When Less Is More: The Harms of Overfertilizing Your Cannabis Plants
Research shows that plants absorb excess nutrients even when they're not needed (called ‘luxury consumption’), which can even inhibit uptake of other nutrients. Learn how to meet plants’ needs while avoiding problems and saving money in the process.
Luxury uptake can result in nutrient antagonism. For example, high phosphorus can result in iron deficiency (A), and high calcium can result in magnesium deficiency (B) even when adequate fertility is available. Photo by Brian Whipker
By Patrick Veazie, Paul Cöckson and Brian E. Whipker
Plants, much like humans, require certain nutrients and minerals to thrive. These macro and micronutrients help plants grow properly, obtain optimal yields, and complete their lifecycles. When creating a fertilizer recipe or selecting which pre-blended fertilizer to utilize, understanding what is being supplied to the plant is important. Just as important, is selecting a fertilizer rate that will promote plant growth without overfertilizing, which leads to both fertilizer and money waste.
With the recent increase in fertilizer prices, a goal for commercial growers should be to minimize inputs while still achieving optimal plant growth and yield. Extensive research has been conducted at North Carolina State University (NSCU) (Cockson et al., 2020; Veazie et al., 2021; Veazie et al., 2021) and by Dr. Nirit Bernstein (Shiponi and Bernstein 2021) examining fertilizer rate effects on plant growth, nutrient uptake, and cannabinoid concentrations.
One of the largest takeaways from this research is that cannabis exhibits “luxury consumption” of many nutrients including magnesium (Mg) and phosphorus (P). Luxury consumption occurs when the plant is supplied with a greater quantity of nutrients than what the plant needs to complete its life cycle. This excess will result in the plant still taking up the available nutrients which leads to elevated leaf tissue concentrations.
Nitrogen (N), potassium (K), P, and Mg are mobile within the plant. As a result, these elements can be moved from one portion of the plant to another to satisfy the needs of developing parts if their availability is limited. Plants accumulate these elements in the lower foliage and then move them to the new developing foliage or the floral material tosustain plant growth.
Research conducted at NCSU examined a range of P fertility rates ranging from 15 to 180 ppm and determined that there was no increase in total bud weight or plant growth for plants grown utilizing a P fertility rate greater than 15 ppm, despite increasing foliar tissue concentrations. (See Fig. 1.) Similar results were also observed for Mg when rates ranging from 0 to 100 ppm Mg were examined. No increases in plant growth or cannabinoid concentration were observed when the Mg fertility rate exceeded 50-75 ppm, although Mg foliar concentrations continued to increase. (See Fig. 2.)
Figure 1. Phosphorus fertility rates were evaluated at North Carolina State University by growing ‘BaOx’ plants at a constant feed for 12 weeks of total growth (4 vegetative + 8 reproductive) at 15, 60, 120, and 180 ppm phosphorus with all other elements held constant.
This research suggests that even though the plant will continue to absorb the available nutrients, that uptake isn’t correlated to an increased yield or cannabinoid concentration.
Antagonistic Luxury Additionally, luxury consumption can result in antagonistic relationships among nutrients. As a result, growers should focus on minimizing their inputs to decrease their fertilizer costs.
When nutrients are supplied through fertilizers, elements come in pairs to even out their charges, such as potassium (K+) or calcium (Ca2+) and nitrate (NO3-). When one element is applied in excess, an antagonistic relationship–whereone element is prevented from being taken up even if it is being supplied through the fertilizer–can be observed. This is common for most macronutrients where you will not see a P toxicity but will observe an iron (Fe) deficiency in some species when phosphorus accumulation becomes too high. (See Fig. 3a.)
Figure 2. Magnesium fertility was evaluated at North Carolina State University by growing ‘BaOx’ and ‘Suver Haze’ plants at a constant feed for 12 weeks of total growth (4 vegetative + 8 reproductive) at 0, 12.5, 25, 50, 75, and 100 ppm magnesium with all other elements held constant.
Another example is maintaining the balance among K, Ca, and Mg at a ratio 4:2:1 to avoid antagonisms among them. When Ca concentrations are excessive, Mg deficiency can be observed even if fertility rates are adequate. (See Fig. 3b.)
Understanding what fertilizer salts are being used and the paired elements can allow you to reduce the luxury uptake of certain elements and still achieve the necessary fertility rate to yield optimal plant growth and cannabinoid concentrations.
Planning fertilizer recipes and fertigation rates is a balancing act. As a grower, you mustprovide the essential elements that a plant needs to complete its lifecycle and promote optimal growth while ensuring that you are not promoting luxury consumption, where the plant is taking up nutrients that are not needed, or inhibiting the uptake of other nutrients.
Figure 3: Luxury uptake can result in nutrient antagonism. For example, high phosphorus can result in iron deficiency (A), and high calcium can result in magnesium deficiency (B) even when adequate fertility is available.
Scaling back fertility to meet the plant's demand but not promoting luxury uptake will prevent potential problems while also lowering the inputs for the plant leading to potential financial savings.
Patrick Veazie is a graduate research assistant pursuing an M.S. in the Department of Horticultural Science at North Carolina State University. His studies focus on perlite alternatives for horticultural substances.
Paul Cöckson is a graduate research assistant and Ph.D. candidate at the University of Kentucky Department of Plant and Soil Sciences. He is a part of the Hemp Agronomy team and is focusing on early germination and establishment of industrial hemp.
Dr. Brian E. Whipker, Ph.D., is a professor of floriculture at North Carolina State University specializing in plant nutrition, plant growth regulators and diagnostics. He co-authored eight scientific journal articles on the impact of fertilization with greenhouse species and three disorder diagnostic guides. Dr. Whipker has more than 30 years of greenhouse experience working with growers.
