Cannabis plants can suffer various deficiencies, excesses and nutrient lockouts throughout their cultivation, both indoors and outdoors. They require a number of specific nutrients to successfully develop. They could be dying at faster than normal rates, changing color in strange ways, withering, or curling under or over. Generally speaking, leaf death is the result of one of three things: the pH level being off, you’re watering being too much or too little, or the wrong amount of nutrients (either toxicity or deficiency).
Different problems in your marijuana plant will be evident in a variety of ways on your plant’s leaves, so it’s important to understand how these two things correlate.
To diagnose nutrient deficiency symptoms, we usually separate all the mineral nutrients into two groups depending on its mobility. If the element is mobile, it can move from one part of the plant to another as needed. When there is a nutrient deficiency, we see symptoms on older or lower leaves first because the plant will transport the elements to support its younger tissue.
If the element is immobile, younger leaves or new tissues will show the deficiency symptoms first because the elements will remain in the older tissues. The growing medium plays a big part in whether you are overwatering or under watering. For example, coco coir or other soil less media could cause a drainage issue rather than it being the fault of your watering routine. It’s a good rule of thumb to have a fifth of the water you put in come out from the bottom of the pots your plants are growing in. Once this has been done, you should wait until the top inch or so is dry before you water again. Poking your finger into the soil until the first knuckle is a good way to check if the top layer is dry enough.
The elements that are mobile include nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), molybdenum (Mo) and nickel (Ni).
The elements that are immobile are sulfur (S), calcium (Ca), iron (Fe), manganese (Mn), zinc (Zn), boron (B) and copper (Cu).
Macronutrients (N, P, K, Ca, Mg, and S) are the elements that plants use the most, and therefore are the nutrients most often found deficient. Other micronutrients (Fe, Mn, Cu, Zn, and B) also cause abnormal growth that are relatively easy to observe.
Nitrogen (N) deficiency: yellowing of the leaves start showing in the older leaves near the base of the plants. If the condition persists, these leaves become completely yellow and fall off the plants.
Phosphorus (P) deficiency: different from nitrogen deficiency, phosphorus deficiency does not turn the leaves into yellow color. On the contrary, leaves turn dark green and even purple color from the margins. The petioles also turn red-purplish color.
Potassium (K) deficiency: leaves margins and tips turn yellow and rusty and may also curl and crinkle from the older leaves. Stem branches may increase and plants become slender and weak.
Calcium (Ca) deficiency: yellow-brown irregular spots start developing on young leave margins. Young leaves may also appear curled and deformed.
Magnesium (Mg) deficiency: a classic magnesium deficiency symptom is leaves yellowing between the veins, while the veins remain green. Irregular yellow-brown spots may appear on older leaves, and eventually lower leaves curl and drop.
Sulfur (S) deficiency: young leaves turn yellow and plants become short and small. Similar to nitrogen deficiency but starts with the younger leaves.
Boron (B) deficiency: yellow-brown necrosis spots show between leaf veins on younger leaves and terminal buds. New stems and growing shoots grow abnormally and burned. Leaves may become thick and brittle.
Iron (Fe) deficiency: leaves yellowing between the veins start with younger leaves. Very similar to magnesium deficiency, but iron deficiency (being an immobile element) starts with young leaves and magnesium deficiency starts with older leaves.
Copper (Cu) deficiency: young leaves become twisted and deformed. Leaf tips and margins become dark green and contain necrotic spots.
Manganese (Mn) deficiency: a typical deficiency symptom is yellowing between the veins with the development of small necrotic spots. It is relatively uncommon for cannabis plants.
Zinc (Zn) deficiency: new shoots and young leaves become very small and clustered. Yellowing between veins start from the tips and margins of the young leaves. Internode spacing (the length between two nodes on the stem) become short and restricted.
In fact, some pH level fluctuation helps maintain your plant’s ability to absorb different types of nutrients at different rates, improving the overall health of your marijuana plant and helping to avoid marijuana leaf issues. The key is to always maintain and manage the pH level of the soil at your plants’ roots. Do this by testing the solution of nutrients and water before feeding it to your plants. This is quite easy to maintain with pH Up/Down. Keep adjusting and testing until the pH level is right.
Some people prefer to use more natural means of adjusting the pH level. You can lower the pH level by using vinegar, and you can raise the pH level by whisking the water (and therefore adding oxygen). You can also mix dolomite lime into the soil to bring up the pH level (and also add magnesium and calcium).
Remember, a healthy pH level is between 6 and 7. Problems arise if the pH level leaves these bounds. Also, a range between 5.5 and 6.5 is better if you are growing your marijuana plants hydroponically.