Cannabis sativa is not nitrogen fixing
Lorelei Alvarez, PHD Environmental Science, University of Virginia
Introduction: Joseph B. Carringer, CEO CMG
February 28, 2020
I have been involved with the industrial hemp industry and cannabis-related activism for nearly three decades. During this time, I have heard the following statement repeated over and over again,
“HEMP IS A NITROGEN FIXER!”
Every major name in industrial hemp that I knew in my early days and have known over the decades has professed this “fact” at least once in their lives as gospel. So, when I was questioned as to the accuracy of the statement by my colleague Professor Alvarez, I was initially shocked. Even worse was the moment when I became god smacked with the realization that we, the hemp industry, had been peddling an inaccuracy as fact for at least the duration of my tenure. So, I did the only responsible thing I could do and ask Professor Alvarez to write an article to teach us the truth about cannabis sativa and nitrogen-fixing.
Cannabis sativa is not nitrogen fixing
Organic nitrogen is one of the most limiting nutrients in soils and is necessary for plant growth and development. Although inorganic nitrogen (N2) is the most prevalent gas in the atmosphere, it cannot be used by plants. It must first be converted to an organic form, such as ammonium (NH¬4) or nitrate (NO3). Some N2 is converted to NH4 by lightning during storms, but the amount is relatively small. The availability of nitrogen in its organic form in soils is often the limiting factor for plant growth. A particular group of organisms called prokaryotes (bacteria) can convert inorganic nitrogen into its organic form that can be used by plants for growth and development (Deacon, n.d.). This process is known as “nitrogen-fixing”. In agriculture, where plants are usually annuals and are replanted every year, the soil can quickly become depleted of organic nitrogen as crops take up available organic nitrogen for growth. Organic and inorganic fertilizers are often added to the soils to replenish this necessary nutrient.
“Nitrogen-fixing plants” are plants that form a symbiotic relationship with bacteria, such as rhizobia, that can convert inorganic nitrogen to organic nitrogen for use by the bacteria and by the associated plants. These bacteria infect plants’ roots, where they grow and thrive in nodules that form on the roots. The bacteria benefit from the relationship by accessing sugars from the plants to use as energy, and the plants benefit by receiving nitrogen in a usable form for growth and development (Wagner, 2011). By far the most common example of this relationship is found in the plant family Fabaceae or the legumes. There are other examples of plants that form symbiotic relationships with nitrogen-fixing bacteria, such as actinorhizal trees and shrubs like alders, which form a symbiotic relationship with the actinomycete, Frankia (Wagner, 2011). It is, however, uncommon to see this relationship outside of the Fabaceae family. Plants that form these symbiotic relationships are often used in crop rotations with other nitrogen-depleting plants because they add organic nitrogen to the soil. Soybeans, for example, are in the legume family and are often rotated with corn crops to replenish organic nitrogen that is depleted by the corn.
The idea that Cannabis sativa is nitrogen fixing and therefore can increase the nitrogen levels in soils has been around for some time. Articles on the web discussing the benefits of industrial hemp, such as the one posted by Blue Ridge Outdoors (Ruffing, 2015), making claims that the plant is nitrogen-fixing and therefore good for soil restoration can still be found. Despite these claims, Cannabis sativa (including both industrial hemp and marijuana varieties) are not plants that form the symbiotic relationship with nitrogen-fixing bacteria.
Cannabis sativa is a member of the genus Cannabis in the plant family Cannabaceae. Some scientists recognize two subspecies of C. sativa, C. sativa subsp. sativa and C. sativa subsp. Indica. (McPartland, 2018). These two subspecies differ in terms of growth patterns and level of Tetrahydrocannabinol (THC) present in the plant, although not all agree that these differences are enough to justify categorizing them as subspecies given the wide range of variation in each. The Sativa subspecies or variety is commonly referred to as industrial hemp and is grown for many uses, including fiber, wood products, food, biomass and CBD oil. The Indica subspecies is commonly referred to as marijuana and is grown for the psychoactive properties of THC. There are several other genera in the family Cannabaceae, including Humulus (hops) and Parasponia (a small group of evergreen, tropical trees). The trees of the genus Parasponia are the only other known species outside of the Fabaceae family to form nitrogen-fixing nodules with rhizobia bacteria (Velzen et. al, 2018). The misconception of Cannabis sativa as nitrogen-fixing might have originated with the discovery that another genus in the Cannabaceae family could form this symbiotic relationship.
