Transfer factor: how much of the soil load reaches the harvest
In short
The Transfer factor describes what share of a substance present in the soil arrives in a given plant part. It depends on the substance, the species and the tissue, and it is the quantity people ask about when the marketability of a harvest is at stake.
Why low means two different things
For radiocaesium, a Belgian study based on greenhouse and lysimeter trials arrived at a value into the hemp stem of roughly 0.6 × 10⁻³ m²/kg. Two conclusions follow at once, and both hang on that same number. First: clean fibre and oil products can be obtained on contaminated land, because very little passes into the usable parts. Second: for exactly that reason, growing removes nothing worth mentioning from the soil. The authors consequently treat fibre crops as an alternative land use rather than a decontamination method.
Limits remain. Because hemp fibres are separated mechanically rather than by retting, the same study restricts their use to areas below 740 kBq/m², and it advises caution with seed products because of higher values there.
Where the data are missing, the rule is missing
Since 1 August 2023 the German federal soil protection ordinance has listed trigger values for seven individual PFAS, but only for the soil-to-groundwater pathway. For the soil-to-crop pathway there are none, and the stated reason is precisely this figure: the data behind transfer factors are considered insufficient. The responsible authority in Baden-Württemberg regards precise predictions of uptake as barely possible at present, even where soil concentrations are known.
Link to cannabis
For growing on contaminated land this is the decisive figure, not the total soil load. It separates a harvest that may be sold from one that may not, and it explains why the same field can serve for fibre and not for seed. What arrives in which part of the hemp plant is covered in the article on contaminant uptake in hemp.