Soil Sample Testing: What to Collect and Why It Matters

A soil test result is only as good as the sample it came from. Contamination during collection, incorrect depth, or inadequate mixing of a composite sample can all produce a result that does not reflect the actual site condition. This article covers the basics of soil sample collection for analytical testing in the UK.

Sampling depth and location ¶

The depth you sample at depends on what you are looking for. For surface contamination from atmospheric deposition or recent spills, the top 10cm is usually the relevant zone. For legacy contamination from industrial use, deeper sampling at 30-50cm or below may be needed. If you are unsure, take samples at two depths and label them clearly.

Location matters as much as depth. A single sample from one point on a site is rarely representative. For a site assessment, collect samples from a grid pattern or from areas of suspected contamination and combine them into a composite, or keep them separate if you need spatial data.

Containers and preservation ¶

Use clean glass or HDPE containers. Do not use containers that previously held other chemicals. For metals analysis, a clean plastic container is fine. For volatile organic compounds (VOCs), use glass vials with minimal headspace and seal immediately after filling.

Refrigerate samples at 4 C if they cannot be sent to the lab within 24 hours. For VOC analysis, samples should reach the lab within 48 hours of collection. Note the collection date and time on the container.

What to tell the lab ¶

Include: the site location (district level is sufficient, you do not need to give a precise address), the land use history if known, the depth and location of each sample, and what contaminants you are concerned about. If you have a previous site investigation report, a summary of its findings is useful context.

If you do not know what contaminants to look for, say so. A broad screening panel (metals plus VOCs plus selected pesticides) is a reasonable starting point for a site with unknown history.

Good sample collection takes an extra 20 minutes on site. It is the part of the process that most affects whether the result is actually useful.