When trying to optimize yield while minimizing inputs, it can be challenging to tease apart which nutrients might be limiting yield. This is largely because certain nutrient deficiency symptoms will mimic each other. By learning about the role of these nutrients and how they interact with the plant’s physiology, we can narrow down which nutrient may be limiting yields. Two nutrients that share very similar nutrient deficiency symptoms are potassium (K) and magnesium (Mg).
In plant physiology, K is involved in many important processes such as long- and short-distance transport of water in plants, and helps with evapotranspiration by regulating stomatal opening and closing in plants. Potassium is also essential for certain enzyme activation, as well as aiding in the transport of photosynthates (e.g., sugars) within the plant. It also is a mobile element, which means it can move (i.e., translocate) from older portions of the plant to satisfy the demand for developing new growth. This means that in plants that do not receive enough K, the initial symptoms will appear on the older leaves or lower branches of the plant.
Magnesium is also very important in plant physiology. Most notably, it is involved in photosynthesis as the central atom of the chlorophyll molecule. This means that if an Mg deficiency is too profound, it can greatly reduce photosynthesis in the plants and will reduce plant sugar production. This reduction would mean there are fewer resources overall for the plant to use in its growth and development. Magnesium is also important in enzyme activation and can also help the plant take up other ions such as phosphorus. Magnesium is another mobile element, so deficiency symptoms will appear on the older leaves or lower foliage, the same location as a K deficiency.
To help distinguish between these two nutrient deficiencies, we have to look at the early and late-stage symptoms to help us diagnose which nutrient might be limiting production.
Deficiency Symptoms of Potassium
Given the large quantities of K that most cannabis and hemp plants need, a lack of K ions will quickly manifest in plants. In previous research, K deficiency symptoms occurred as early as 28 days after K was removed from the fertilizer regimen (Cockson et al., 2019). When K is limited, foliar deficiency symptoms will first manifest as a general yellowing of the leaflet margins in lower branches (Fig. 1). If K continues to be limited, this yellowing will continue, resulting in necrotic margins that appear burnt (Fig. 2). In the late stages of a severe K deficiency, older and/or lower leaves will develop severely necrotic margins and eventually abscise (i.e., fall off) (Fig. 2).
Figure 1. This six-week-old Cannabis sativa ‘BaOx’ plant received 0.0 ppm K fertility. The leaflet margins developed a yellowing, which progressed inward toward the midrib.Photo: Paul Cockson
Figure 2. This composite photo shows the deficiency symptom progression of recently matured leaves from a Cannabis sativa ‘BaOx’ plant receiving 0.0 ppm K fertility. The leaflet margins developed a yellowing, which progressed inward toward the midrib, and eventually became necrotic.Photo by Paul Cockson
Deficiency Symptoms of Magnesium
Magnesium will also show deficiency symptoms quite early. In research conducted at North Carolina State University (NCSU), Mg deficiency symptoms were already present after just six weeks (Cockson et al., 2019; Veazie et al., 2021). Early stages of Mg stress will appear on the lower leaves as a yellow netting of interveinal chlorosis (the areas between green plant veins are yellow). This yellowing will be concentrated around the center of the leaf and radiate outward as symptoms progress (Fig. 3). In advanced Mg starvation, the netting becomes extremely pronounced and will spread upward toward the middle of the plant. In some cases, small tan necrotic spots can appear in between the yellow veins of the lower leaves.
Figure 3. Magnesium deficiency symptoms in their early stages (left) and late stages (right) of development.Photo by Patrick Veazie
A Nutrient Whodunit
To distinguish between K and Mg deficiencies, first determine if the lower leaves are showing signs of green veins with yellow areas between (Mg deficiency), or if the leaves are more uniformly yellow (K deficiency). In later stages, both deficiencies will have necrotic or dead tissue, but in K deficiency, these spots will be larger and around the margin of the leaf, whereas these will present as smaller tan spots often between the veins of the lower leaves in Mg deficiency.
Additionally, taking leaf tissue samples can be a very useful tool. From previous literature, adequate K should be between 1.54 – 3.41% of the leaf, and Mg should be within 0.25 – 0.81% for vegetative plants (Kalinowski et al., 2020). While these values will vary slightly based on cultivars and growing conditions, they can be a good range to start with.
