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Nutrient Deficiencies in Outdoor Cannabis Plants

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Whatever the type, gardeners need to be able to identify plants that are sick. They are able to take a look at their plants and determine exactly what is wrong. This is even more important for marijuana growers as they are not usually able to seek advice from others to help them.

You should not just be able to diagnose the problem and correctly, you must also have the knowledge and tools to remedy all that makes your plants sick. If you are able to do this properly, you will have a greater chance of succeeding during your first marijuana grow op.

This article about nutrient deficiencies in outdoor marijuana plants will help you improve your skills and increase your chances of success.

Do not go with your plants?

Deficiencies are not the only possible problems your plants may have with nutrients. They can also have too much of something, including the three essential nutrients (Nitrogen, Phosphorus, and Potassium). So how to identify it?

First, consider whether what you gave to feed your plant was completely balanced, specifically looking at the NPK balance. After that, test the pH value of the soil and the water you gave it. If the pH levels are not perfect, correct them. If they are balanced and your plants show symptoms of poor health, then you probably have a problem that is related to the sun, as it is difficult to control.

Before being too confused or overwhelmed by these possibilities, make sure that you identify them correctly. If your plants receive plenty of sun and balanced water, there is a good chance that the problem comes from nutrients.

Identify a lack of nutrients

The most likely explanation for the poor health of your plants is a lack of nutrients in the soil, or the inability of the roots to absorb the nutrients that are present. The challenge at this time is not to actually enrich the soil as they will easily provide the necessary nutrients to your plants. In fact, the challenge is to identify which nutrients are problematic.

You can look at the different symptoms your plants exhibit, but just watching can be difficult as some nutrient deficiencies will seem the same to an untrained eye. If after reading what the different deficiencies look like you are still not sure which nutrient is missing from your plant, you can try rinsing the soil with water before doing anything else. This is because the deficiency of one nutrient can be caused by the surplus of another nutrient. Too much potassium, for example, can prevent your plant from absorbing iron. The process is as follows: rinse the soil, test the pH level, add what you think is necessary and test the pH level again.

Testing the pH level is always important, but especially when you need your plants to quickly absorb one or more nutrients. The closer the pH level is to 7, the more your plant will absorb all types of nutrients. Whatever the plant, the best way to give it a specific nutrient is to use “foliar nutrition”. It means to make a tea and spray the leaves of your plants with. Just be sure not to do that in the middle of the afternoon when the temperature is high. It’s better to do it early in the morning or just after sunset.

Continue reading in order to understand what nutrients they are missing from your plants, as well as some information on what exactly each nutrient does, why it is important and how to correct the deficiency.


Nitrogen is the nutrient that produces chlorophyll and amino acids. Chlorophyll is essential for the process of photosynthesis while amino acids are responsible for the creation of proteins, so you can understand that this nutrient is absolutely essential to keep your plant healthy

Deficiency symptoms
If your plant is lacking in nitrogen, these symptoms will be seen mostly in old leaves, starting from the base to the middle of the plant. Your nitrogen-deficient cannabis will look perfectly green on top, but will yellow the further you look down. Your plant will be most at risk of having nitrogen deficiency during the flowering phase. This is because cannabis plants store the nutrients in the leaves, and during the flowering phase they need these stocks, using everything they own.

Your must be more worried if your plant is in the growing phase. It’s simply because your plant needs green, healthy leaves to keep growing. If the leaves are yellow, they will not help this process.

How to fix it
You will need to find a way to quickly increase the amount of nitrogen your plant absorbs. Blood meal is an easy way to do this, as well as dried blood, cottonseed meal, bat guano (also called bat manure), fish meal (also known as fish emulsion name), or earthworm droppings (“manure” of earthworms). You can find each of these items in your local garden center, or else they are available on the internet.

Although the yellow leaves will never return to their green state but will simply fall out of the plant, this does not mean that your plant will not recover. If you treat it properly and quickly, your plant will recover in a few weeks, and new green leaves will replace the yolks that have been lost. Before and after adding products to the soil, be sure to test the pH level, as it could increase or decrease when you treat nutrient deficiency.


Phosphorus is mainly used to help your plant to have roots, as well as to increase the strength of these leaves and stems. It also helps seed germination making it especially important during the flowering period of your plants. Do not be tricky when you feed your phosphorus plants, it is often necessary to put large quantities.

Symptoms of Deficiency
If your plant does not absorb enough phosphorus, its growth will slow down, and it will usually appear frail and lifeless. The edges of the leaves of your cannabis plant will lose their bright green color – they will turn brown – and begin to curl up. These symptoms usually occur during the coldest days of the growing season because this is the time when marijuana plants most often have difficulty absorbing phosphorus from the soil. In addition to the cold, if the soil is too wet or too basic, the same problems will appear.

How to fix it
A larger infusion of phosphorus should be needed during cold weather. You can buy fertilizers and plant foods containing phosphorus. As long as the NPK ratio does not exceed 5, the substance will be useful for the phosphorus deficiency of your plants. There are also some all-purpose foods (Miracle-gro for example) that can also work. If you use them, you will need to use only half of what is recommended on the package – an overdose could be fatal for your plants.

Bone meal, earthworm droppings and bat guano can also bring some phosphorus. A more difficult solution would be to use crab shell or crabmeal. Many farmers use them for any problem that arises with their plants, but they have a special ability with phosphorus deficiencies. The results are visible in one week.


As with the other two nutrients of the three fundamentals, potassium is extremely important for your plant to function properly. Potassium is mainly responsible for the water respiration of your plant as well as resistance to most diseases. But not only, it is also useful for photosynthesis production and conversion processes. Finally, potassium assists the circulation of water, helping to move water throughout the plant, making it especially important for flowering and vegetation phases.

Symptoms of deficiency
Plants that lack potassium will have very slow leaf growth that may look like burnt leaves on their edges and tips. Marijuana plants that are not very stiff, that can easily be twisted or broken (by you or the wind) are probably a lack of potassium. For existing mature leaves, they may be spotted and yellow in some places (between the veins first, then on the entire leaf). These leaves will also become completely yellow then die. Because marijuana plants with potassium deficiency grow more slowly, its most harmful effect is a delay in the flowering phase.

How to fix it
Because marijuana plants simply and easily absorb potassium, this particular deficiency should be easily fixed. Even if the pH level is not perfect, they can continue to absorb the nutrient quickly and efficiently. One way to fight the lack of potassium is to add a fertilizer that has potassium in your garden center (if you already use it). If you prefer an organic method, you can also use wood ash, kelp flour, granite dust, or potassium sulphate. You should see the results in a week.

These specific deficiencies are not the only problems that your plant may encounter, but it is a first starting point to make sure your plants have the minimum level of health they need. As long as your plants have these three basic nutrients, they will be healthy enough to make a decent crop. As with every problem you may encounter when you grow marijuana, early detection is the most important factor in keeping your plants healthy and having a successful harvest.

Dr. Kanika Singla

Ph.D., IARI Postdoctoral Scholar, UC Berkeley

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