Problem: Calcium is an important nutrient that helps provide structure to the cannabis plant and helps it withstand stress like heat. When your cannabis plant has a calcium deficiency, the main symptom that you’ll be able to notice is brown or bronze splotches or spots on your leaves. It is also often associated with yellowing (though leaves don’t always turn yellow). Occasionally the splotches may appear purple, especially under LEDs. Calcium deficiencies are often caused by low pH at the roots, but also occasionally from growing in soft/filtered water.
Calcium deficiencies are often associated with yellowing in addition to the brown spots, (though not always!). This is an example of the beginning of a calcium deficiency that has yellowing in addition to the brown spots.
Calcium deficiencies are common when growing in coco or using soft or filtered water, but the most common reason to see a calcium deficiency is low pH at the roots.
A cannabis calcium deficiency can sometimes be difficult to diagnose since calcium deficiencies are often accompanied by magnesium, iron, and/or other cannabis deficiencies.
Under LED grow lights, calcium deficiencies may even appear purple at first. Later in the flowering stage, it’s normal for leaves to become more colorful, but if all the hairs are still white it’s too early for leaves to be discolored (leaves that lose their green too early can’t photosynthesize as well which can reduce yields). Normally when you see something like this it’s because the pH is too low.
Calcium moves relatively slowly through the plant (it is a semi-mobile nutrient), which means it tends to “stay put” after it’s been given to a leaf. It tends to show up on leaves that are actively growing and getting some amount of light.
Calcium deficiencies most often show up in the following places:
Newer growth (upper leaves)
Parts of fan leaves that have been exposed to the light
Calcium deficiencies tend to appear on newer or growing leaves, which means calcium deficiencies first appear on leaves where there’s rapid vegetative growth.
Some of the most noticeable signs of a calcium deficiency will appear on newer or growing leaves which may display:
Spotting / Mottling
Small brown spots
Small or distorted new leaves
Affected leaves may appear green besides the spots
Other Symptoms of Calcium Cannabis Deficiency
If a cannabis plant is affected by a calcium deficiency for too long, it may begin to show the following symptoms due to the lack of calcium.
Stems become weak or flimsy and may crack easily
Stems become hollow or show inner signs of decay
The plant does not stand up well to heat
Flowers/buds do not develop fully, or development is slow
Roots appear weak or under-developed
In severe calcium deficiencies, parts of roots may even die off or turn brown
Roots are more susceptible to root problems like slimy root rot
Cannabis tends to like high levels of calcium, so it is unusual to feed too much calcium when using normal amounts of nutrients and/or regular soil. There are not many known cases of cannabis calcium toxicity (too much calcium), however too much calcium can cause the plant to lock out other nutrients, so it’s important not to go overboard.
Calcium deficiencies are more likely to appear when…
Grower is using filtered or reverses osmosis (RO) water to feed plants – the amount of calcium found in tap water varies, but some tap water has enough calcium to prevent calcium deficiencies
PH is Too Low – If grow medium is acidic (below 6.2 pH, Calcium can get locked out even if the Calcium is physically there)
Growing cannabis in hydroponics or coco coir without supplementing extra Calcium (it’s recommended to use a Calcium supplement in hydro or coco)
When growing in soil that hasn’t been supplemented with calcium (usually from dolomite lime)
Too much potassium can also sometimes cause the appearance of a calcium deficiency
Outdoors – calcium deficiency is more likely to appear in acidic soil (below 6.2 pH)
Different strains of cannabis tend to have different nutrient problems. Some cannabis strains (or even specific plants) tend to use much higher levels of calcium than others, and so you may see calcium deficiency problems with one plant even when all the other plants (which are getting the same nutrients and environment) aren’t showing any signs of deficiency.
Your cannabis plant may show signs of a calcium deficiency if the pH at the roots is too low. That is because when the pH of your root zone is too acidic, your cannabis cannot properly absorb calcium through its roots. Therefore the first step is to ensure that you have the correct pH for your growing medium.
Please note: After a calcium deficiency is cleared up, the problem (brown spots and unhealthy new leaves) will stop appearing on new growth, usually within a week. Please note that leaves that have been damaged by a calcium deficiency will probably not recover or turn green, so you want to pay attention to new growth for signs of recovery.
In soil, calcium is best absorbed by the roots in the 6.2 – 7.0 pH range (in soil, it’s generally recommended to keep the pH between 6.0 – 7.0, but calcium specifically tends to be best absorbed above 6.2)
In coco or hydro, calcium is best absorbed by the roots in the 6.2 – 6.5 pH range (in coco/hydro, it’s generally recommended to keep the pH between 5.5 – 6.5, but calcium specifically tends to be best absorbed above 6.2)
If you suspect your growing cannabis plant has a calcium deficiency, flush your system with clean, pH’d water that contains a regular dose of cannabis-friendly nutrients that includes calcium. This will remove any nutrient salts that may be affected the uptake of calcium and help restore pH to the proper levels.
To supplement with extra Calcium… (it’s very rare to give a cannabis plant too much calcium, however, too much calcium can lock out other nutrients so don’t go overboard)
Calcium and magnesium deficiencies often appear together in cannabis. Many growers decide to purchase some sort of Calcium-Magnesium (often called Cal-Mag) supplement for their grow room in case this common deficiency appears.
After supplementing with Cal-Mag and correcting the pH, you should expect to see new healthy growth within a week. Remember, the old leaves will probably not recover, but new growth should be green and healthy.