How to Detect Deficiencies and Toxicities in Cannabis Plants

Deficiencies occur in your plants when they are not given enough nutrients, or they are given too much of one nutrient and it causes a nutrient lockout. Your plant can show signs of nutrient toxicity if given too much of one nutrient. When an excess of nutrients is given, salt accumulates in the plant.

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Here we will go over the most common nutrient deficiencies and toxicities and how to detect them. Most deficiencies and excesses will overlap, proving it hard to pinpoint the exact cause. This is why it's always important to follow a strict feeding schedule when feeding fertilizer to cannabis plants. 

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Click HERE for Future Harvest’s feed schedules. 


 


There are 12 essential nutrients when growing cannabis. We call these macro-nutrients and micro-nutrients. There are also some non essential nutrients that will provide certain benefits as well. Click HERE to learn about the essential nutrients for your plant 


The 3 most essential nutrients are Nitrogen, Phosphorus, and Potassium. These macro-nutrients are essential to plant growth. Any fertilizer will display on the label how much of each of these nutrients is included. Click HERE to learn how to read a fertilizer label.


 

An excess, sometimes known as a toxicity, is when an excess of nutrients is given to the plant. This can result in salt accumulating.

 

 

How do you know if your plant is suffering from nutrient toxicity? 

Your plant will show signs of distress, discoloration, drooping, etc. Read on for the specific symptoms of the different nutrient toxicities!

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EXCESSES: 

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Nitrogen Excess: It’s common for growers to give their plant too much nitrogen in the flowering stage. Nitrogen toxicity can result in your plant producing smaller buds. 

How to Detect: Leaves will turn dark (dark green) and will curl downwards like a claw. Leaves may eventually turn yellow and die. You may also experience tip burn. 

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Phosphorus Excess: This can cause a nutrient lockout in your plants which can lead to deficiencies in other important nutrients (copper, magnesium, zinc, copper, and iron). A Phosphorus toxicity can be hard to detect because it is similar in appearance to a Cal-Mag deficiency.

How to detect: Upper leaves will yellow in the veins. Lower leaves will develop dark spots and curl. You may experience nutrient tip burn. 

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Potassium Excess: This can cause a nutrient lockout in your plants which can lead to deficiencies in other important nutrients (Manganese, zinc, magnesium, and iron). It can present similar to a cal-mag deficiency. 

How to Detect: Leaves will have tip burn. The top leaves will grow thin blades. The lower leaves may develop dark spots and curl. 

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Magnesium Excess: Magnesium excesses are rare. When they do happen they will cause a calcium lockout. 

How to Detect: Look for signs of a calcium deficiency. Dark spots on new or growing leaves. Older leaves may die and new leaves will grow in small. Leaf tips may curl and brown spots will develop. 

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Sulfur Excess: High levels of sulfur can increase the salt content of your growing medium. An excess will accumulate in the soil and cause salt damage to your plant. 

How to Detect: Leaves may turn dark and growth will be stunted.

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Calcium Excess: Calcium excess is rare but can cause lockout of other nutrients if it does happen. 

How to Detect: Nutrient tip burn will occur. Signs of a magnesium or potassium deficiency may present. 

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How do I know what deficiency my plant has?

Each nutrient deficiency will come with specific physical symptoms. However, many of the symptoms are similar. Read on to find out which nutrient deficiencies cause what symptoms. If you suspect your plant is suffering from a deficiency, you should narrow down the possibilities of what could be causing it and start to treat it as soon as possible. 

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DEFICIENCIES: 

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Sulfur Deficiency: When growing in soil a sulfur deficiency is very rare. It is more common with hydroponic systems. It may present like a nitrogen deficiency.

How to Detect: Older leaves turn light green. Stem may take on a purple hue. As it progresses, leaves may turn yellow. 

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Magnesium Deficiency: Magnesium deficiencies are common in soils with a low pH (7 or lower). 

How to Detect: lower leaves will turn yellow and eventually dark green. Eventually, as the deficiency worsens, younger leaves will begin to be affected. After awhile, the leaves will curl upwards and fall off. 

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Phosphorus Deficiency: A phosphorus deficiency can leave the plant vulnerable to pests and diseases. 

How to detect: leaves will be slow growing and become blue / green with brown spots. The stems might turn a purple color and the leaves will curl downwards. 

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Potassium Deficiency: Because potassium helps the plant protect itself against pathogens, a potassium deficiency can result in the plant being vulnerable to diseases. 

How to Detect: Older leaves will turn yellow or brown and the edges will appear as if there is nutrient burn. Both yellowing in the middle of the leaves and burnt looking edges is a sign of a potassium deficiency. 

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Nitrogen Deficiency: causes loss of leaves and growth stunting. A nitrogen deficiency will affect the lower leaves first. Sometimes while your plant is flowering, it may have some leaves turn yellow as it uses its nitrogen to create buds. Keep an eye out for yellow leaves ascending your plant in the vegetative stage as this is a sign of nitrogen deficiency.  