How to Avoid Deficiencies
While diagnosing problems can help save time and money, the best defense against production issues is knowing what range to keep K and Mg within your fertilizer solution. From previous literature, we know that providing 50 – 75 parts per million (ppm) Mg will optimize plant biomass production, while higher rates (75 – 100 ppm Mg) will optimize Mg concentrations in the plant itself (Veazie et al., 2021).
In separate literature, Dalit Morad and Nirit Bernstein found that in the vegetative stage plant growth was optimized at 35 – 70 ppm Mg and that 2 – 20 ppm Mg negatively impacted plant growth and development (Morad & Bernstein, 2023).
For K fertilization, a study from Canada found that there was no upper limit to K fertility with regard to floral yields (Bevan et al., 2021). However, in other literature, optimal K fertility was found to be at 60 – 175 ppm K, with concentrations under 15 ppm negatively impacting plant growth and yield (Saloner & Bernstein, 2022).
Different cultivars and production systems will require certain adjustments to these ranges, but by keeping K between 60 – 175 ppm, and Mg between 35 – 75 ppm growers should be able to avoid nutrient deficiencies.
Conclusions
Ultimately, the optimal rate for potassium fertility depends largely on your end goal as a producer—whether you are producing cannabis for floral material, an extraction market, or clonal material. By having an in-house nutrient monitoring system and taking periodic leaf tissue analysis, you can avoid issues before they arise.
If issues do arise, looking to see if the lower leaves are overall yellow (K deficiency), or if the interveinal regions are yellow (Mg deficiency) will help diagnose between K and Mg deficiencies.
Paul Cockson1,3, Patrick Veazie1, and Brian E. Whipker1,2
1 Department of Horticultural Science, North Carolina State University: Raleigh, NC
2 Floriculture Extension and Research
3 Department of Plant and Soil Sciences, University of Kentucky: Lexington, KY
Works Cited
Bevan, L., Jones, M. and Zheng, Y., 2021. Optimisation of nitrogen, phosphorus, and potassium for soilless production of Cannabis sativa in the flowering stage using response surface analysis. Frontiers in Plant Science, 12, p.764103.
Bryson, G.M.; Mills, H.A.; Sasseville, D.N.; Jones, J.B., Jr.; Barker, A.V. Plant Analysis Handbook III: A Guide to Sampling, Preparation, Analysis and Interpretation for Agronomic and Horticultural Crops; Micro-Macro Publishing, Inc.: Athens, GA, USA, 2014.
Cockson, P., Landis, H., Smith, T., Hicks, K. and Whipker, B.E., 2019. Characterization of nutrient disorders of Cannabis sativa. Applied sciences, 9(20), p.4432.
Kalinowski, J., Edminsten, K., Davis, J., McGinnis, M., Hicks, K., Cockson, P., Veazie, P. and Whipker, B. 2020. Augmenting nutrient acquisition ranges of greenhouse grown CBD (Cannabidiol) hemp (Cannabis sativa) cultivars. Horticulturae 2020, 6(4), 98; https://doi.org/10.3390/horticulturae6040098
Kalinowski, J., Edminsten, K., Davis, J., McGinnis, M., Hicks, K., Cockson, P., Veazie, P. and Whipker, B. 2020. Augmenting nutrient acquisition ranges of greenhouse grown CBD (Cannabidiol) hemp (Cannabis sativa) cultivars. Horticulturae 2020, 6(4), 98; https://doi.org/10.3390/horticulturae6040098
Morad, D. and Bernstein, N., 2023. Response of medical cannabis to magnesium (Mg) supply at the vegetative growth phase. Plants, 12(14), p.2676.
Saloner, A. and Bernstein, N., 2022. Effect of potassium (K) supply on cannabinoids, terpenoids and plant function in medical cannabis. Agronomy, 12(5), p.1242.
Veazie, P., Cockson, P., Logan, D. and Whipker, B., 2021. Magnesium’s impact on Cannabis sativa ‘BaOx’and ‘Suver Haze’ growth and cannabinoid production. Journal of Agricultural Hemp Research, 2(2), p.1.
