Some processors are asking growers to test their fields for heavy metals. There are no calibrations that can tell you what the level of a specific heavy metal will be in the plant based on soil test levels. Heavy metals are naturally occurring in soils. In other words nobody can tell you how much arsenic or cadmium in a soil is too much at this point. Below is a chart that shows the natural range of heavy metals found in soils in North Carolina taken from Minimizing Risks of Soil Contaminants in Urban Gardens. This publication also gives you an idea of what prior activities could lead to high levels of heavy metals in soils.
Reported ranges in natural soils follow (remediation goal for contaminated site cleanup in parentheses)
As 0.1-97 ppm (4.4)
Cd 0.01-2 (14)
Cr 1-2000 (24,000)
Pb <10-700 (0.0012-400 depends on specific chemical form)
Hg <0.01-4.6 (0.98-4.7 depends on specific chemical form)
Se <0.1-4.3 (7.8)
Why are processors asking for heavy metal soil testing since we have no way to predict what a given soil heavy metal level will equate to in the plant? I cannot speak for all processors and I suspect there are several reasons:
1. Other processors are asking for it.
2. They are trying to make sure growers avoid contaminated field with levels higher than those found naturally.
3. The processors want to start building baselines to help predict what levels might cause problems in extracted products for developing future recommendations.
I had a conversation with one processor I respect. They admitted that while they do not know what levels are critical, they might be able to suggest to growers to test alternate fields if heavy metals are higher than average or normal. You can see in the chart above that there is quite a range for “normal” heavy metal soil levels.