How to treat calcium deficiency/toxicity in cannabis plants

Published Mar 12, 2019 12:04 p.m. ET

Every cannabis grower’s worst nightmare is waking to plant’s covered in ugly spots that signal the plant's overall health is diving. The most common culprit responsible for yellow or brown spots on weed leaves is a calcium deficiency. Calcium is one of the essential nutrients a marijuana plant requires to have the best shot at flourishing. Calcium for weed strengthens the overall structure of the plant, the leaves, and helps it to hold up to extreme heat and parasites. A calcium deficiency in a cannabis plant can be difficult to diagnose since a calcium deficiency will often be accompanied by other nutrient levels falling dangerously low such as low iron or magnesium.

How to diagnose calcium deficiency

Calcium is absorbed slowly over time by a plant and tends to stay inside it once it is consumed and fed to a leaf. A calcium deficiency will appear mostly on the higher leaves and new growth of a cannabis plant. If you see spots on cannabis leaves located only on new growth, large fan leaves most exposed to light, and the highest sections of branches then you are likely facing a calcium deficiency. When searching for spots on cannabis leaves watch for:

  • Holes or completely dead spots.
  • Crinkling or wrinkles.
  • Unusually yellow or brown spots on weed leaves.
  • Tiny brown spots.
  • Distorted or oddly formed new leaves.
  • Stunted plant growth.
  • Curled edges or tips on leaves.
  • Dead leaves which may appear yellow or brown.
  • Leaves may appear a darker green where spots are present.
  • Weak stems.
  • Hollow stems.
  • Wilting leaves
  • Small unestablished root system.

Calcium toxicity cannabis

Marijuana plants like more than average amounts of calcium exposure, so calcium toxicity in cannabis plants is incredibly rare. However, it can and does happen and will present itself in a similar fashion as a calcium deficiency. The only way you will be able to obtain calcium toxicity in marijuana is by overfeeding your crop nutrients in a condensed form. If you think you have a deficiency but have also been feeding high amounts of condensed nutrients, you may want to perform a flush on your plant before attempting to solve the problem with more calcium which could potentially make things worse.

How to treat calcium deficiency in marijuana plants

Calcium for weed is so essential that a deficiency can completely kill a plant in a relatively short window of time. Since it usually takes a plant suffering for a great deal of time before symptoms like brown spots on leaves are severe enough to raise concerns, it is essential you act fast once you realize what is going on, but first, there are two things you should know.

Plants grown in soil- Marijuana plants grown in soil do best with a 6.0-7.8 ph range with calcium being absorbed the most efficiently at 6.2.

Hydroponics- Cannabis plants grown using hydroponics prefer an ideal range of 6.2-6.5 ph with calcium still being more effectively absorbed by the plant at 6.2.


Step 1- The first thing to do when faced with a calcium deficiency is to perform a complete flush of your system with clean water that contains an added amount of liquid calcium.

Step 2- Choosing a supplement is a trial and error task. Though most of the time it is safe to assume that if your plant is calcium deficient, it is also suffering from a lack of magnesium or iron as well. Pick a supplement that offers a broad range of additives to help your plant heal itself including any of the above-listed nutrients. Two of the most popular supplements on the market are Cal-Mag, and Dolomite Lime with Cal-Mag being suitable for any grow method including hydro and Dolomite being strictly for those growing in soil.

Step 3- Using the Nutrients

Cal-Mag- To use Botanicare Cal-Mag Plus you will need to add one teaspoon or five milliliters per gallon of water. This supplement can be used daily until your soil reaches an optimal ph balance and restores your plant’s intake of calcium to one that is sufficient for growth.

Dolomite Lime- This multivitamin is used by adding it to the water before watering and is not meant to be applied directly to the soil. You will need six teaspoons of it for every single gallon of soil. It takes around 30 teaspoons to make 5 gallons of nutrient-enriched water. This can be used to water your plants every other day. Be sure to keep checking the ph balance of your soil, so you know when you have finished the treatment.


To prevent calcium deficiency before it even begins always be sure to provide your plant with all three of the primary essential nutrients including iron, magnesium, and calcium. You can do this by using supplements to add once a week to your regular watering schedule, keeping your marijuana plant’s healthy and fed. Whenever any of these minerals are in short supply, they seem to affect the plant's ability to absorb any nutrients until the imbalance is corrected. If you are keeping them well fed and providing plenty of light and water, you can avoid ever having to experience a deficiency of any kind.


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