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  • Writer's pictureAlexis Mellons

Using Color to Detect Nutrient Deficiencies in Cannabis

Growing cannabis is a multi-sensory experience. We use our noses to witness terpene development, our hands to feel for brittle stems. Most of all, we use our eyes to monitor growth patterns, detect pests, and importantly, recognize nutrient deficiency.

Nutrient Deficiency Symptoms by Color

Just as we humans grow frail and weak in illness, so do cannabis plants. Discoloration of your cannabis plant’s stems and leaves can tell you a lot about the health of your plant, but with some basic knowledge of nutrient deficiencies, you can tackle your cannabis health issues simply and with confidence.

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Nutrient Deficiencies in Cannabis

Calcium Deficiency in Cannabis

Calcium is important to the cannabis plant throughout the entire growth cycle. Not only does it aid in germination and constructing a healthy root system, it also bolsters the structure of a mature plant, strengthening it against stressors, such as heat.

A calcium deficiency in the cannabis plant is uniquely identifiable by a brown- or rust-colored spotting on the leaves. Additionally, it can include crinkling, yellowing, stunted growth, distorted or dead leaves, and curled tips. Primarily, though, small brown spots or dead spots are your sure sign of a calcium deficiency.

These symptoms typically show themselves during the flowering stage, and almost always appear on leaves that are actively growing. For this reason, you’ll notice a calcium deficiency more so at the top of your plant or where it’s close to a light source.

When it comes to the cause and curing of a calcium deficiency, there are a couple options you should explore. If you believe you’ve given your plant the perfect blend of nutrients, your regimen is likely not the source. Rather, your soil’s pH may be the culprit. As with most nutrients, cannabis absorbs best when its soil pH is between 6.0 and 7.0; specifically, calcium is best taken in a pH above 6.2. If too acidic or too alkaline, the plant’s roots simply cannot absorb enough nutrients to keep itself healthy. Not only does this waste the nutrients you’ve sourced and purchased, but your plant (and, therefore, your product) suffers along with you. The best solution here is to completely flush your system, using the corrected pH and your standard nutrient regimen, and see how your plants react.

If you continue to see signs of calcium deficiency—or if you’re confident in your pH from the start—there are supplements you can buy. Cal-Mag is a popular option among growers, containing calcium, magnesium, and iron. Calcium and magnesium deficiencies specifically tend to appear as a unit within cannabis, so this type of dual supplement is recommended when you’re seeing signs of either.

Unfortunately, symptomatic leaves are most likely not going to reinvigorate themselves back to health. However, you should see healthy, standard growth on new leaves. With calcium—and other nutrient—deficiencies, it’s safe to give your plants about a week’s time to bounce back to health after applying your solutions. Keep a close eye on your affected plants during this time; if symptoms persist, you may have another deficiency to explore.

Iron Deficiency in Cannabis

In the cannabis plant, iron plays an important role in photosynthesis. Iron is key to the production of chlorophyll, which gives plants their lush, green coloration and allows them to absorb oxygen. Iron also assists in delivering other nutrients through the cannabis plant’s circulatory system, and it plays an important role in the growth of younger tissue.

Recognizing a lack of iron is fairly simple, as the symptoms are unique to this deficiency. In mature leaves (namely at the top of the plant), you’ll notice color loss at the base. This starts as a yellowing, but eventually, the basal section of the cannabis leaves turn completely white due to a lack of chlorophyll. New leaves will sprout as yellow or even white, and when ignored for too long, affected leaves and shoots will halt growth altogether and die off.

This is an important differentiation in nutrient deficiencies, as an iron deficiency starts at the centers, not the tips, of cannabis leaves. Also unique to iron is a plant’s ability to bounce back after a mild deficiency. Leaves that were once completely yellow can return to their saturated greens after a few days, whereas most deficiencies render leaves completely useless. This is only in minor cases, however, when iron can be added back quickly.

