October 05, 2020 9 min read

By George Mouratidis

Like every other living organism, marijuana needs "food" to survive. If your cannabis strain doesn't get the nutes it needs, it won't produce high-quality weed.

Thankfully, cannabis has evolved many "warning signs" to let home-growers know when it's extra hungry. Unfortunately, it's pretty tricky to determine what weed wants once it starts changing colors.

We won't lie: it can be challenging to diagnose a nutrient deficiency. Warning signs of deficiencies often overlap with other issues like overwatering, improper pH, and cold temperature.

It takes a keen eye to differentiate between the many problems pot could be signaling. There are, however, a few key strategies anyone could use to gauge their ganja's strange symptoms.

The Primacy Of pH Levels

We can't talk about nutrient deficiencies without first mentioning pH. Short for "potential of hydrogen," pH helps cultivators determine their grow medium's acidity or alkalinity.

The pH scale goes from 0 – 14, with 0 equivalent to battery acid and 14 to bleach.

Cannabis plants thrive in a grow medium that's slightly acidic.

If you're using soil, you should keep pH between 6.3 – 6.8 at all times. Hydroponics growers should aim to maintain a pH between 5.5 – 6.1.

Thankfully, it's not difficult to check or adjust the pH. There are many pH swabs and electronic scanners you could find at any gardening store. You could also find many pH adjusters that could raise or lower your medium's pH.

Remember, cannabis can only absorb nutrients in a tight pH range. Even if you believe your plant has a nutrient deficiency, you must always triple-check your pH levels. You'd be surprised how often improper pH levels go hand-in-hand with nutrient deficiencies.

Hydroponics vs. Soil – Which Is Better For Nutrient Absorption?

A big positive associated with using soil is that it already has plentiful nutrients.

While this doesn't mean you can't run into nutrient deficiencies when growing in soil, it does mean you have more room for error. Plus, soil cultivators won't have to spend as much on supplemental nutrients versus hydroponics growers.

While novice growers often prefer the "buffer zone" soil offers, that doesn't mean you should write off hydroponics. Indeed, now that there are many automated hydroponics units, it has never been easier for home-growers to maintain a healthy pH.

If you'd like to learn more about the pros and cons of hydroponics, be sure to read through this previous blog post. You could also check out the impressive range of hydroponics units now available in Everything But The Plant's catalog.

How Do Your Minerals Move? – Mobile vs. Immobile Nutrients

Before we get into specific nutrient deficiencies, it's essential to distinguish between mobile and immobile nutrients. Knowing whether a chemical is mobile or immobile is the first step towards making a proper diagnosis.

The terms "mobile" and "immobile" are self-explanatory: nutrients can either "move" within the plant or not. If your plant has a mobile nutrient deficiency, you'll probably see warning signs first appear on old leaves near the plant's bottom. On the other hand, immobile nutrient deficiencies usually appear all at once in new leaves.

As you're scanning your plant, note where the symptoms appear and whether they resemble a mobile or immobile issue. For easy reference, here's an overview of the "mobility" of significant nutrients:

  • Nitrogen: Mobile
  • Phosphorus: Mobile
  • Potassium: Mobile
  • Magnesium: Mobile
  • Zinc: Mobile
  • Calcium: Immobile
  • Boron: Immobile
  • Copper: Immobile
  • Iron: Immobile
  • Sulfur: Immobile
  • Selenium: Immobile
  • Manganese: Immobile
  • Silicon: Immobile

Diagnosing The "Big Three" – NPK Deficiencies

The three most significant nutrient deficiencies you need to watch out for are nitrogen, phosphorus, and potassium.

Often abbreviated NPK, these three macronutrients are the key ingredients in any cannabis strain's diet.

Since these three nutrients play the largest role in marijuana growth, all home-growers should learn the warning signs of deficiencies.

"Yellow Fever" – Signs Of A Nitrogen Deficiency

Of the "big three" nutrients, nitrogen deficiencies are the most common. You should keep your eyes peeled for this deficiency at all growth stages, but especially during the vegetative stage.

The telltale sign of a nitrogen deficiency is vivid yellow leaves on the plant's bottom layer. Since nitrogen is a mobile nutrient, this warning sign will first appear on your oldest leaves. As this disease progresses, the yellow leaves will work their way up the plant.

In addition to the yellow leaves, nitrogen deficiencies will stunt your plant's growth. You might also notice a sharp reduction in the new foliage that appears on the top of your plant.

