Published on 03/02/2023
Introduction
Plant nutrition is crucial for crop growth, development, and yield. For cannabis, nutritional needs and subsequent fertiliser recipes are dependent on the growing system, cultivar demands, climate conditions, etc. Of course, growers strive to provide their crop with a balanced recipe that contains all the nutrients needed for optimal growth. However, nutrient disorders can still arise, even with a properly formulated fertiliser recipe. For example, due to suboptimal rootzone pH, nutrient antagonism, lack of transpiration, and more (Llewellyn et al., 2023). To the benefit of growers and researchers alike, there is a growing number of peer-reviewed studies that investigate the effects of nutrient supply on the yield and potency (=chemical makeup) of cannabis inflorescences.
In this article, we will look at two studies with direct practical use for growers. Namely, “Characterisation of Nutrient Disorders of Cannabis sativa” by Cockson et al. (2019) and “Foliar Symptomology, Nutrient Content, Yield, and Secondary metabolite Variability of Cannabis Grown Hydroponically with Different Single-Element Nutrient Deficiencies” by Llewellyn et al. (2023). These studies describe the progression of nutrient disorders, accompanied by pictures and leaf tissue analysis. We chose to highlight these articles as they provide us with high-quality photos and descriptions of the disorders during different crop phases.
Macronutrient deficiencies
Before we look at nutrient disorders, it is important to understand what a healthy plant looks like. Normal cannabis plants have a dark green colour for most of the production cycle. Don’t be alarmed if you see fan leaves turning yellow (a process called chlorosis) during the final weeks of flowering. This natural type of chlorosis often occurs even when the plant receives all its nutritional needs. Keep this in mind when diagnosing nutrient disorders in the last weeks before harvest.
While the authors of the abovementioned papers go into detail about all important macronutrients (Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), and Sulphur (S)), we will focus on their findings for N, P, K, and Ca deficiencies in this article.
Let’s start with N, a major component of chlorophyll and used for the synthesis of new proteins. It’s also the mineral nutrient that is needed in the greatest amount by plants. N-deficiency is first shown at the bottom of the canopy. It will then gradually progress to the middle canopy layers. Leaves will first turn a pale yellow, and in advanced stages, they become bright yellow and senesce (Fig. 1). Due to the importance of N for plant growth, a deficiency will lower biomass production and have a large impact on inflorescence yield.
Another macro plant nutrient is phosphorus. P-deficiency induces the formation of olive-coloured necrotic lesions on the fan leaves and later also on sugar leaves (Fig. 1). The lesions start small but can cover large parts of the leaves in advanced stages. P-deficiency also causes leaf curling, turning individual leaflets inward and upward. P is used throughout the plant to facilitate energy transfer, for example, in the process of photosynthesis. Interestingly, Llewellyn et al. (2023) found that the limited supply of P increased THC concentration in inflorescences. However, the greatly diminished inflorescence yield outweighs the increase in potency.
The initial symptoms of K-deficiency are the yellowing and browning of leaflet edges and possibly some older fan leaves. Plants rely on K for the functioning of stomata. Stomata are crucial for gas exchange and the transport of water and nutrients. This makes K so vital to plant health. In flowering plants, larger sugar leaves exhibit dark brown lesions surrounding secondary vein branches. If the deficiency persists, yellowing of the edge expands to the midrib of the leaf. As the yellowing of leaves continues, leaves, mainly from the upper and middle canopy, will turn brown, and the leaf edges will curl up and dry out (Fig. 1). At advanced stages, whole-plant chlorosis is observed. Llewellyn et al. (2023) found the highest concentrations of THCA and CBGA in K-deficient inflorescences. However, the increased potency comes at the cost of lowered inflorescence yields.
Ca-deficiency manifests itself in different ways. According to Cockson et al. (2019), Ca-deficiency induces stunting and irregular growth patterns in growth tips and expanding leaves. The base of the leaflets is then of a lighter green colour than the leaf tip. As time progresses, the yellowing expands, and interveinal chlorosis develops. Emerging leaves and growth tips are now significantly malformed and can become necrotic (Fig. 1). Ca deficiencies often cause malformation of leaves, as Calcium is essential for cell wall integrity. Ca-deficiency occurs when transpiration is limited. This is because the transport of Ca relies mainly on transpiration. Ca deficiencies can also appear when the balance of K, Ca, and Mg is upset.
Restoring the balance is challenging; therefore, it is essential to consider the concentrations of these nutrients when selecting a fertiliser recipe. Llewellyn et al. (2023) noted a different symptomology. Their flowering plants developed yellow rings with necrotic brown centres on lower fan leaves. On the most affected leaves, the spots coalesced. In more advanced stages, young leaves also developed spots, and leaf tips started to curl upwards. Authors from both studies note moderate negative effects on biomass and inflorescence production.
Micronutrient deficiencies
While Cockson et al. (2019) go into detail about many micronutrients (Boron (B), Copper (Cu), Iron (Fe), Manganese (Mn), Molybdenum (Mo), and Zinc (Zn)), we will only cover Fe and Mn as these micronutrients are covered in both studies.
Mn deficiencies can be recognised by interveinal chlorosis. Over time, the green veins will become more distinct from the interveinal tissue, which will turn yellow. Some necrotic regions can also develop around leaf edges (Fig. 2). Llewellyn et al. (2023) did not note a significant response to Mn-deficiency in flowering plants, even though upper foliar concentrations were extremely low. Plants use Mn in several processes essential for photosynthesis. However, researchers observed no differences in biomass production between the control and Mn-deficient treatments. Therefore, they suggest that cannabis can very efficiently take up and utilise Mn.
If an Fe-deficiency occurs, the base and middle sections of leaflets of newly expanding leaves and growth tips will become a pale green colour (Fig. 2). Because plants treat iron (Fe) as a relatively immobile nutrient, you’ll see symptoms appear most clearly on young leaves at the top of the plant. In contrast, the lower and mid sections of the plant will look healthy. A side effect of Fe deficiency may be the accumulation of other cationic elements, such as Zn. Still, neither study reported a deleterious impact of Iron deficiency on yield or potency. Plants use iron (Fe) to synthesize chlorophyll, and they depend heavily on pH levels to absorb it effectively.Therefore, it is important to check the pH of your nutrient solution.
Diagnosing and correcting nutrient deficiencies is an important aspect of growing cannabis. The studies presented today help growers and researchers connect symptomology with nutrient deficiencies. Still, visual assessments, even when combined with foliar tissue analysis, won’t guarantee a perfect diagnosis due to the complex nature of plant nutrition. It’s important to gather as much information as possible to realise the highest chance for a correct diagnosis. We highly recommend reading the full articles, as they present more pictures and nuances in the original texts. You can read and download both articles for free, as they are open-access. We’ve provided the links to the cited articles below.
Articles cited
Llewellyn, D., Golem, S., Jones, A. M. P., & Zheng, Y. (2023). Foliar Symptomology, Nutrient Content, Yield, and Secondary Metabolite Variability of Cannabis Grown Hydroponically with Different Single-Element Nutrient Deficiencies. Plants, 12(3), 422. https://doi.org/10.3390/plants12030422
Cockson, P., Landis, H., Smith, T., Hicks, K., & Whipker, B. E. (2019). Characterisation of nutrient disorders of Cannabis sativa. Applied sciences, 9(20), 4432. https://doi.org/10.3390/app9204432
Author: Kjell Sneeuw MSc.