Cannabis Nutrient Deficiencies: Grower’s Guide

Like all living things, cannabis plants require certain nutrients to survive. The most crucial of these are nitrogen, phosphorus, and potassium (or NPK), although numerous secondary nutrients ensure that your plants thrive and produce a bountiful harvest every time.

Failing to provide the necessary nutrients can lead to plant stress and reduced yields. Therefore, it is essential to understand cannabis nutrient deficiencies and how to remedy them as quickly as possible.

Read on for our guide to the most common deficiencies and how to recognize them before they become severe.

Cannabis Deficiency Chart

The table below summarizes the most common cannabis nutrient deficiencies. The following sections describe each one in more detail.

Normal Aging in Cannabis Plants

Before we dive into the signs of cannabis nutrient deficiency, it is crucial to understand that your plants will naturally change with age. Some of the signs of aging can look similar to certain nutrient deficiencies, and it is crucial to know the difference.

Normal aging in cannabis plants causes the oldest fan leaves (towards the bottom of the plant) to turn yellow and possibly wilt and fall off. Throughout the flowering period, this yellowing can begin to make its way up the plant as it diverts its energy into creating potent cannabinoid-rich buds.

You can distinguish this natural aging from a nutrient deficiency in the following ways:

• Yellowing starts at the bottom of the plant and slowly progresses upward
• Yellowing begins at the edges of leaflets and progresses toward the center
• Yellowing is most distinct on the large central leaflet and progresses outward to the smaller lateral leaflets
• Yellowing may increase after performing a pre-harvest flush

Now, we will compare this to the signs of some common nutrient deficiencies. Note that the signs of deficiency listed below may differ from those described in other sources. They are based on the results of a 2019 study by Cockson et al. and a 2023 study by Llewellyn et al., both of which investigated the impact of various cannabis nutrient deficiencies when applied under strict laboratory conditions.

Nitrogen Deficiency

The first sign of nitrogen deficiency in weed is usually stunted growth, resulting in smaller plants and a lower yield. As the deficiency progresses, the leaves may begin to turn yellow, with this symptom being most apparent on the bottom ⅔ of the plant.

At a glance, the signs of nitrogen deficiency might look similar to normal aging. However, it can be recognized by the following features:

• Yellowing starts at the bottom of the plant and slowly progresses upward
• Yellowing begins at the bases of leaflets and progresses toward the tips
• Yellowing affects the interveinal regions first, then the whole leaf
• As the deficiency progresses, sugar leaves may also become yellow
• Fan leaves may eventually turn brown and fall off
• Stem tissues may also turn dull green and then brown
• Symptoms progress rapidly

Phosphorus Deficiency

Phosphorus deficiency may not become apparent in cannabis plants until about midway through flowering. One of the first signs is slightly stunted growth, although it may not be as obvious as with nitrogen deficiency.

Next, the lower fan leaves may develop yellow spots, which gradually increase in severity and expand into large, gray necrotic lesions. These lesions can then merge into irregularly-shaped necrotic regions, accompanied by mild yellowing of the other leaves.

Eventually, all leaves, including sugar leaves, may be affected. Finally, the leaves dry out and begin to curl upward. The plants’ stems may also have some purple coloration.

Potassium Deficiency

The first signs of potassium deficiency in cannabis are dark brown lesions around the secondary branch veins of larger sugar leaves. This may be accompanied by light yellowing or browning of the fan leaves, predominantly at the leaflet tips and margins and gradually extending towards the leaflet centers.

This may progress to increased yellowing or browning of the fan leaves approximately one week after the symptoms occur in sugar leaves. The marijuana leaves may then become dry and have a scorched appearance. Symptoms will be most severe at the top ⅔ of the plant.

In severe, late-stage potassium deficiency, the only remaining green tissues will be the flowers and midribs of the sugar leaves and possibly some of the lower fan leaves. There may also be some browning of the flowers’ uppermost tips.

Potassium deficiency severely affects yield, and plants exposed to these conditions may produce half the average weight of bud compared to a healthy plant.

Calcium Deficiency

At first glance, calcium deficiency in cannabis may resemble phosphorus deficiency. However, there are some key differences to note.

In cases of calcium deficiency, the plants’ leaves develop yellow spots with a necrotic brown center. These spots primarily occur on the lower ⅔ of the plant, with increasing severity towards the lower fan leaves. The spots affect the entire leaf but are most concentrated towards the margins and tips.

As the deficiency progresses, these spots can grow and merge to form irregular necrotic areas along the leaf margins. This is accompanied by a general yellowing of the lower fan leaves, increasing from the leaflets’ bases to their tips. Eventually, the upper fan leaves and sugar leaves may also be affected, with the upper leaves browning at the margins and curling upwards.

The best way to distinguish calcium deficiency from phosphorus deficiency is to look at the plant’s new growth. In calcium deficiency, new growth will be irregular, and the leaflet bases may be pale and narrow compared to the tips.

Magnesium Deficiency

The first sign of magnesium deficiency in cannabis is mild yellowing in the interveinal regions of lower fan leaves. This rapidly progresses to affect smaller, younger fan leaves, while older fan leaves and sugar leaves are less affected.

As the deficiency becomes more serious, necrotic regions form in the interveinal areas, increasing in severity from the leaflet bases to their tips. At this stage, it may affect the entire plant, with significant yellowing of the smaller fan leaves and sugar leaves in all areas except the midrib and margins.

Magnesium deficiency can reduce yield by as much as 30% compared to healthy plants.

Zinc Deficiency

Zinc deficiency may only become apparent in cannabis during the later stages of flowering. The signs include yellowing at the margins of newer leaves, mainly concentrated along the leaflet margins. As the deficiency progresses, irregular, tan-colored necrotic regions may form along the leaflet margins.