If you suspect low nitrogen levels, consider adding a fertilizer like bat guano to your soil mix. Those who are growing hydroponically could add more of their vegetative nutrient formula.

Typically, it takes one week before you'll notice your plant recover from a nitrogen deficiency. Just be forewarned: you can't save any leaves that have already turned yellow. You could either let these leaves fall naturally or snip them off of your plant.

"Purple Haze" – Signs Of A Phosphorus Deficiency

Like nitrogen, phosphorus is a mobile nutrient, which means deficiencies first appear on old leaves near the bottom of your plant.

Unlike nitrogen, this rare deficiency first appears during the flowering phase. Affected leaves will most likely take on a bluish tinge, and vertical growth will halt. Another warning sign of phosphorous deficiency is that branches and stems turn a deep purple.

As phosphorus deficiency gets worse, you might notice copper or purple blots start to appear on old leaves.

These symptoms will travel upwards if you don't address this deficiency promptly. FYI: phosphorous deficiencies also make your plants more susceptible to harmful fungi and micronutrient issues.

Interestingly, adding more phosphorus isn't the ideal first strategy for fixing this deficiency. Instead, expert cultivators recommend double-checking your pH level. Plants could only absorb phosphorus within a narrow pH range.

Please ensure your soil's pH is between 6 – 6.5 for optimal phosphorus absorption. Only after you've tweaked your pH should you add more flowering nutrients or phosphorus-rich additions like worm castings.

A Bad Burn – Potassium Deficiency

Potassium is another rare nutrient deficiency, but it's not unheard of during the flowering stage. Since potassium is mobile, you should be on the lookout for symptoms on older leaves near the plant's bottom.

Unfortunately, it's downright difficult to determine whether you have a potassium deficiency until it reaches its final stage. This is because early symptoms (e.g., dark leaves and spots) could mimic many other conditions.

The most obvious sign of a potassium deficiency is a strange "burn-like" discoloration. Unlike a standard "leaf burn" that affects the tips of a plant's leaves, "potassium burn" leaves rusty scars around the edges of your leaves.

One standard solution for potassium deficiency is to give your cannabis a high-quality flush.

Often, potassium deficiencies occur when there's too much sodium in your soil. Flushing out this excess salt could be all you need to relieve these symptoms. If you're growing in soil, you could also add chicken manure or seaweed to boost potassium levels.

Should You Worry About Micronutrient Deficiencies?

Just because there's "micro" in the name doesn't mean cultivators should ignore micronutrients. True, plants don't need as much of these nutrients compared with NPK, but they still play a significant role in plant development.

However, if you're growing your plants in a nutrient-dense soil mix, chances are slim you’ll run across a micronutrient deficiency. Micronutrient deficiencies are more common in hydroponics, especially if you're using nutrient-depleted water like reverse osmosis.

The best way for hydroponics growers to avoid micronutrient deficiencies is to use tap water. Although tap water might seem "impure," it has higher traces of the micronutrients your plants need. Trust us, using tap water will save you a ton of time, aggravation, and money.

Even though micronutrient deficiencies are rare, that doesn't mean they're impossible. Here are two of the most common micronutrient deficiencies cannabis cultivators should know about.

How To "Spot" A Magnesium Deficiency

Magnesium deficiency isn't all that frequent, but it's more common in a plant's vegetative stage. Like nitrogen, magnesium deficiency first appears as yellow leaves near the bottom of the marijuana plant. What distinguishes magnesium from nitrogen deficiency are tiny brown "spots" that form on the affected leaves. Magnesium-deprived leaves will eventually curl at the tip and die off.

The simplest way to treat a magnesium deficiency is to mix a few teaspoons of Epsom salt with water and feed it to your plants. Keep re-applying this mix until you notice healthy growth resuming.

Also, it would help if you double-checked the temperature in your grow room. Plants have more difficulty absorbing magnesium at lower temperatures, so this could be contributing to the deficiency.

The "Silver Sign" Of An Iron Deficiency

Iron deficiencies aren't as common or deadly as other disorders, but they are one of the easiest problems to diagnose. The prime sign of iron deficiency is a silvery-white discoloration that gradually works towards the tips of your leaves.

In most cases, this "deficiency" is caused by poor iron uptake. Please check that your pH isn't too high, as cannabis plants have difficulty absorbing iron at high pH levels. Iron deficiencies could also be related to overwatering or feeding your plant too much phosphorus.

