Effect of nitrogen on tetrahydrocannabinol (THC) content in hemp (Cannabis sativa L.) leaves at different positions
I. Bуcsa, P. Mбthй, and L. Hangyel
GATE “Fleischmann R.” Research Institute, Kompolt 3356, Hungary
Bуcsa, I., P. Mбthй, and L. Hangyel 1997. Effect of nitrogen on tetrahydrocannabinol (THC) content in hemp (Cannabis sativa L.) leaves at different positions. Journal of the International Hemp Association 4(2): 78 -79. The effect of different levels of nitrogen fertilizer, of physiological age of leaves and of the interaction between these factors on the Δ 9 -tetrahydrocannabinol (THC) content of leaves from different positions on the hemp (Cannabis sativa L.) plant were analyzed by gas liquid chromatography. High nitrogen levels reduced the THC content of leaves, and older leaves contained less THC than younger ones. There was no significant interaction between these two factors.
Hemp (Cannabis sativa L.) is a traditional and important raw material for the textile industry and is currently of interest as a wood fiber supplement in the paper industry (de Meijer and van der Werf 1994). A significant increase in cultivation of hemp in Europe is anticipated for future fiber production. Furthermore, drug-type Cannabis may play an important role in future therapeutics (Clarke and Pate 1994.). However, since one of its cannabinoid compounds is Δ 9 -tetrahydrocannabinol (THC), the psychoactive agent of the plant, its cultivation is presently limited. Breeders have developed low-THC and high-fiber content varieties, but some of these still contain a THC concentration verging on the EU limit for cultivation subsidy in Europe (de Meijer et al. 1992).
Since hemp has a high nitrogen (N) requirement, it is important to determine the relationship between N fertilization and THC content, and (for the purpose of analytical sampling) the interaction between N fertilizer and leaf position, in relation of leaf THC content.
Cannabinoid content of the leaves is known to decrease gradually from the top to the bottom of the plant (Hemphill et al. 1980). Nitrogen content in vegetative parts of the plant has been thought to correlate positively with its THC content (Coffman and Gentner 1975, Haney and Kutscheid 1973).
Table 1. Fertilizer treatments in mg/kg soil.
Table 2. The effect of nitrogen treatment on the fresh weight and plant height of hemp and on the N content of the phytomass.
Materials and methods
The experiment was performed using 5.5 liter pots, with two plants of the variety ‘Kompolti Hibrid TC’ per pot. This high-fiber variety has a THC content (0.5-0.7%) exceeding the EU 0.3% subsidy limit. Pots were placed in a glasshouse under ambient environmental conditions. A chernozem brown forest soil from Kompolt was used (Krisztian and Hollo 1992). Treatments are shown in Table 1.
Chemicals used for the nutrient supply : anal. grade NH4NO3, KH2PO4, KCl. Doses were supplied as solutions. Date of sowing: 22 April. Date of harvesting: 13 August. The lowest N level treatment was considered as the control.
The number of replications was six. Plants were grown until the end of flowering for staminate plants. Leaf samples were collected on 13 August in the following way: every leaf of both the staminate and pistillate plants was collected, dried at 40°C for 24 hr, ground, weighed, homogenized, and stored in a refrigerator at 3°C for 90 days. Leaves were placed into three groups: (a) older leaves that occur along the middle part of stem, (b) leaves from the side branches, and (c) younger leaves that occur near the top of the stem.
THC was extracted from the dried leaves with petroleum ether for 3 hr at RT. (Hanus et al. 1981). Analyses were performed with a Hewlett-Packard 5890 series II gas liquid chromatograph. Parameters of analyses were: HP-1 capillary column, 0.3 ID x 25 m length; injector temperature 260°C ; split ratio 1:70; detector temperature 300oC. Analyses were programmed from 190 to 265°C at 15C/min with an 11 min and a 5 min internal plateau at 235°C and 260°C, respectively.
Initially, an analytically pure THC standard was not available, so areas of THC peaks were used for characterization of THC content. The peak identification of cannabidiol and THC was carried out according to Hanus et al. (1981). Subsequently, CBD and THC standards were obtained and the original data were verified with these standards. The back-calculation of the data to an absolute value is analytically incorrect in our opinion, so this determination was not made.
The statistical significance of factors presumably affecting the THC content of leaves was determined by analysis of variance. The first factor was (a.) the leaf position on the plant, and the second factor was (b.) the nitrogen treatment.
Table 3. Analysis of variance of N fertilizer experiment (in a randomly arranged bifactorial split-plot experiment with six replications).
