Cadmium. Nickel. Lead. Mercury.

These are probably not the substances you think of when discussing the quality of your next harvest.

We talk about THC. About terpenes. About calcium and magnesium. About VPD, light output, microorganisms and yield.

But surprisingly few growers ask another question:

What else, apart from nutrients, is actually present in our soil and fertilizers?

This question is more important than it may initially sound.

Because cannabis can take up various metals from its root zone. Most of them remain in the roots. However, some can be transported further.

All the way into the flowers.

A study published in 2026 involving four medical cannabis cultivars shows exactly this. And it makes clear why raw material quality in cannabis does not begin with NPK alone.

Cannabis does not only take up what we want it to

A plant does not distinguish according to the label we put on the fertilizer.

It possesses transport mechanisms for ions and minerals. It needs these for calcium, magnesium, potassium, iron and numerous other elements.

However, these pathways can also allow substances into the plant that we do not want there.

The current study examined four drug-type cannabis cultivars. The plants were exposed to a mixture of four metals:

• Cadmium
• Lead
• Nickel
• Cobalt

The researchers then examined where these elements accumulated within the plants.

As expected, they found the highest concentrations in the roots.

At first, this sounds reassuring.

But only to a certain extent.

All of the metals examined could also be detected in aboveground plant parts. The translocation depended heavily on the element in question. Nickel was transported particularly efficiently from the roots to the aboveground plant parts. This translocation was lowest for lead.

The root therefore functions as a kind of protective barrier.

But it is not an impenetrable barrier.

Cadmium and nickel reached the flowers

Now things get interesting for growers.

At the higher exposure level examined, the scientists found cadmium concentrations of 6.67 to 11.5 µg per gram in the flowers.

For nickel, the values were between 4.39 and 6.71 µg per gram.

For comparison, the authors used WHO guideline values for medicinal plants. These specify 0.3 µg/g for cadmium and 1.5 µg/g for nickel.

Cadmium and nickel were therefore above these values used by the authors for all four cultivars examined.

Lead, on the other hand, remained well below them.

This is not proof that commercially available cannabis is generally contaminated with heavy metals.

For the experiment, the plants were deliberately supplied with defined metal concentrations.

However, the study demonstrates the crucial mechanism:

If certain metals are available in the root zone, they can reach the cannabis flower.

And that is precisely why we need to discuss where these metals can come from in the first place.

The dangerous part is that the plant does not have to collapse

Perhaps one would expect a contaminated plant to immediately look obviously diseased.

It was not quite that simple in the experiment.

At the higher metal concentration, the plants did develop visible chlorosis. At the lower concentration, the visual changes were considerably less pronounced.

Even more interesting:

Plant height, number of internodes and stem diameter did not differ significantly.

Flower biomass was also not significantly reduced by the treatment.

This leads to an important insight:

A good harvest does not prove that a harvest is clean.

A plant can grow.

It can produce flowers.

It can appear to function normally at first glance.

That still says nothing about which unwanted elements are present in its tissue.

Appearance is not an analytical certificate.

That is precisely why hemp is of interest for contaminated soils

Now we come to the seemingly paradoxical side of this characteristic.

The fact that Cannabis sativa can take up metals is not only a problem.

It can also be used deliberately.

Industrial hemp has been studied for years as a possible plant for phytoremediation.

In this process, plants are grown on contaminated sites to take up pollutants from the soil or influence their mobility.

The uptake of various metals by hemp has been described. These include cadmium, lead, nickel, chromium and zinc, among others. The uptake of, or tolerance to, mercury has also been described in research.

From a soil remediation perspective, this is fascinating.

A fast-growing plant produces a great deal of biomass and can take up substances that one wants to remove from the soil.

But this is exactly where the crucial difference lies.

Industrial hemp for soil remediation is not a crop for consumption.

If a plant deliberately takes up pollutants from contaminated soil, these substances do not disappear at harvest.

They are subsequently present in the biomass.

That is why the subsequent handling of the contaminated plant material is also a separate issue in phytoremediation.

What may be desirable for remediating soil is precisely what we do not want in a cannabis flower.

Where can heavy metals enter a grow?

The answer is not simply:

"From poor soil."

That would be an oversimplification.

Metals and other problematic elements can generally enter a growing system through various pathways.