Iron deficiencies, comparatively, aren’t that common, and they typically indicate another imbalance. If you’re noticing symptoms, you first want to check the status of outside influences. Phosphorous, for instance, can block a plant’s absorption of many nutrients, including iron, at too high a concentration. If your soil pH is above 7, the plant won’t be able to absorb iron (and most other nutrients). Insufficient light, traumatic transplanting, excessive pruning, or harsh temperatures can also lead to iron deficiency.

Testing your pH and flushing your system, again, is a logical first step in acutely identifying the source of your plant’s problem. For those looking to supplement their nutrient regimen with iron, Cal-Mag is a popular option. As the name suggests, it contains calcium and magnesium as well. With iron, these three nutrient deficiencies typically occur together, making Cal-Mag a fix-all for common symptoms.

Magnesium Deficiency in Cannabis

Like iron, magnesium plays an important role in the development of chlorophyll, making it a key ingredient in photosynthesis and the overall health of your cannabis plant. Additionally, it helps to deliver phosphorus to other parts of the plant; importantly, magnesium aids in phosphate metabolism, an important process during the flowering stage of cannabis growth.

With a magnesium deficiency, you’ll notice yellowing at both the edges and the veins of your cannabis leaves, with possible red stems. This is an important change to notice, as a magnesium deficiency can act quickly, taking your plant down before you’re ready to fix it! As the cannabis plant pulls what magnesium it does have out of old leaves and into fresh leaves, this lack will be noticeable first in leaves located toward the bottom of your plant.

As with calcium and iron, start by checking your soil pH if you think you’re experiencing a magnesium deficiency. At the improper pH, cannabis roots cannot absorb the magnesium you’re providing. Another source of deficiency could be that you’ve added calcium, but not magnesium, typically through agricultural lime or another magnesium-rich source that excludes calcium. Cal-Mag is a great supplement for cannabis if you’re seeing signs of illness.

Phosphorus Deficiency in Cannabis

Phosphorus is a crucial nutrient throughout the growth cycle of cannabis, and is known as one of the “Big Three” (Nitrogen, Phosphorus, and Potassium). It’s used by the plant for the process of photosynthesis, and it’s a key nutrient when plants are working to create buds during flower.

A lack of phosphorus presents itself in a few unique ways, but in general, cannabis isn’t heavily prone to this deficiency. Cannabis plants seem to love an abundance of phosphorus, and standard nutrient regimens tend to include a large dose of it. However, you should watch for signs of a phosphorus deficiency, especially during flower, because it can greatly affect your final harvest if left untreated.

You can be sure your plants aren’t getting enough phosphorus if you notice a significant darkening of its leaves. Dark greens, blues, or even grays will appear, often accompanied by brown or purple-hued spotting. Bright red or purple stems can be another sign your plant needs phosphorus; however, this isn’t always the case, and you shouldn’t rely upon this alone to determine your deficiency. Beyond variations in color, you may notice your leaves thickening, curling, appearing shiny or waxy, or feeling dry and stiff.

If you’re seeing signs of a phosphorus deficiency, start first by analyzing other factors in your soil and water. If you’re using a standard nutrient regimen, chances are your plant has the phosphorus, but can’t use it. Start first with checking your pH, and properly flush your system if you discover an imbalance. At the right pH, your plants will be able to absorb the phosphorus you’re surely feeding it. An excess of zinc or iron also hinders phosphorus absorption, which could cause these symptoms. Then, there are environmental stressors that could cause your plant to lock out its phosphorus. These include compact soil conditions, overwatered roots, and temperature extremes or swings (especially in temperatures cooler than 60°F).

At the first sign of a phosphorus deficiency, don’t panic! Take the time to investigate what other factors could be causing these symptoms in your plant, and only add more phosphorus if you’re sure your other conditions are perfect. In the end, it all comes down to balance. As much as cannabis loves its phosphorus, too much can prevent the absorption of other nutrients.