Iron Deficiency

Iron deficiency does not have many observable signs in cannabis, although the sugar leaves and buds may appear paler than usual during the early flowering stage. The upper half of the plant is generally affected first.

As the deficiency progresses, smaller, younger leaves may develop mottling and yellowing at the margins. The middle portion of the leaflets may also appear paler than the bases and tips.

The later stages of iron deficiency can sometimes cause the lower fan leaves to turn pale brown at the margins, although the lower leaves are often unaffected.

Copper Deficiency

The signs of cannabis copper deficiency do not become apparent until late in the growth cycle. The first noticeable issues include slightly stunted growth and distortion of new and expanding leaves, which appear narrower at the base and display slight yellowing.

These signs become more severe as the deficiency progresses, with more significant yellowing of the leaves, especially at the margins. At its most advanced stage, copper deficiency causes entire leaves to become yellow with distorted margins that curl inwards and downwards. The new and expanding leaves may also be floppy and wilted.

Copper deficiency can cause a 45% decrease in yield.

Boron Deficiency

The first signs of boron deficiency in cannabis include slightly stunted growth, with new leaves and growing tips showing a distorted growth pattern. Leaflets may appear small and narrow at the base compared to the tips, with the most obvious distortion in the new and expanding leaves.

This deficiency progresses to cause necrosis at the leaflet margins and leaves that curl in and down at odd angles from their stems. Late-stage deficiency results in necrotic growing tips and wilting of the entire plant.

Sulfur Deficiency

The first sign of sulfur deficiency in weed is a general yellowing of the upper sugar leaves. As the deficiency progresses, the upper fan leaves may grow at an odd upward angle while the lower fan leaves become yellow. The yellowing will start from the leaflets’ midribs, progressing to the margins.

In late-stage sulfur deficiency, the sugar leaves may develop brown tips and margins, while the fan leaves become completely yellow. These signs can look similar to nitrogen deficiency but begin in the upper parts of the plant and progress downward, whereas nitrogen deficiency symptoms start in the lower parts of the plant and progress upward.

Sulfur deficiency can reduce yield by as much as 34% compared with healthy plants.

Manganese Deficiency

Manganese deficiency in weed does not have any obvious signs, and deficient plants may appear exactly the same as healthy ones throughout their growth cycle. This deficiency does not appear to affect yield and, therefore, is not usually a major cause for concern.

Nutrient Lockout

The best way to prevent nutrient deficiencies in cannabis is to provide ample nutrients in the form of plant fertilizers. However, paradoxically, providing too much fertilizer can cause further nutrient deficiencies.

This is because using chemical fertilizers alters the pH of your growing medium and affects how easily your plants can absorb the nutrients you provide. The abbreviation “pH” stands for “potential of hydrogen,” and it denotes the acidity or alkalinity of a substance. A pH of 7 is neutral, while a lower pH is acidic, and a higher pH is alkaline.

Cannabis plants prefer a slightly acidic growing medium, approximately 5.5 to 6.5 for hydroponics and 6 to 6.8 for soil. Adding chemical fertilizers that are high in mineral salts makes your growing medium more alkaline, and it becomes more challenging for your plants to absorb nutrients through their roots, leading to deficiency. This scenario is known as nutrient lockout.

Here’s how to identify nutrient lockout in weed and how to fix and prevent it.

How to Identify Nutrient Lockout

The signs of cannabis nutrient lockout can look very similar to those of nutrient deficiency. They include:

• Stunted growth
• Yellowing of leaves, affecting the lower leaves first and moving up the plant
• Other discolorations of leaves, including brown, purple, or red
• Leaves curling upward or downward
• Leaf tips appearing burnt or scorched

The best way to distinguish nutrient lockout from a true deficiency is to check the pH of your growing medium. You can do this using an electronic pH meter or pH testing strips. If the pH is above 6.5 for hydroponics or 6.8 for soil, you could be dealing with nutrient lockout.

How to Fix Nutrient Lockout

You can fix nutrient lockout in cannabis by adjusting the pH of the growing medium so it falls back within the optimal range.

If you are growing hydroponically, you can achieve this relatively easily using “pH Up” and “pH Down” solutions. If you are using soil, you will need to flush it with plain water to remove any excess fertilizer. Repeat this over several days, and keep rechecking the pH until you are back within the optimal range. You may then need to re-fertilize your plants, but go low and slow and be careful not to overdo it.

You could also consider using a foliar feeding spray, which allows you to administer nutrients directly to the plant’s leaves.

How to Prevent Nutrient Lockout

One of the best ways to prevent nutrient lockout in cannabis plants is to create a detailed feeding chart so you can keep track of what nutrients you have administered and when. This will help you to troubleshoot if things go wrong and hopefully rectify the issue before it becomes severe.

Another excellent prevention method is to use organic fertilizers, such as worm castings or compost tea. These amendments release their nutrients slowly over time and reduce the risk of lockout. Using an enriched “super soil” as your growing medium is a great starting point and may reduce the need to add extra nutrients later on.

Summary: Cannabis Nutrient Deficiencies

You should now have a solid understanding of marijuana nutrient deficiencies and how to recognize them. If you feel your plants are struggling, you should be able to identify the cause and take the appropriate action.

The best way to avoid nutrient deficiencies is to provide your plants with all the elements they need to thrive, either in the form of organic soil amendments or synthetic fertilizers. However, synthetic fertilizers can lead to nutrient lockout, and their application should be monitored carefully. Therefore, many experts consider growing cannabis organically using a nutrient-rich supersoil to be the simplest and most effective method.