How to

27 May 2021

Diagnosing Cannabis Nutrient Deficiencies

Cannabis plants require regular uptake of nutrients to develop into healthy specimens capable of producing dense, trichome-laden, terpene-rich flowers concentrated with cannabinoids such as THC and CBD. Growers who keep their crop on schedule with the correct application of plant nutrients should expect smooth sailing as the plant progresses through the different stages of its life cycle. 

Cannabis plants will require different kinds and levels of nutrients as they reach growth milestones: nitrogen (N) for the vegetative stage, phosphorous (P) for the transition to flowering, and potassium (K) for the development of healthy, juicy buds during the flowering stage. The balance of these macronutrients (and other micronutrients, detailed below) is critical to the development of cannabis plants, and the deficit or absence of even a single key nutrient can have detrimental effects on plant health and crop yield.

An important skill to develop is an eye for plant health, as cannabis plants are quite capable of communicating nutrient deficiencies and any health issues. It is up to the grower to take the time to observe their crop regularly for any signs of stress.

Where to Look for Cannabis Nutrient Deficiencies

Nutrient deficiencies will occur when cannabis plants are lacking essential nutrients for vigorous growth. At best, these may manifest as mildly as yellow leaves that indicate a deficit of nitrogen during veg, and at worst, nutrient deficiencies can jeopardise an entire crop as unhealthy plants over time may develop viruses or attract pests that can spread to healthy specimens, reducing yield and quality of flowers.

The cultivator who takes the time to regularly scout their plants will be able to distinguish visual cues from cannabis leaves and overall plant appearance that indicate plant health. A good starting point is to begin observations from a “wide” or “broad” perspective by considering the plants as a whole crop in their grow room and to narrow it down from there.

Are some plants growing slower than their counterparts, or perhaps appear less green or vibrant? If so, the grower can then “zoom in” and assess the individual plant’s qualities.

When observing individuals, the cultivator has several points of reference to consider. These include leaf colouration and lustre, stem strength, overall plant “green-ness”, leaf edges and tips, the status of new growth, and internodal spacing. It’s a long list to keep track of, but over time, the grower who regularly scouts their plants will be able to spot anything that seems out of order as they lock in their eye for plant health.

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The Importance of pH in (Mis)diagnosing Deficiency Symptoms

Nutrient deficiencies can occur even when the grower is regularly feeding their plants with nutrients: just because the nutrients are available at the root level doesn’t mean the plant is absorbing them. This can be frustrating for growers as nutrients can be expensive, especially for larger growing operations. The key element to consider here is the acidity and alkalinity of the growing medium and water that is being used to maintain plants.

Nutrient absorption at the root level can be maximised by ensuring that the pH range of the growing medium and water is between 5.8 and 6.2. It is easy enough to order pH pens or pH testing solutions that the grower can use to inform their decision-making. If the growing medium and water are too acidic (below 5.5) or too alkaline (above 7), then the nutrients simply will not be absorbed in the root zone. Generally speaking, between 5 and 7 is safe for marijuana plants, with the range of 5.8 to 6.2 being the most optimal.   

Growers should keep in mind that the pH of their water source is also variable, depending on the time of the year, supplier-treatment levels, and amount of rainfall. A weekly pH test of water before applying to any cannabis plants is worth considering. When troubleshooting nutrient deficiencies, it is good practice for the grower to pH their growing medium, nutrient solution and water to rule out nutrient lockout caused by unbalanced pH levels.

Mobile Nutrient Deficiencies

Nutrients are defined as either mobile or immobile depending on whether they can be transported to different parts of the plant after absorption. For example, a plant will relocate mobile nutrients stored in fan leaves to address a deficiency in another part of the plant. True to their name, immobile nutrients cannot be transported where needed and instead remain in place where they were first deposited or absorbed.

Distinguishing between mobile and immobile nutrients is one of the ways growers can diagnose any potential nutrient deficiencies. Signs of stress found on older leaves lower down on the plant may indicate mobile nutrient deficiencies as the nutrients will have been transported to where they would be more useful. Stressed younger leaves or leaf chlorosis (loss of green pigmentation) higher up in the canopy may indicate a deficit of immobile nutrients.

Nitrogen deficiency can be identified in plants that are showing pale or yellowing lower leaves down their stems. Following the lightening of the leaf colour, nitrogen-deficient leaves will eventually curl and drop off. The yellowing can progress further up into the middle and canopy of the plants, eventually resulting in earlier-than-expected harvests and reduced yields as the plants lacking in nitrogen will develop fewer nodes for flowers to grow.

Phosphorus deficiency is characterised by dry leaves that contain brown patches of discolouration on their surfaces, as well as red or purple leaf stems. Green leaves may also become dark green, with a bluish tinge, and seem less vibrant. If left unchecked, growers may notice leaves curl before they die.

Potassium deficiency will show as curling leaves with browning and yellowing on the leaf edges and tips. Unlike nitrogen deficiency, it may occur higher up in the plant canopy. Cannabis plants lacking potassium may also seem leggier/stretchier than their healthy counterparts, with more brittle leaf stems and stalks.

Iron deficiency may at first appear on new plant growth and shows as a stark yellowing at the base of young leaves. Yellowing of leaves in between leaf veins is a good indicator of an iron deficit as iron is required in the production of chlorophyll and plays an important role in photosynthesis.

Diagnosing cannabis nutrient deficiencies of a marijuana pant

Immobile Nutrient Deficiencies

A calcium deficiency will result in new growth looking “warped” or twisted. Calcium plays a role in strengthening cellular walls, which give cannabis plants their structure. Curling leaves and the appearance of brown or yellow spots on the leaf surface is another symptom of the absence of calcium.

Magnesium deficiency is characterised by the yellowing of newer leaves (this is an important distinction, as nitrogen deficiency is found primarily in older leaves). New leaves will show yellow spots, which will slowly brown before dying. On older leaves, areas of the surface in between veins will yellow and may show brown or rust-coloured spots.

Boron deficiency is quite rare in soil-based growing mediums (hydroponic growers should remain vigilant) as most soils contain sufficient amounts. A boron deficit will cause wilting – growers should be careful not to attribute this to stress from under-watering and risk overwatering their plants – and new growth to appear twisted and non-uniform. Cannabis plants lacking boron may also show brownish/yellow discolouration on their leaves.

Manganese deficiency tends to appear at the base of new growth as pale discolouration. Newer leaves will feature a more vibrant green along their edges while the yellowing shows in the interveinal area of the leaves. When left unchecked, this will spread to leaf tips, causing brown spots to appear.

As mentioned above, checking pH on any growing media and water is a good first step when diagnosing deficiencies. Once optimal pH is established, the most important step is to add the nutrients that the plants are needing. Slow-release, organic nutrients are a good alternative to liquid nutrient feeds for replenishing nutrients in soil-based grows.

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Post author
Martin
Martin is a production horticulturist with experience in commercial cannabis cultivation and sustainable farming from his time with Emerald Cup Award-winning farmers Esensia Gardens in northern California's Emerald Triangle.
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