Preliminary findings from a study conducted by a Portland State University geology professor show a significant connection between the chemical makeup of cannabis plants and the native soils they’re grown in.
The results move the cannabis community one step closer to a better understanding of how regional terroir (pronounced ter-wahr) affects the unique character of outdoor-grown cannabis crops.
Terroir, a French word that translates to “land” or “territory,” includes several distinctive features of a place, including soil composition, climate and topography.
John Bershaw and his graduate students at PSU focused their research on how five native soils in different parts of Southern Oregon impacted the chemical composition of two strains of marijuana plants that were grown and harvested last year.
“The five soils used were chemically unique, and our results show that this is impacting plant chemistry,” Bershaw said. “For example, a soil with a high amount of magnesium from the Illinois Valley produced plants with relatively high magnesium concentrations.”
Magnesium is one of several minerals found in varying amounts in soil. It’s an important nutrient because it enables plants to absorb the sun’s energy during photosynthesis. Magnesium deficiencies, as well as excessive levels of magnesium, in soil can have detrimental effects on plants.
Bershaw worked with the staff at Alter Farms in Grants Pass to collect soil samples from five cannabis farms that represented the diversity of native soils in Southern Oregon. Each soil was analyzed for its composition of organic matter and inorganic minerals.
Two of Alter Farms’ strains, Fire Runner and Purple Hindu Kush, were selected for the study, and three clones of each strain were planted in outdoor raised beds, each bed filled with a different soil type.
After harvesting last fall, cured flower samples from the six study plants were analyzed for cannabinoid and terpene concentrations, as well as levels of macronutrients (nitrogen, phosphorus, potassium, calcium, sulfur and magnesium) and trace minerals (such as iron, manganese, boron, copper, iodine and zinc).
Data analysis is still underway; however, Bershaw said initial findings support the theory that growing plants in native soils contributes to the unique characteristics of cannabis grown in different parts of Southern Oregon. This is good news for craft cannabis growers in our area, who want to market their flower and tinctures as distinctive.
In addition, knowledge of the relationship between soil composition and cannabis characteristics will inform the growing practices of cannabis farmers, including how soils can be amended for optimal results.
Until now, evidence of the effects of terroir, including soil composition, on cannabis has been largely anecdotal. However, marijuana studies have been increasing after 46 states in the U.S. have legalized medical marijuana and 11 states have legalized recreational marijuana. Oregon legalized recreational marijuana in 2014.
Research on the effects of soil characteristics on cannabis may still be in its infancy, but different soil types are known to have significant effects on other crops such as wine grapes. For example, sandy soils produce grapes that are processed into lightly colored, aromatic wines, whereas soils with high clay content produce grapes that become wines with deep colors and rich, complex flavors.
Bershaw’s study included three of 12 soil types — Mollisols, Alfisols and Ultisols. Samples of three different Mollisol soil subtypes were collected from farms in the Bear Creek Valley, Illinois Valley and Grants Pass. Samples of Ultisol soil were collected from a cannabis farm in Grants Pass, and samples of Alfisol soil were taken from a farm in the Applegate Valley.
The three soil types have different physical and chemical properties. Mollisols are grassland soils that have high fertility and are rich in calcium and magnesium. Ultisols are acidic forest soils with relatively low fertility; a lot of the macronutrients have been leached from Ultisol soils. Alfisols are also forest soils that are similar to Ultisols, but they are less acidic and more fertile.
Bershaw said further details of his research findings will be available later this year, and then we’ll know more about how different soils impacted the chemical composition of the cannabis tested.
“There’s still a lot of data and ongoing analysis,” Bershaw said. “That being said, the initial finding is significant as it suggests strongly that there is a significant connection between soil and plant chemistry.”
Rhonda Nowak is a Rogue Valley gardener, teacher and writer. Email her at Rnowak39@gmail.com. For more about gardening, visit her blog at http://blogs.esouthernoregon.com/theliterarygardener/ and check out her podcasts and videos at https://mailtribune.com/podcasts/the-literary-gardener, and her website at www.literarygardener.com.