There was a significant increase in fresh weight of shoot (80-130%) and plant height (28-39%) due to N supplimentation (Tab. 2). THC was highest in leaves near the shoot tip and on side branches, and lowest in oldest leaves (Fig. 1a). THC content of leaves of each plant part decreased in response to N fertilizer (Fig. 1b).
The decrease was significant in the case of the highest N dose (Fig. 1b). THC contents of leaves from various plant regions were significantly different, independent of the N level (Tab. 3, see: F-value of Factor “a”) The other factor, N fertilizer treatment, also had a statistically significant effect on THC content of leaves (Tab. 3, F-value of Factor “b”). There was no interaction between the two factors (Tab. 3, F-value of Factor “a X b”) in relation to leaf THC content.
Figure 1. Effect of examined factors on THC content of hemp leaves :
a.) THC content of leaves from different positions on the hemp plant (means of three levels of N fertilizer).
b.) Effect of N fertilization on the THC content of hemp leaves (means of three positions on the plant).
These experiments show that the THC content of leaves decreases with increasing N doses. This phenomenon is favorable for agricultural production, because nitrogen fertilization will increase stem yield and simultaneously decrease THC content of the plant significantly. Additional studies are necessary to determine optimal N dose/ha, time of application, fertilizer type and the lowest THC content achievable under field conditions.
Problem: A cannabis nitrogen deficiency will cause the older, lower leaves on your plant to turn yellow, wilt away and eventually die. The plant typically appears pale or lime-colored.
The yellow leaves of a nitrogen deficiency may show signs of brown, and they will usually become soft and sort of “fold” in, before possibly turning crispy but ultimately falling off on their own.
Example of cannabis Nitrogen deficiency – yellow bottom leaves. Almost all plant nutrients contain Nitrogen
Nitrogen-deficient plants often appear pale or lime-colored. The leaves on this marijuana plant don’t have obvious leaf symptoms like spots or markings, but they are pale all over the whole plant. Almost lime green. The light-colored leaves are a sign the plant needs more Nitrogen (and nutrients in general). On the flip side, plants that are receiving too much Nitrogen turn dark.
If the yellowing leaves are at the top of your plant or the yellow leaves are mostly new growth, then you probably don’t have a nitrogen deficiency. Nitrogen deficiencies usually affect the oldest, lowest leaves first, or the entire plant becomes light colored.
Nitrogen is a mobile nutrient, which means it can move throughout the plant as needed. Cannabis needs nitrogen to keep leaves green and make energy from light. All new leaves get plenty of nitrogen to make them green and help with photosynthesis. The leaves that get the most light are the newest, youngest leaves, so the plant “wants” to give those leaves priority for getting light.
If new leaves aren’t getting enough nitrogen, the plant will start to “steal” nitrogen from the older, lower leaves, so that it can give it to newer leaves. This is what causes the yellowing and wilting of a nitrogen deficiency.
It’s relatively normal for your cannabis plant’s leaves to start turning yellow towards the end of your flowering cycle as the plant becomes nitrogen deficient while creating buds.
However, if your cannabis plant is losing lower leaves fast due to yellowing (if yellowing and dying leaves is “climbing” up the plant from the bottom), especially in the vegetative stage before plant is making buds, you have a problem that you will need to fix as soon as possible.
You don’t want a nitrogen deficiency in the vegetative stage!
If you notice your lower cannabis leaves turning yellow in the vegetative stage or in the beginning part of the flowering stage, your plant may be experiencing a nitrogen deficiency which will need to be treated.
It is not good if your cannabis plant is showing signs of an advanced nitrogen deificiency while still in the vegetative stage. It’s normal to lose a few yellow leaves off the bottom of your plant here and there, especially with very big plants. But if you are losing a significant amount of yellow leaves, and the yellowing seems to be moving up the plant quickly, then you have a problem.
As a grower, you’re interested in how much nitrogen to give your plants at what time. The ratio of nitrogen to other nutrients has a huge effect on growth and bud formation.
Vegetative Stage – higher levels of Nitrogen (pretty much any plant food will do)
Most complete plant foods that you get at a gardening store contain high levels of nitrogen (N). These nutrient system tend to work well in the vegetative stage.
Some examples of cannabis-friendly one-part Vegetative nutrient systems…
Pretty much any complete plant food
Flowering Stage – lower levels of Nitrogen (use “Bloom” or Cactus nutrients)
It’s extra important to find a nutrient system with lower levels of nitrogen for the last part of your plant’s life. Many “Bloom” or “Flowering” style base nutrients are just the ticket.
