Nutrient deficiencies in cannabis: How to spot and address them

As a cannabis cultivator, you want to grow the healthiest cannabis plants with the highest yields possible. Giving your plants the right balance of vital nutrients is key to good cannabis cultivation. Not only must you give your plants essential nutrients, you must also give them the correct amount of nutrients — not so much that they suffer nutrient burn but not so little that they suffer nutrient deficiencies.

Nothing slows down strong plant growth faster than a nutrient deficiency. In serious cases, nutrient deficiencies will leave cannabis plants more vulnerable to attacks from pests and mold. Severe nutrient deficiencies can ultimately result in the death of the plant. And while the best approach is to focus on prevention, there are also many ways to identify and treat different types of nutrient deficiencies in your cannabis plants to encourage new growth and better overall plant health.

Photo by: Gina Coleman/Weedmaps

This guide to cannabis nutrient deficiencies will show you how to pinpoint and address deficiency symptoms to keep your cannabis plants in top condition.

What causes cannabis nutrient deficiencies?

Improper pH levels are a common cause of nutrient deficiencies in cannabis plants. Even if your soil contains all the nutrients your plants need, out-of-whack pH will prevent the roots from absorbing the needed fuel. Cannabis does best in a slightly acidic environment. Aim for a range of 5.8 to 6.8 soil pH, with the midpoint of 6.3 considered optimal. But it's not just the soil pH level you need to monitor. Make sure to test the pH levels of your water as well. For hydroponic solutions, the ideal pH falls between 5.5 and 6.5, with the optimal level differing depending on the product. 

Signs of a healthy cannabis plant

Just as important as knowing how to spot nutrient deficiencies is knowing what a healthy cannabis plant looks like. This gives you a standard to keep in mind and to compare your cannabis plants against to make sure they are strong and healthy.

To assess the health of your cannabis plants, look for these key indicators:

• Green leaves free of spots and other discoloration, such as yellowing between leaf veins
• Sturdy plant structure with no apparent drooping
• Prolific, resinous buds at harvest time

Photo by: Gina Coleman/Weedmaps

If your marijuana plants exhibit these three key indicators of good health, it is unlikely that a nutrient deficiency exists. On the other hand, there are also clear warning signs that make it easy to spot cannabis deficiencies early on. The key is identifying nutrient deficiencies and adjusting the way you feed your marijuana plants before any cannabis nutrient deficiencies do permanent harm.

How to spot and diagnose a nutrient deficiency

Marijuana plants can't tell us what's wrong verbally (unless you smoked way too much) but cannabis deficiencies will present themselves if you know what to look for. Deficiencies tend to produce easily observable symptoms, so pay close attention to how your plants communicate to you.

Cannabis plants require many of the same essential nutrients to thrive that humans do. Just like us, they need calcium, iron, and potassium. But a nutrient deficiency in a marijuana plant will present in different ways. Whereas a lack of calcium can make our bones brittle, such a deficiency can completely stunt the growth of cannabis plants and ultimately lead to death. Yellowing leaves are a universal sign of vitamin and mineral deficiencies, but lack of each nutrient may cause additional symptoms to appear. It's possible that nutrients are missing from the grow medium or water but the more likely cause for any of these is a pH problem preventing the plants' roots from accessing existing nutrients.

Here are three general nutrient deficiency symptoms to watch out for, as well as possible diagnoses for each symptom.

Discoloration

Lightening and/or yellowing of leaves, particularly near the base of the plant, can signal a nutrient deficiency. This one can be a bit tricky to pinpoint back to a single specific deficiency, as discoloration can arise from a lack of several important nutrients, including:

• Nitrogen deficiency: Nitrogen is the primary element of the NPK (nitrogen, phosphorus, and potassium) powerhouse of macronutrients, and a nitrogen deficiency is the most common nutrient problem in cannabis plants. It presents as yellowing of older leaves and eventually the entire plant, along with curled leaves and scant buds.
• Phosphorus deficiency: Necessary for photosynthesis, phosphorus also contributes to overall plant growth and the production of resin, the plant's natural pest control. In addition to the tell-tale yellowing, marijuana plants with a phosphorus deficiency may have a dark purple hue and blackish spots on the leaves. 
• Potassium deficiency: Potassium keeps marijuana plants flowering and wards off pests, such as spider mites. Dull leaves are one of the most obvious signs of a potassium deficiency, but brown spots and brownish leaf tips that appear burnt are also common signs of a potassium deficiency. 
• Iron deficiency: Cannabis plants need ample iron for chlorophyll production and overall health. Look for yellowing between the leaf veins. An iron deficiency is often the domino effect of other types of nutrient deficiencies, namely a lack of copper, manganese, and zinc, so make sure you treat an iron deficiency right away. 
• Copper deficiency: Copper helps your plant develop healthy buds. Since copper is needed only in trace amounts, a copper deficiency is rare but not unheard of. Dark blue leaves, bright yellow or white tips, and leaves with a metallic sheen are all signs of copper deficiency. Leaves may also curl under. 
• Manganese deficiency: As with copper, a manganese deficiency is uncommon. It can result from high iron or pH levels. The first signs of a manganese deficiency appear in new growth and spread to older leaves, resulting in dead spots on all the leaves. 
• Zinc deficiency: A zinc deficiency is more common in cannabis than copper and manganese deficiencies and, again, high pH levels can be the culprit. Scrutinize the delicate leaf tips to determine if your plants have a zinc deficiency. The leaf tips will turn a burnt brown shade and eventually rotate 90 degrees to one side.

