the same sampling instrument to take the five to ten samples to mix for a composite sample.
- For individual samples, put each sample into a different container (double plastic bags, or containers provided or recommended by the laboratory). For composite samples, mix the individual samples in a clean container (such as a clean plastic bag placed inside a bucket) and then transfer the mixed sample to the container that will be sent to the laboratory. Follow the instructions provided by the laboratory regarding how to package and label the soil samples.
(From the Cornell Waste Management Institute)
Understanding Your Test Results:
For recommendations for garden soils and guidance values for metals, please read Understanding Your Test Results: Metals in Garden Soils and Vegetables by Healthy Soils, Healthy Communities. Please follow these Best Practices for Healthy Gardening to reduce contact with metals and other chemicals in soil.
Soil Testing Fact Sheets
Healthy Soils, Healthy Communities
Soil Testing for Nutrients and Other Properties
Cornell University Cornell Nutrient Analysis Laboratory (CNAL)
Cornell Waste Management Institute >
Environmental Sciences Analytical Center >
University of Massachusetts
Soil and Plant Tissue Testing Laboratory
Environmental Testing of Soils for Lead and Other Toxic Metals >
See laboratories that are certified under NYS’s Environmental Laboratory Approval Program
(Select a laboratory certified for “solid and hazardous waste”.)