To further complicate the issue, C. sativa does form a symbiotic relationship with arbuscular mycorrhizal fungi. These fungi also penetrate the roots of plants, taking up sugars and sharing nutrients. They are not, however, able to fix atmospheric nitrogen into a usable form for plants. When these mycorrhizae were first discovered they were widely believed to have the ability to fix nitrogen, but that belief has been refuted by research on the fungi (Mikola, 1986). Although arbuscular mycorrhizae do not fix nitrogen into an organic form, they are still beneficial to plants because they increase the surface area in root systems for taking up nutrients and water. Plants with this symbiotic relationship are more resistant to drought, stress and nutrient deficiencies in soil (Ahangar et. al, 2014; Chen et. al, 2018).
Because C. sativa does not form a symbiotic relationship with nitrogen-fixing bacteria, it will deplete the soil of organic nitrogen over time. Russo (2018) in “Marijuana not monoculture” makes the argument for companion planting C. sativa plants with plants in the legume family that have the symbiotic relationship with nitrogen-fixing bacteria. This companion planting would have multiple benefits, including restoring nitrogen to the soil, increasing diversity and decreasing chances of crop loss to disease and pests. C. sativa does have many other benefits for soil health even though it does not form the symbiotic relationship with nitrogen-fixing bacteria. The plants have deep roots, which can break up compacted soils. Their water and nutrient needs are low compared to many other agricultural crops. Additionally, the tall, fast-growing plants shade out weeds, which reduces the need for herbicides, and some studies indicate that they are naturally resistant to pests (Hobbs, 2019).
Ahanger, M. A., Hashem, A., Abd-Allah, E. F., & Ahmad, P. (2014). Arbuscular Mycorrhiza in Crop Improvement under Environmental Stress. Retrieved from https://www.sciencedirect.com/science/article/pii/B978012800875100003X
Chen, Arato, Miguel, Borghi, Lorenzo, Nouri, … Didier. (2018, August 10). Beneficial Services of Arbuscular Mycorrhizal Fungi – From Ecology to Application. Retrieved from https://www.frontiersin.org/articles/10.3389/fpls.2018.01270/full
Deacon, J. (n.d.). The Microbial World: The Nitrogen cycle and Nitrogen fixation. Retrieved from http://archive.bio.ed.ac.uk/jdeacon/microbes/nitrogen.htm
Hobbs, J. (2019). American hemp: how growing our newest cash crop can improve our health, clean our environment, and slow climate change. New York, NY: Skyhorse Publishing.
John M. McPartland. (2018). Cannabis and Cannabinoid Research. 203-212. http://doi.org/10.1089/can.2018.0039
Mikola, P.U. (1986). Relationship between nitrogen fixation and mycorrhiza. Mircen Journal 2, 275–282 https://doi.org/10.1007/BF00933493
Ruffing, R. (2015). Industrial Hemp: Think Green in 2015. Retrieved from https://www.blueridgeoutdoors.com/go-outside/industrial-hemp-think-green-in-2015/ Russo, S. (2019). Marijuana Not Monoculture! Retrieved from https://www.projectcbd.org/environment/marijuana-not-monoculture
Velzen, R. van, Holmer, R., Bu, F., Rutten, L., Zeijl, A. van, Liu, W., … Geurts, R. (2018). Comparative genomics of the nonlegume Parasponia reveals insights into evolution of nitrogen-fixing rhizobium symbioses. Retrieved from https://www.pnas.org/content/115/20/E4700
Wagner, S. C. (2011). Biological Nitrogen Fixation. Nature Education Knowledge 3(10):15. Retrieved from https://www.nature.com/scitable/knowledge/library/biological-nitrogen-fixation-23570419/