Drooping

In many types of cannabis nutrient deficiencies, leaves will curl, droop, and ultimately drop. Typically, this type of stunted growth is evidence of the following nutrient deficiencies:

• Magnesium deficiency: Magnesium is essential for adequate energy absorption from light as well as for creating carbohydrates and sugars that yield flowers. Observable as many as six weeks after development, magnesium deficiency symptoms cause rusty spots to form on the leaves and, ultimately, for the whole plant to droop. 
• Calcium deficiency: Without calcium, cells cannot grow sufficiently nor can vital nitrogen and sugars circulate through the plant. If a cannabis plant has a calcium deficiency, you'll notice curling lower leaves followed by withered root tips at an advanced stage. 

Photo by: Gina Coleman/Weedmaps

Low yield

Above all, a low yield of flowers will tell you (unfortunately after it's too late) that your plants have been suffering from a nutrient deficiency. The only way to prevent this unwanted outcome is by learning how to spot and fix a nutrient deficiency before your plant gets to the flowering stage. However, if you're already this far along and your plant has an extremely low yield,  it will often be the result of the following deficiencies:

• Nitrogen: Since it's a key macronutrient, it should come as no surprise that nitrogen deficiency affects cannabis yield by slowing. Yellowing of older leaves and eventually the entire plant are signs of nitrogen deficiency. 
• Copper deficiency: Since copper is needed only in trace amounts, a copper deficiency is rare but not unheard of. Look for dark blue or purplish leaves with bright yellow or white tips. Leaves may also develop a metallic sheen and roll under. Finally, buds are slow to develop and may not ripen at all. 
• Sulfur deficiency: Sulfur is crucial for the development of potent oils and terpenes, many of which may offer therapeutic benefits after harvest. The symptoms of a sulfur deficiency are subtle, starting with pale green to yellow leaves and culminating with slow and limited flower production. 

Photo by: Gina Coleman/Weedmaps

How to treat nutrient deficiencies in cannabis

They say an ounce of prevention is worth a pound of cure and that's true with cannabis nutrient deficiencies, too. As stated above, minding your pH is the best way to prevent nutrient deficiencies and be sure your cannabis plants are able to absorb the nutrients you're providing. But cultivating cannabis can be an experience in trial and error, especially for beginners. 

If your plants aren't in peak condition, here are some very general steps you can take to address deficiencies, promote healthy root growth, and help your plants absorb nutrients more effectively.

• Experiment with nutrient-rich potting soils and consider investing in a better quality soil if your budget allows. 
• Test the pH levels of your soil and/or hydroponic nutrient solution. Always ensure your grow medium falls within the optimal range of the ideal pH levels. 
• Move your cannabis plants if too much or too little light could be an issue. 
• Prune any damaged leaves to inhibit the spread of disease to new leaves.
• Always remember that a little extra TLC can go a long way in nursing a sick cannabis plant back to health and giving you a plentiful harvest.

If you've identified specific nutrient deficiencies and reached a clear diagnosis, here are some specific ways to treat many of the ones that harm marijuana plants.

• Nitrogen deficiency: Amend your soil with fish meal, manure, feather meal, or organic plant compost. Spray your plants with compost tea.
• Phosphorus deficiency: Increase the pH levels of your grow medium to the upper end of the allowable range, add worm castings and fish meal, and increase the temperature around your plant since cannabis has trouble absorbing phosphorus at lower temperatures. Don't overwater and be sure your soil is aerated.
• Potassium deficiency: Flush your grow medium to get rid of excess potassium, then adjust the pH, and add chicken manure to the medium. Spray your plants with a seaweed solution.
• Iron deficiency: Treat iron deficiencies by flushing your grow medium and giving your plant an iron supplement combined with a bit of nitrogen-rich fertilizer to improve nutrient uptake.
• Manganese deficiency: Flush the grow medium, cleanly cut away damaged growth, and spray your plants with a seaweed solution.
• Zinc deficiency: Zinc deficiencies can typically be treated by boosting zinc uptake through the process of adjusting pH to the proper range and avoiding too much water. It may also help to spray your plants with fish- or seaweed-based emulsions.
• Magnesium deficiency: Magnesium deficiencies can be addressed by first flushing out your grow medium with 6.0 pH water. You can also try dissolving Epsom salts into the water and feeding the solution to your plants.
• Calcium deficiency: To address calcium deficiencies, add a calcium-magnesium supplement to your plant's diet, add hydrated lime to your watering solution, and adjust pH to 6.2
• Sulfur deficiency: Try dissolving Epsom salts in the water before watering your plants and adjust your regular water to the proper pH levels.

Signs of too many nutrients

Spotting and correcting cannabis plant deficiencies is vital for maintaining healthy growth. But be careful not to overcorrect in your attempts to address a nutrient deficiency. When plants absorb too many nutrients, they can suffer from a nutrient burn, which impedes a healthy flow of water and nutrients and can do serious harm to your plants.

If you're giving your cannabis plant too many nutrients, you will notice the tips of the leaves begin to turn light brown, then become bronze-colored. Leaf tips will then become crispy and start to curl. If left untreated, these symptoms will spread from the tips of the leaves down toward the center of the leaves until the entire leaf is crispy and unhealthy looking.

If you notice signs of nutrient burn, make immediate adjustments to how you're feeding your plant. First, flush cannabis plants to get rid of excess nutrients. If you're growing in a hydro setup, flush out all water and replace it with clean water at the proper pH level. If you're growing in soil, heavily irrigate the medium to flush out all excess nutrients. From there you can resume feeding your plants with water at the right pH and a better-balanced mix of essential nutrients.