The current cannabis study explicitly identifies soil, contaminated fertilizers and irrigation water as possible sources during cultivation, among others. Foliar fertilizers and plant protection products can also contribute, depending on their composition.

The following can therefore be relevant in a grow, among other things:

• native soil
• substrates and soil mixes
• irrigation water
• mineral fertilizers
• organic fertilizers
• composts
• rock powders
• phosphate sources
• lime and mineral amendments
• other natural raw materials

This does not mean that these materials are automatically problematic.

But it does mean:

Their origin and purity matter.

Natural does not mean free of pollutants

We encounter a common misconception, particularly in organic cultivation.

Natural is equated with clean.

Chemically, the world does not work that way.

Cadmium occurs naturally.

Lead occurs naturally.

Nickel occurs naturally.

Mercury occurs naturally.

Arsenic occurs naturally.

What matters is not whether a substance comes from nature.

What matters is its concentration, chemical form, bioavailability and the amount that ultimately enters the growing system.

Mineral raw materials in particular originate from natural deposits.

As a result, they bring with them not only the minerals for which we use them.

Depending on their origin, they may also contain accompanying elements.

A rock powder is therefore not automatically good simply because it comes from a mountain.

A phosphate raw material is not automatically clean simply because it formed naturally.

And an organic raw material is not automatically free of pollutants simply because an animal or plant was involved in its formation.

That is precisely why limit values for pollutants exist for fertilizers

This is not a new topic in Germany.

The Fertilizer Ordinance explicitly addresses pollutants.

For products placed on the market under German fertilizer law, the DüMV contains limit values for, among other things:

• Arsenic
• Lead
• Cadmium
• Chromium VI
• Nickel
• Mercury
• Thallium

The general limit values are, for example, 150 mg/kg dry matter for lead, 1.5 mg/kg for cadmium, 80 mg/kg for nickel and 1.0 mg/kg for mercury. Special rules apply to individual starting materials and product groups. For rock powders, for example, the nickel limit may be exceeded by 50 percent under the conditions specified in the ordinance. Additional requirements for cadmium apply to phosphate-rich fertilizers.

The details are complex.

The fundamental statement, however, is very simple:

Fertilizer quality is not just about NPK.

A product that can legally be placed on the market must also comply with the relevant pollutant requirements.

Anyone selling fertilizer bears responsibility

An important point is often misunderstood in the grow sector.

In Germany, not every fertilizer automatically undergoes a state approval procedure with a complete individual analysis of every batch before it is sold.

Responsibility initially lies with the entity placing the product on the market.

The Bavarian State Research Center for Agriculture explicitly states that the entity placing the product on the market is responsible for compliance with the requirements of fertilizer law. This also includes pollutant limit values. At the same time, fertilizers are inspected as part of official market surveillance.

This does not mean that the law generally requires every product to undergo a new, complete heavy metal analysis for every individual batch.

But it does mean:

A manufacturer must know what it is placing on the market.

Anyone purchasing raw materials and using them to manufacture fertilizers cannot rely solely on a material being described as "natural," "organic" or "suitable for plants."

They must ensure that the product meets the applicable requirements.

And this is precisely where "laboratory-tested" becomes interesting

Analysis certificates are becoming increasingly common in the grow market.

In principle, we welcome this.

More transparency is good.

But a PDF with a laboratory logo does not automatically answer the crucial questions.

Because:

An NPK analysis is not a heavy metal analysis.

A CalMag analysis is not a heavy metal analysis.

A microbiological examination is not a heavy metal analysis.

And even an extensive analysis only says something about the parameters that were actually examined.

When a manufacturer advertises a product as "laboratory-tested," it is therefore worth taking a second look.

Not only:

Was anything analyzed?

But:

What was analyzed?

Which heavy metals were determined?

Which detection limits were used?

Which raw material or sample does the analysis refer to?

How relevant is this sample to the product being sold today?

An analysis is valuable.

But only if it answers the right question.

CannaSelection: For us, raw material quality does not end with nutrient content

That is precisely why we go one step further at CannaSelection.

When we select a starting material, we are not only interested in:

How much calcium does it provide?

How much phosphorus?

How much magnesium?

Which trace elements?

How does it behave in the soil?

We also want to know what we are not introducing into the system with it.

That is why we have the starting materials we use analyzed for relevant heavy metals.

For us, this is part of raw material selection.

Not a marketing gimmick.

