AUTHORS:
Dalit Morad 1,2 and Nirit Bernstein 1,*
1Institute of Soil Water and Environmental Sciences, Volcani Center, 68 HaMaccabim Road, P.O. Box 15159,
Rishon LeZion 7505101, Israel 2 The Robert H. Smith Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot, Israel * Correspondence: Nirit@volcani.agri.gov.il
Plants 2023, 12(14), 2676; https://doi.org/10.3390/plants12142676
Submission received: 14 January 2023 / Revised: 29 January 2023 / Accepted: 7 February 2023 / Published: 18 July 2023

Practical Breakdown
Plant nutrient requirements are a well-studied topic in plant science and horticulture. For many commercial crops, optimal nutrient levels have been extensively researched and formulated. However, cannabis, being a relatively new crop in legal commercial cultivation, is still undergoing scientific exploration to determine its precise nutrient requirements.
This paper focuses on a single element, magnesium (Mg), classified as a secondary macronutrient. Magnesium plays a critical role in various developmental and physiological processes in plants, and cannabis is no exception.
Like most nutrients, plants require a minimum threshold of available magnesium to facilitate normal growth and development—a range often referred to as the sufficiency range. Beyond this, an optimal range exists where plant performance is maximized. For commercial cannabis producers, understanding and achieving these optimal nutrient requirements is essential for enhancing plant performance and yield.
In this study, five Mg concentration levels (2, 20, 35, 70, and 140 mg L⁻¹) were tested through fertigation. All other essential nutrients were kept constant, with Mg levels adjusted using magnesium sulfate (MgSO4). Sulfur was chosen as the accompanying ion due to prior evidence indicating that sulfur uptake in cannabis does not increase with higher supply levels.
Under controlled environmental conditions, a Type-II cannabis variety was cultivated vegetatively over 30 days. Perlite was used as the growing medium due to its inert chemical properties, which minimized interference and complexities in the root zone.
Both destructive and non-destructive sampling methods were employed throughout the trial to collect data on plant development and nutrient uptake. These results were then analysed to identify statistically significant trends in plant performance and nutrient dynamics.
Key Findings
1. Optimal Range: The results indicated that the optimal Mg supply range for cannabis plants is between 35–70 mg L⁻¹ during the vegetative growth phase.
2. Deficiency and Toxicity: Concentrations of 2–20 mg L⁻¹ resulted in deficiency symptoms, while 140 mg L⁻¹ caused toxicity for the vegetative growth phase of cannabis.
3. Cation Interactions: An increase in magnesium (Mg) supply resulted in an impairment of Calcium (Ca) and Potassium (K) uptake as well as root-shoot translocation.
Application For Cultivators
1. It is strongly advised to avoid a reactive approach to nutrient management in your cultivation environment. Waiting to observe visible symptoms of nutrient deficiency or toxicity before acting and adjusting your fertigation program can lead to irreversible damage.
By the time symptoms appear, plant performance and yield potential may already be compromised. Instead, it is recommended to adopt a proactive strategy by regularly conducting elemental testing of your fertigation solution and plant material, such as leaf tissue and leaf sap.
This allows for precise monitoring and optimization of your plant nutrition program, ensuring consistent and healthy plant growth.
2. Based on the findings of this study, the recommended sufficiency range for magnesium (Mg) supply during the vegetative growth phase is 35–70 mg L⁻¹.
However, caution must be exercised when increasing Mg supply, as its antagonistic interactions with calcium (Ca) and potassium (K) may lead to deficiencies of these nutrients in plant tissue.
Additionally, excessive Mg levels, such as 140 mg L⁻¹, can result in toxicity symptoms, which typically manifest first on the lower leaves, as observed in this study.
This information provides a valuable framework for refining the nutrient requirements of your specific commercial cannabis variety, helping to optimize plant health and productivity.
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