Some examples of good one-part Flowering nutrient systems…
If you can’t order online and can’t find a good one-part base Bloom formula locally, you do have other choices. Though not an ideal choice, most Cactus plant foods will contain good nutrient ratios for growing cannabis during the budding stage. So in a pinch, you can use the cactus nutrients that can be found at most gardening stores.
The first cannabis plant pictured below is showing signs of nitrogen deficiency late in flowering; nitrogen deficiency in late flowering is completely normal and even desired. The last picture is an infographic about nitrogen and your marijuana plant.
It’s normal for plants to show signs of a nitrogen deficiency as the plant gets close to harvest. This is actually a good thing! Too much nitrogen can actually prevent proper budding, and can reduce the overall taste and smell of your plant. This is why all “bloom” and flowering nutrient formulas are relatively low in nitrogen.
Don’t worry about yellow leaves close to harvest! It’s normal to see a few Nitrogen-deficient leaves in the flowering stage. Nothing to worry about unless you see the yellowing leaves start climbing up the plant.
So don’t sweat it if you see your cannabis show some signs of nitrogen deficiency late in the flowering stage! Relatively low levels of nitrogen in the late flowering stage help promote proper cannabis bud development and will increase your yields!
Solution: You can find many pre-mixed nutrients from the store which contain nitrogen or you could use nitrate of soda or organic fertilizer which are both good sources of nitrogen. In fact almost all plant nutrients of any kind will include nitrogen. If you haven’t been providing any nutrient to your plants, try supplementing your regular nutrients with a bit more nitrogen and see if the plant starts recovering.
If you’ve already been using nutrients, then you probably don’t have a nitrogen deficiency. If you’re seeing the signs of spreading nitrogen deficiency even a week or two giving nitrogen to your plants through nutrients, then you need to figure out what else is causing the yellowing so you can stop it.
More About Nitrogen and Your Marijuana Plants
Sometimes you can get the signs of a cannabis nitrogen deficiency if the pH at the plant root zone is too low, even if the nitrogen is there. This is because when the pH at the roots is not right, your plant roots can’t properly absorb nutrients. If you aren’t sure about your root pH, learn more about pH & growing cannabis plants here.
Nitrogen is especially important during the vegetative stage of your cannabis plants. As your plants start flowering, they will need lower amounts of nitrogen.
Nitrogen is one of the 3 nutrients that is included in almost every kind of plant food.
When looking at plant nutrients, you’ll almost always see 3 numbers listed, like 3-12-6 or 5-10-5. These numbers represent the percentage of Nitrogen (N), Phosphorous (P) and Potassium (K) contained in the bottle. Just about all plant life on Earth needs these 3 elements to grow.
The 3 numbers on the front of plant nutrient bottles list the amount of Nitrogen, Phosphorus, and Potassium.
The very first number, “3” in the case of the picture to the right, always displays the proportion of nitrogen in this nutrient bottle compared to the other 2 nutrients (Phosphorus and Potassium respectively).
Nitrogen is in all plant nutrient formulations because it’s vital to plant processes.
Note: During the last few weeks before harvest, marijuana plants start pulling all the remaining nitrogen from her leaves as part of the bud-making process. This causes yellowing leaves starting towards the bottom of the plant. This is part of the natural flowering process and you don’t need to fight it. You may notice that marijuana leaves are yellowing in almost all pictures of marijuana plants with big buds that are close to harvest. You tend to get smaller yields from nitrogen-toxic plants with dark green leaves at harvest.
Remember: It’s Normal For Marijuana Leaves To Start Turning Yellow As Harvest Time Approaches
Occassionally a nitrogen toxicity is mistake for a deficiency. Could your plant actually be nitrogen toxic? (pictured below)
This picture shows a Nitrogen Toxicity
- Bronze or brown patches
- Brown or slimy roots
- Brown or yellow leaf tips/edges
- Buds dying
- Buds look odd
- Bugs are visible
- Curling or clawing leaves
- Dark leaves
- Drooping plant
- Holes in leaves
- Mold or powder
- Pink or purple on leaves
- Red stems
- Shiny or smooth leaves
- Spots or markings
- Twisted growth
- Wilting leaves
- Yellow between leaf veins
- Yellow leaves
This page is part of our Plant Doctor series. You can use our tool to filter by symptom and help diagnose your plant.A nitrogen deficiency causes the lower/older leaves of a cannabis plant to start yellowing, wilting, and dropping off on their own. Learn how to fix it. ]]>