But because we view Living Soil as a system.

Every raw material we add becomes part of this system.

And the plant does not stand beside it.

It lives in it.

It takes up elements from it.

It builds tissue from them.

Ultimately, these become the flowers that are later consumed.

That is why it is not sufficient for us to evaluate a raw material solely on the basis of its desired nutrients.

Purity is part of quality.

With Living Soil in particular, every starting material matters

Living Soil depends on combining different components sensibly.

Compost.

Worm castings.

Mineral raw materials.

Calcium sources.

Phosphorus sources.

Rock powders.

Biochar.

Organic nutrient carriers.

Microbiology.

This diversity is a strength.

At the same time, however, it increases the responsibility involved in selecting starting materials.

The more components a system contains, the more important it becomes to ask what quality each individual component brings with it.

That is precisely why we do not think much of using any raw material simply because its nutrient analysis looks good on paper.

The origin must be right.

The composition must be right.

And the pollutant profile must also be right.

Outdoor cultivation also requires a closer look

Anyone cultivating indoors with a defined substrate has at least a large part of their starting materials under control.

Outdoors, things are different.

A garden soil may have decades of history behind it.

Former commercial use.

Imported fill.

Construction debris.

Proximity to heavily trafficked roads.

Industry.

Previous agricultural use.

Unknown materials.

For this reason, German soil protection law explicitly takes into account the soil-crop plant pathway and contains investigation and action values for various inorganic pollutants.

Anyone planting cannabis directly in the soil on a property with an unknown or suspicious history should therefore not focus solely on whether tomatoes were previously grown there.

When in doubt, a soil analysis may be far more worthwhile than the next bottle of booster.

But do I really smoke these heavy metals then?

We need to be precise here.

The current 2026 study examined which metals end up in the plant and its flowers.

It did not examine what proportion actually enters the human body during combustion or vaporization.

The measured flower values therefore cannot simply be used to calculate a specific absorbed dose.

However, heavy metals have already been detected in cannabis products and cannabis smoke. This is precisely why scientists treat contamination of cannabis flowers as a relevant consumer protection issue.

The sensible approach is therefore not panic.

The sensible approach is:

Avoid allowing unwanted substances to reach the plant in relevant concentrations in the first place.

The plant knows nothing about marketing

Ultimately, biology is rather matter-of-fact.

The plant does not care whether a package says "Premium," "Natural," "Organic," "Living Soil" or "100% natural."

It responds to chemistry.

To available ions.

To concentrations.

To its genetics.

To its root environment.

And that is precisely why good plant nutrition must go beyond:

What does my plant need?

The second question is at least just as important:

What do I absolutely not want in my plant?

For us, both belong together.

Conclusion: A strong plant is not enough

Current research shows very clearly:

• Cannabis can take up heavy metals from its root zone
• most of them often remain in the roots
• some can nevertheless be transported all the way to the flowers
• nickel showed particularly strong translocation in the current study
• at the higher experimental exposure, cadmium and nickel exceeded the WHO guideline values for medicinal plants used by the authors
• growth and flower biomass do not necessarily have to decline significantly as a result of such contamination
• industrial hemp may be of interest for remediating contaminated soils precisely because of its uptake capacity
• for cannabis intended for consumption, this same characteristic makes the quality of soil, water, fertilizer and raw materials all the more important
• the German Fertilizer Ordinance therefore contains clear pollutant limit values
• CannaSelection has its starting materials analyzed for relevant heavy metals and takes these values into account when selecting raw materials

When growing, we like to focus on what we put in.

Perhaps we should talk at least as much about what we want to keep out.

Because ultimately, it is not only the size of a flower that matters.

Not only its THC content.

Not only its terpene profile.

A harvest is only truly good when the starting materials are right, too.

Or, more simply:

What is in your soil can later end up in your flower.

Sources and further research

The key current study is Heavy metals in cannabis: plant contamination and effects on cannabinoid production by Beigel et al. It was published on July 27, 2026, in the Journal of Cannabis Research. The study examined four medical cannabis cultivars under defined exposure to cadmium, lead, nickel and cobalt.

For German pollutant limit values, Annex 2 of the Fertilizer Ordinance is particularly relevant.

There is now a broad body of research literature on hemp as a possible phytoremediation plant, focusing on the uptake and accumulation of various metals by Cannabis sativa.