Cannabis growers know that balancing nutrients is critical in hydroponic cannabis cultivation. In order to grow well, cannabis needs the right balance of the macronutrients nitrogen (N), phosphorus (P), and potassium (K). It is also important to provide the right levels of the micronutrients calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), and manganese (Mn). The 2023 study, “Foliar Symptomology, Nutrient Content, Yield, and Secondary Metabolite Variability of Cannabis Grown Hydroponically with Different Single-Element Nutrient Deficiencies,” monitored and documented single nutrient deficiencies in hydroponic cannabis plants over their lifespan. This study also provided useful photographs for visual diagnosis of nutrient deficiencies and evaluated the effect of deficiencies on cannabis productivity at harvest. Here are some of the key findings of the study:
- Nitrogen deficient cannabis plants produced 73% less vegetation
- Phosphorus deficient cannabis plants produced 59% less vegetation
- Other single nutrient deficiencies resulted in 33% to 72% less vegetation at harvest (by fresh weight)
- Correcting nutrient deficiencies is critical to preserving cannabis yields
- Some chlorosis (yellowing) in leaves starting at the bottom of the foliage is normal as cannabis plants mature
While deficiencies substantially reduced vegetative and floral growth, effects on secondary metabolite production (cannabinoids and terpenes) were minor overall. However, other studies have noted a reduction in cannabinoid and terpene production in malnourished cannabis plants.
Visual Signs of Nitrogen Deficiency in Cannabis
Stunted vegetative growth was the first symptom of nitrogen deficiency that the research team observed. At week three, slight yellowing at the tips of lower leaflets was visible. By week four, yellowing of the fan leaves on the lower part of the plants was obvious. By the fifth week, leaves were turning brown and dying. By the end of the study, stem tissue had turned brown and green color only remained in the top-most leaves. Since nitrogen is one of the nutrients most critical to cannabis plant growth, nitrogen deficiency symptoms appeared most rapidly in this study.
Visual Signs of Phosphorus Deficiency in Cannabis
The study did not observe significant signs of phosphorus deficiency in cannabis foliage until the mid-flowering growth stage. Yellow spots on foliage began to appear in week three of flowering and progressed in severity. Necrotic lesions and necrosis along leaf margins also appeared as time progressed. Necrosis like this can also be a sign of water logging. In these phosphorus deficient plants, most foliage had died by the seventh week of flowering, and leaf petioles also had a purple coloration. Another visual indicator observed was curling of leaf tips. Phosphorus is a key nutrient for tetrahydrocannabinol (THC) production, so correcting deficiencies is important for cannabinoid production as well as plant health.
Visual Signs of Potassium Deficiency in Cannabis
Potassium deficiency symptoms in cannabis became visible to researchers during the third week of growth. Foliage on deficient plants displayed two main symptoms. The first was dark brown lesions surrounding secondary branch veins of large sugar leaves. The second indicator was yellowing, then browning at the leaflet tips of lower fan leaves. Leaf tips and margins progressively displayed drying and scorching. By the sixth week of growth, green tissue mainly remained at the inflorescences and most leaves had become brown and dry along the ridges. However, plants with lower potassium did produce higher levels of cannabinoids which is consistent with other research that has linked higher cannabinoid production with lower potassium supply and lower overall yields.
Micronutrient Deficiency Signs in Cannabis Plants
If you are looking to diagnose cannabis nutrient deficiency, check out the full text of this article linked in the reference section to view photographs and read the researcher’s full descriptions of single nutrient deficiencies. Here’s a brief summary of micronutrient deficiency signs that the researchers observed in cannabis plants:
Calcium: Deficiency signs appeared in week three and included yellow spots with necrotic lesions in the center. Leaf margin discoloration and curling were also indicators of calcium deficiency in cannabis plants. Symptoms tend to appear first in younger plant tissue. Calcium-deficient plants had the lowest cannabinoid potency of all plants in the study.
Magnesium: Magnesium deficiency signs in cannabis foliage appeared in the fourth week of growth. The first signs were yellowing along the leaf veins and quickly progressed to necrotic regions in young fan leaves. Yellowing of the whole leaf with green left along the main leaf veins was apparent in most of the foliage by week six. Magnesium deficient cannabis leaves showed dark color along the leaf margins and the leaf veins themselves, with inner-tissue losing color before leaves become fully yellow.
Sulfur: Sulfur deficiency in cannabis leaves looks similar to nitrogen deficiency where leaves turn yellow. However, sulfur deficiency signs did not appear until week four of growth which is later than the appearance of nitrogen deficiency. Another difference observed was that chlorosis in sulfur deficient plants started with the upper canopy instead of the lower canopy where nitrogen deficient chlorosis first appeared.
Iron: Iron deficient cannabis leaves appeared lighter green in color during early growth. By week six, leaf margins displayed browning.
Manganese: Manganese deficient cannabis plants did not display any significant visual indicators of deficiency.
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.