Nitrogen Deficiency in Cannabis

Nitrogen is a fundamental component of cannabis, especially in the vegetative stage of growth. Like iron and magnesium, it’s important to the process of photosynthesis and production of chlorophyll. It also plays a role in cell development and other biological reactions. Watching for signs of a nitrogen deficiency is crucial, especially during veg, as it can cause serious stunting if left untreated, affecting your plant through the rest of the growth cycle.

Signs of nitrogen deficiency start at the tip of the leaf and work their way inward. Greens give way to yellows that grow paler with time. Eventually, your leaves will turn brown, may wrinkle, and even fall off completely. You’ll notice this first toward the bottom of your plant, but when left untreated, this deficiency will quickly work its way up as the plant takes what nitrogen is available from newer and fresher leaves.

A nitrogen deficiency is one of the more important to watch for; it causes irreparable damage to the plant’s development for the rest of its growth cycle. Plants are less able to fight against pests and diseases, and they’re less able to adapt to stressors, such as temperature changes. If you’re unfortunate enough to see this in your plants, it’s best to act immediately so the least amount of damage can be done.

Scary as a nitrogen deficiency sounds, you really only need to worry about this until the flowering stage. Once in flower, the plant naturally uses less nitrogen, which plays a smaller role in bud production. You may see mild signs, such as yellowing leaves, during this stage, and that’s perfectly normal. Additionally, a nitrogen deficiency is one of the easiest to reconcile, as long as you can catch it early. A water soluble fertilizer packed with nitrogen will penetrate the roots directly and swiftly, and you’ll see your leaves return to green in just a few days. You can use an organic nitrogen-rich fertilizer to improve your soil, or you can use a mineral fertilizer, which should be cheaper and absorbed faster (but does contain chemical components). Worm castings are another rich and organic additive often recommended by growers. With a simple worm tea, you can easily feed your plants a healthy dose of nitrogen.

Potassium Deficiency in Cannabis

Potassium plays a huge role in cannabis cultivation, and is one of the most common deficiencies we see in the industry. Not only does potassium aid in photosynthesis, disease prevention, cellulose production, and protein development; it also helps transport water and nutrients throughout the plant, prevents energy loss, and proliferates root growth. This is one nutrient you don’t want to lose.

Signs of a potassium deficiency typically appear during the flowering stage of cannabis growth. Leaves will turn yellow at the tips, slowly making its way to the veins and base of the leaf. As the plant loses more potassium, it is less able to regulate its own temperature, which results in a trademark “burn” that many would compare to a nute burn. You can discern a potassium deficiency from an actual nute burn, however, as a nute burn shouldn’t be accompanied by yellowing of the leaves. When left untreated, a potassium deficiency will cause further deterioration, including browning, dead spots, wrinkling, and eventual death of the leaf.

If you’re seeing signs of a potassium deficiency, there are simple fertilizers you can use to reconcile the issue. A flowering fertilizer usually contains a healthy dose of both potassium and phosphorus, and it’s typically sufficient for correcting the levels of either. You can use either an organic booster, which works a little slower but is great for your soil; or you can use a mineral booster, in which case, just make sure it's acceptable for use on edible plants. After a few days (or a week, for organic), you should see signs that your plants returning to health.

Copper Deficiency in Cannabis

Copper deficiencies aren’t very common, though you may see symptoms if the plant is unable to absorb the copper it does have access to. Within cannabis, this nutrient plays a key role in many enzymatic processes, as well as photosynthesis, respiration, and metabolism. Interestingly, copper is also responsible for enhancing colors and flavors in plants and flowers.

Identifying a copper deficiency is arguably very simple, as its symptoms are fairly unique. You’ll notice your plant’s leaves turn darker in color, namely to blue or purple tones. This will be in stark contrast to the leaves’ edges and tips, which turn a bright yellow or white. Beyond this color change, you may notice a waxy or shiny effect, as well as a stiffening of l