Calcium Deficiency in Cannabis & How to Fix it

Just like in humans, plants need calcium to survive. Cannabis is no exception. It isn't immune to fluctuations in calcium and, when grown hydroponically, is especially susceptible to calcium deficiency. Knowing the value of calcium to the plant's life cycle is a fundamental lesson for successful cultivation. 

What are the signs of calcium deficiency in cannabis? More importantly, what can you do to rectify the nutrient levels in both soil and hydroponic systems?

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Calcium is an essential nutrient for plant development, from seed to harvest. If nitrogen, phosphorus, and potassium are the primary nutrients, calcium is a critical secondary one. Calcium helps a seed germinate and form a healthy root system. With enough calcium, cannabis plants can also improve the intake of other nutrients through its robust network of roots. Imagine calcium as an essential building block, which helps the plant absorb, use, and create many other vital nutrients. 

As the plant matures, calcium keeps doubling down to grow a robust root system but also shores up the structure of the plant's cell walls. Stronger cell walls help plants stay healthy, even in the face of pathogens. This secondary nutrient will also help individual cells communicate with one another.

If there isn't enough available calcium for the plant to use, it can cause long-lasting impacts all the way into the flowering stage. Not only will new leaf growth suffer, but the final harvest will as well. 

Routine checks of the canopy will give you the best indication of calcium levels. Most of the signs and symptoms of calcium deficiency in cannabis begin at the canopy, affecting new growth sitting directly under direct light.

Partially shaded leaves will only begin to show signs of deficiency in the areas exposed to light. The flower calyx is another area to keep a close eye one. Use a LED Loupe to check these issues beyond what's visible to the human eye, including proper calyx development.

Some of the most common signs include:

• New growth slows over the entire plant
• Stems and branches weaken. In severe cases, stems die off and become hollow.
• New leaves begin to mottle, eventually dying off and curling under. 
• Leaves begin to spot (brown, yellow, burnt appearance), especially those under direct light. 
• In the flowering stage, flower development is distorted. Look closely at the calyces. They will appear crinkled or deflated.
• Hydroponic growers will note serious issues with root development, even root rot.

Hot tip: If a plant has a calcium deficiency, it may suffer from other nutrient deficiencies as well. When the balance of nutrients is wrong, it can impact how well the plant absorbs all vital nutrients and minerals.

As mentioned, outdoor and soil-based growers will have fewer issues with nutrient deficiencies than within a hydroponic system. Soil is much more forgiving and will help even out any nutrient issues.

However, how can you solve a calcium deficiency if it arises in your outdoor or soil-based system? The first step is to test the pH level of the soil, as acidic soil can lock out calcium absorption through the roots. Depending on where you live, acidic soil may come directly from tap water. You may also wish to test your water source. Adjust for a pH level between 6.2 to 7.0 pH, which is a range ideally suited for calcium absorption. 

A secondary test is a Caption Exchange Capacity test or CEC number, which comes from a soil test. You'll want to see a higher CEC number, which will indicate less sand and more organic matter. The organic matter helps soil retain water, and therefore nutrients. While you may not be able to correct the CEC for your current crop, it's worth fixing for your next harvest. In the current crop, you may need to increase how much you water (and add nutrients) as the soil will dry out quickly with a lower CFC.  

Once pH and CEC are within the ideal range, there are two common ways to add calcium to a calcium-deficient plant: adding nutrients to the water and using a foliar spray.

Calcium comes in many forms, including regular garden lime, dolomite lime, and pre-made cannabis-specific nutrient combinations such as calcium magnesium, calcium acetate, or some combination of all three. These may be mixed into the soil before planting or added to a watering schedule.

Foliar sprays are faster acting as they absorb via the canopy instead of the root system. Again, they come in a variety of nutrient combinations and could offer an easy fix to your calcium deficiency cannabis plants.

Most likely, if you are witnessing the signs of calcium deficiency, it is within a hydroponic system. Hydroponic systems require careful attention to every detail, including the nutrient formulas. If the nutrients are out of balance, the harvest will suffer. It's not just about the N-K-P rations. Secondary nutrients are also needed.

Just like in an outdoor operation, you'll want to test the water for pH level. Chances are, if you are growing hydroponically, you'll already have a good grasp of the pH, but it never hurts to double-check the numbers before adding additional nutrients. 

Once the pH is under control, there are two options for hydroponic growers: adding hydrated lime or a commercial 'calcium-booster' to the nutrient tank. Most growers suggest that it is nearly hard to over-do it with calcium, but that doesn't mean it's impossible. Check labels for the product to confirm ratios and follow the parts per million (ppm) guidelines.

Unless something has gone horribly wrong with the soil, most of the time, only soilless or hydroponic growers will encounter calcium deficiencies. A nutrient deficiency, if caught early enough, is thankfully not disastrous. By checking pH, possibly flushing the system, and introducing a hydroponic nutrient formula containing calcium, your grow should be back on track in no time.