What Else Could Cause Dope To Droop?

Unfortunately, many cannabis symptoms mimic nutrient deficiencies. If your plants aren't responding to pH adjustments or increased nutrition, you could be dealing with one of the following issues.

Light Burn Is A Bummer

"Light burn" is the “skin burn” of the cannabis world. As the name suggests, this condition occurs when indoor plants are too close to the light source and begin "roasting."

Unsurprisingly, this condition first affects leaves closest to your grow light. You'll notice affected leaves turn yellow and develop brown spots on their tips. These tips could curve upwards as light burn becomes more serious. Light burn could also cause your buds to turn bleach-white if not addressed during the flowering stage.

The easiest way to treat light burn is to move your light further away from your plants. If space is limited, you should experiment with low-stress training like bending to keep your plants further away from the light. You could also look into grow lights on Everything But The Plant that have dimmer options.

Temperature Control – How Climate Affects Cannabis

Climate plays a crucial role in influencing the appearance of your cannabis plants. In general, cannabis performs best in subtropical regions with warm temps and moderate humidity. If your grow tent's climate veers out of this range, you'll probably see warning signs spring up all over your plant.

If your grow tent's temperature is too high, you might notice fan leaves curling upwards on their sides. Overheated plants are also more susceptible to "light burn," especially near the plant's top. For tips on keeping your grow tent cool, we strongly suggest reading this detailed blog post.  

On the opposite extreme, plants that are too cold will appear droopy with wilted leaves. Cold cannabis plants are also more prone to magnesium deficiency, which causes symptoms like yellowing leaves and brown splotches.

Although every strain has slightly different preferences, you should aim to maintain daytime temps between 68° F – 78° F. As for humidity, try to keep these levels at around 60 percent during vegetation and 40 – 50 percent during flowering.

If you're having trouble maintaining these temps, then you might have issues with your grow tent's insulation. You could read more about advanced insulation strategies on this previous guide.

By the way, Everything But The Plant offers dozens of the highest-quality indoor grow tents. No matter what size or style you're looking for, we bet there's a tent in our catalog that could squeeze into your grow space.

Watch Your Water – The Dangers Of Overwatering

Overwatering is a common issue that beginner cultivators struggle with. This is especially the case if you're growing in soil. If you're still new to growing cannabis, please remember "less is more" when watering your weed.

If cannabis plants are overwatered, you'll notice the fan leaves start to sag and slowly fall off. Unlike many nutrient deficiencies, these affected leaves remain green and will show minimal signs of wilting. Other potential symptoms of overwatering include mold infestations and stunted growth.

If you suspect you're overwatering your cannabis plants, hold off for a few days and continuously monitor your soil's moisture level. When you feel your soil is slightly dry, try adding about ½ the amount of water you'd typically use. This is also an excellent opportunity to double-check your pot's drainage system.

If you're having a difficult time choosing when to add water to your soil, you might want to invest in a soil water meter. These handy devices will give you an accurate read on the amount of moisture in your soil.

Too Much Of A Good Thing – The Consequences Of Overfeeding Cannabis

Like overwatering, it's common for novice growers to "overfeed" their weed. Believe it or not, too many nutrients could cause as much damage as underfeeding them.

If you're giving your plant too many nutrients, you'll probably notice what's called a "nute burn" (i.e., the tips of your leaves will turn copper or brown). As the disease progresses, these leaves will start to curl, and the discoloration will move inwards.

The easiest way to deal with a nute burn is to flush your cannabis plants for a few days and cut your nutrient feeding schedule in half. Throughout this process, be sure to keep close tabs on your pH levels to make sure your roots can absorb the nutrients you're using.

If you need to add high-quality nutrients to your grow room, please don't forget to check out Everything But The Plant's catalog.

A Final Warning: Please Avoid "Nutrient Neurosis"

Diagnosing nutrient deficiencies can be extremely stressful, especially for new home-growers. As a final word of warning, please remember that overfeeding your plants is just as harmful as underfeeding them. We understand how easy it is to "panic dump" nutrients at the first sight of an issue, but you first need to eliminate other problems like pH imbalance, overwatering, and light burn.

Scanning for nutrient deficiencies should be a routine part of every cannabis cultivator's schedule. As you gain more experience growing various strains, you should start to recognize the warning signs of common disorders.


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