Take a soil sample that represents the bed

A laboratory can analyze the material you send. It cannot know whether that handful came from a representative part of the garden or a small pile of spilled fertilizer.

Follow the laboratory's method

Read its instructions before collecting anything. They specify the sampling depth, number of subsamples, tools, container, and amount needed. Take samples from several appropriate points within one reasonably uniform area and combine them as directed.

Keep very different areas separate: a vegetable bed, lawn, and recently amended border may need their own samples. Avoid unusual spots unless the purpose is specifically to investigate that problem area.

Label the question as well as the sample

Provide the requested history and intended planting. State if you are investigating salts or another issue that needs an additional test; a standard fertility analysis does not automatically test for every contaminant or disease.

Use clean tools and prevent contamination from fertilizer, containers, or other soil. Record where and when the sample was collected. When the report returns, keep it with any amendments you apply. Careful sampling turns the result into a useful baseline; careless sampling can produce a precise-looking answer to a question that does not represent the bed you intended to improve.