Toxic soil produces clean hemp flower
The heavy metals taken up by cannabis plants grown in coal mining remediation fields were expressed in the leaves of the mature plants. But, critically, the heavy metals did not appear in the floral buds where cannabinoids, including valuable CBD, is concentrated, the researchers told Leafly.
“We did see metal uptake in the leaves and removal from the soil but not in the floral buds,” said Sairam Rudrabhatla, a professor of biology at Pennsylvania State University-Harrisburg and one of the study’s lead authors.
Details about the analysis of the flower buds was not published in the study. But when the buds were tested for heavy metals, “none were present,” as Hannah George, lab manager at Penn State’s Central Pennsylvania Research and Teaching Laboratory for Biofuels, confirmed to Leafly via email.
Cannabis: A known remediation agent
Cannabis’s ability to remove toxic material from the soil—a technique called phytoremediation—is well known. Cannabis phytoremediation has been used to remove radioactive contaminants from areas around the infamous Chernobyl nuclear power plant in Ukraine, and activists are hoping to use hemp to remove plutonium from land surrounding the former US nuclear weapons factory in Rocky Flats, Colorado.
Researchers are still perfecting their techniques with regard to phytoremediation. At Chernobyl, cannabis proved to be an excellent remediation agent, but the plants that did the best job of removing cesium-137, the radioactive byproduct contaminating the exclusion zone, were cultivars of amaranth.
Opens new fields to hemp cultivation
The findings in the recent PLos One study represent the first demonstration that cannabinoid production can be “remarkably influenced by mine land soil conditions.” And they present a new and potentially lucrative opportunity for hemp cultivators seeking available land in the ongoing CBD-fueled hemp boom.
With agricultural production acreage limited due to existing food production and encroaching suburban development, land that’s currently considered toxic and unsuitable for agriculture can be used to cultivate hemp.
The hemp plants will remove some of the toxic heavy metals, thereby unlocking more potential for the previously “useless” mine land soil. And since the hemp will also produce more CBD than hemp grown in clean soil, there’s a built-in financial incentive to grow in those problematic soils, Rudrabhatla said.
‘We expected the plants to die’
“We were very surprised and then very excited to see this kind of thing can actually work. What we were expecting was that all the hemp plants would die,” Rudrabhatla added. “Honestly, that’s what we were expecting.”
“If you remove the word mine land, and introduce a drought stress, or a soil stress, or deprivation of nutrients like magnesium or phosphorous, the response would be very similar,” he noted. “The only advantage with mine land soil is that that soil is doing nothing right now. We can grow on this soil and remediate this soil, so it can then grow valuable food crops.”
Six cultivars of hemp
The researchers obtained six different cultivars of industrial hemp from the Pennsylvania State Department of Agriculture. Three of those cultivars are recommended to grow for fiber and seed only, while three others are recommended as CBD source material. The researchers grew them in two different types of contaminated soil and in two commercial soils: Miracle-Gro potting mix and PRO-MIX HP Mycorrhizae High Porosity Grower Mix.
Contaminants present in the mine land soil included nickel, cadmium, lead, arsenic, and mercury.
All of the plants were expected to produced less than 0.3% THC (the maximum allowable THC content in hemp) and no more than 2% CBD.
Greenhouse vs outdoor grows
The researchers grew each plant in each soil in greenhouse conditions as well as outdoors. They grew each variety in between 12 and 18 pots of three seeds each per hemp cultivar, for total of 132 pots in the entire study.
The hemp grown in mine land soil produced 2.16% and 2.58% CBD—perhaps not enough to impress a grower, but “a significant increase” over the 1.08% and 1.6% CBD content produced by plants in the Miracle-Gro soil grown outdoors and in greenhouse conditions, respectively.
“Total CBD content in the floral buds grown in mine land 1 soil in both outdoors and in the greenhouse was higher than the floral buds grown in Miracle-Gro in both environmental parameters and in the field, which can be concluded due to the heavy metal stress,” the researchers wrote.
Stress also boosted THC in one strain
However, the overproduction-due-to-stress phenomenon cut both ways: One strain overproduced THC beyond the 0.3 percent limit, which means the crop would have to be destroyed if it were a commercial harvest.
Why and how does the plant do this? The heavy metals seem to trigger a genetic response in the plants that leads to an overproduction of the acids that determine later cannabinoid production.
“Notably, Cannabidiolic acid synthase (CBDAS) was expressed 18 times higher in the mine land soil,” the researchers wrote, adding that “hemp increased total CBD content under high heavy metal conditions and was a result of enhancement of CBDAS and OAC (oliveatolic acid synclase) gene expression.”