Why are my cannabis leaves turning yellow? It’s a very common question, and possibly the result of a cannabis nutrient disorder in your soil. Cannabis nutrient disorders are extremely common with cannabis gardens. Indoor cannabis gardens are more commonly affected by nutrient disorders than outdoor gardens are. There are three macronutrients that must be considered when growing marijuana, (N) Nitrogen, (P) Phosphorus, and (K) Potassium.
During the different growth phases of the cannabis life cycle the plants require different concentrations of these macronutrients. During vegetative growth it is best to have high nitrogen content and low phosphorus content. For the flowering stage it is best to have high phosphorus content and low nitrogen content.
There are also a number of micronutrients needed for proper growth that must be present. Most fertilizers for soil do not contain these micronutrients because they assume it will be provided in the soil.
1. pH Level
The pH level in any grow environment can have a great effect on the outcome of the cannabis plants. pH is measured on a scale from 0 to 14, with 7 being neutral. 1 is the most acidic and 14 is the most alkaline. Cannabis plants tend to react best when pH levels are in the 6.0 to 6.5 ranges. Nutrients do not properly dissolve when the pH levels get outside the 6 to 6.5 areas, resulting in plants that have smaller leaves and do not grow very fast. The use of a digital pH meter is the best way to make sure that proper pH levels are kept.
By achieving the right pH levels, you will see the following in your garden:
- Bigger, stronger yields
- Less nutrient deficiencies
- Healthy, well-developed roots
How To Control pH Levels
- Use a pH digital meter to keep an accurate range
- Use pH up and pH down to get to the right level
- Do not mix nutrients together, instead, add them directly to the water
- Use dolomite lime if you are making your own soil
2. Iron (FE)
Iron deficiencies sometimes happen in hydroponic systems, outdoor cannabis gardens, and in indoor planting mediums. They can cause new leaves to turn bright yellow due to a lack of chlorophyll. Iron deficiencies can be seen on all new growth except for the lower leaves of the plant. Iron is important, particularly for younger tissues in the cannabis plant. Enzymes need iron to function properly, and iron is important for the synthesis of chlorophyll.
How To Control Iron Deficiencies:
- Use a foliar feeding option with a chelated iron fertilizer that also contains zinc and manganese.
- Flush the system with clean, proper pH level water.
- Add a supplement that contains Iron, Calcium and Magnesium.
3. Magnesium (MG)
Not having enough magnesium in a garden commonly occurs in all types of planting mediums and hydroponic systems. The symptoms begin in the lower leaves, where they will start to yellow and show signs of chlorosis, which is a lack of chlorophyll in the leaves. After this the leaves will curl inward and shortly after will die. Next, it moves upward to the middle and upper section of the leaves. Stems and petioles on the plant will turn purple. Magnesium is important for developing healthy veins and growing strong and healthy leaves.
How To Control Magnesium Deficiencies:
- Use magnesium sulfate and Epsom salts. Use a teaspoon of Epsom salts per every 1-gallon of water.
- Try adding worm castings to the soil
- Add dolomite lime to the medium
4. Nitrogen (N)
A lack of nitrogen is the most common nutrient deficiency found in cannabis gardens. It can first be seen when the lower leaves turn a pale green color. Soon after, the leaves will become yellow and begin to die. Plants with not enough Nitrogen often have small leaves and do not grow properly. The stems usually turn to a purple or red color. Nitrogen is important for the process of photosynthesis and the production of amino acids and chlorophyll.
Nitrogen helps keep the leaves green. The leaves that receive the greatest amount of light are the youngest or newest leaves. If new leaves aren’t receiving adequate nitrogen, the plant will start to “steal” nitrogen from the older, bottom leaves, so that it can give it to newer leaves above. This is what causes the yellowing and wilting of a nitrogen deficiency and it is always seen first on the bottom, older leaves. Plants will die quickly if nitrogen is not sufficient.
Nitrogen is most important to the plants in the vegetative phase when they are growing fast.During flowering, nitrogen levels can be decreased, but not entirely eliminated. Plants often show a nitrogen deficiency during the middle to the end of the flowering phase. This is normal. The plants become nitrogen deficient while they are focusing on the production of flowers. It is actually a good sign if plants show signs of nitrogen deficiency right before harvesting. If you look at pictures of big buds right before harvest, almost always the leaves show yellowing. This is because nitrogen levels drop during this time.
Do not make the mistake of adding more nitrogen right before harvest. Having an excess of nitrogen can decrease the size of the buds and have a negative impact on the smell and taste of the plant. If you notice this during the vegetative phase however, then it is a problem that needs addressing. Keep in mind that it is normal for plants to lose a few leaves to yellow color that drop off and die. But if you see a whole bunch of them doing it during the vegetative phase than it is something that needs to be fixed.
How To Control Nitrogen Deficiencies:
- Add a fertilizer that has a high level of Nitrogen (N-P-K)
- Put Bat Guano to the plants
- Add Fish Meal
If you are looking for Bone Meal & Bat Guano, we recommend checking out “Happy Frog’s Steamed Bone Meal Fertilizer.”
These are a few of the many deficiencies that cannabis plants commonly exhibit. With some studying of the various problems that can arise when growing weed, they can be avoided more times than not.