How to Detect: lower leaves will turn yellow and/or fall off. Your plant will be pale in color (light green). 

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Calcium Deficiency: Calcium deficiencies will appear on leaves that are either new or still growing. 

How to Detect: Dark spots on new or growing leaves. Older leaves may die and new leaves will grow in small. Leaf tips may curl and brown spots will develop. 

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Read our customers most common deficiencies HERE

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While using our nutrients, following our feed schedules will produce best results. However, they are merely a guide. Every plant, grower, and grow is different. Please tweak, change, and manipulate our feed schedule and our nutrients to work for you!
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With excesses we recommend scaling back the nutrient by 10-20%. From there, you can adjust accordingly. 
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With deficiencies, first we recommend making sure you do not have anything causing a nutrient lockout which would have to be addressed first before upping doses. If your plant is in need of more nutrients, up your nutrient feed by 10-20% and scale back or up from there. 
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Always read your plants. They will tell you when they are stressed or lacking proper nutrition. 

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Monitor your pH and PPM closely! We recommend testing these every feeding. 
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When your pH is out of whack, your plant can’t always absorb the nutrients it needs. A pH outside of the optimal range, can result in nutrient lockout. Even with pH buffered nutrients, it is still important to monitor your pH. Other factors, such as soil alkalinity and water acidity can affect your pH even with pH perfect fertilizer. 
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For best practice, flush with straight pH water every third feeding. Some plants can be watered every day, but with others it won’t be necessary. When growing in soil, stick your finger into the soil 4-5cm deep. If the soil is dry, it is time to water. 
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Remember, a young plant will take up less water than a mature one. Small plants might only need to be watered every 2-3 days. 

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How to tell when your plants need to be fed or watered:

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  • Drooping leaves can be a sign of under watering. However, they can also be a sign of over watering. In the case of over watering, your plant should still have green leaves while in most under watering situations the plant will look sickly. 
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  • Yellow or brown leaves can be a sign of under watering. However, it is completely normal for leaves to turn color during flowering. If you have yellow or brown leaves during vegetation, your plant is likely suffering. 
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Under watering is better than over watering! Not only is under watering easier to correct, but when your under watered plants are showing signs of distress it should only take a few hours to perk them back up.
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If your plants have been put under a significant amount of stress, we recommend using our additive Super B+! It is a powerful vitamin supplement for plants designed for stress relief. 

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Temperature and humidity levels are another important aspect to cannabis growing. Many growers will have unhealthy plants due to growing outside of the optimal temperature range. 
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What temperature should I grow cannabis at?

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When growing cannabis you’ll want to monitor your temperature closely. Temperature changes can stress out your plant. 

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Temperature: 

- During the day (lights on) we recommend keeping your grow between 24-27°C (75-80°F)
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- At night (lights off) we recommend keeping your grow between 20-22°C (68-72°F)⠀⠀⠀⠀⠀⠀
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Always closely monitor the temperature and humidity levels in your grow room to ensure your plant is growing in the optimal conditions. When growing hydroponically, you’ll also want to monitor the temperature of your nutrient solution. (Keep it between 17 and 22°C / 63-72°F)

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What humidity levels should I grow cannabis at?

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For clones and seedlings keep humidity at 65%-80%
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During vegetation, keep the humidity between 55-70%
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During early flowering, humidity should be between 40-55%
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Plants in late flowering are most comfortable with a lower humidity. Keep it between 30-40% 
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Every plant is different. Different strains can react differently to humidity levels and temperature. Always monitor your plant and adjust accordingly. 
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When growing outside you have less control over the temperature and humidity levels. You can help your plant not dry out by watering it as often as it needs and using a foliar spray. Click HERE to read about foliar feeding 
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Planting in the correct season to grow outside will also be beneficial. You don’t want to plant too early or too late and have your plant suffer from frost. If you are growing in a glass greenhouse you can control the heat and humidity levels with fans, air conditioners, or dehumidifiers. 
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Lighting:
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Another common mistake with growers is their lighting! If you are growing inside, your light will affect how well your cannabis plants grow. You can easily burn them if the light is too close or too hot. Rotating and switching your plants is recommended so they all grow evenly and receive the same amount of light. 
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Monitor your plants closely. As they grow, you may have to move the light up, or have your plants on the ground. The leaves closest to the light will show you signs of burning first. 
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Cannabis can be an easy plant to grow but it is also temperamental and can get stressed out easily! Growing a healthy plant that produces high quality buds and a big yield requires constant care and attention. Your plant will always tell you what is wrong with it, it’s up to you to notice the signs. 

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Helpful tip! Take photos of your plants daily. This way, if the coloring starts to look off, you can refer back to when it started. The earlier you catch a deficiency or toxicity, the better!

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Any other questions? Email us at marketing@futureharvest.com or send us a message on Instagram @futureharvestdevelopment and we’ll help you as best we can!

    Published by Future Harvest

    Bio:
    Future Harvest | Grow Better |

    E-Mail: marketing@futureharvest.com

  • Aug 31, 2020
  • Category: Articles
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