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  • Mitchell Stern

What's Wrong With My Plant? - How to Diagnose Nutrient Deficiencies in Cannabis

Updated: Jan 24

If you've ever grown cannabis, you know just how easily nutrient deficiencies can occur. Fortunately, most cases of nutrient deficiencies and toxicities have simple solutions.


In this article, we're going to discuss

  • Leaf anatomy

  • Symptoms of essential nutrient deficiencies & toxicities

  • The importance of pH to nutrient availability

  • Treatment options for correcting nutrient imbalances



Table of Contents

Understanding Nutrients

Leaf Anatomy

Macronutrients

Micronutrients

The NPK Ratio

Why is pH Important?

The Plant Nutrient Deficiency Chart

Diagnosing a Nutrient Deficiency

Get to Know Your Nutrients



Understanding Nutrients

Nutrients are vital for plant growth and diagnosing nutrient can be difficult if you don't understand how nutrients interact with each other.


Before we learn how to diagnose nutrient deficiencies in cannabis plants, we'll need to learn a bit about how nutrients work on a cellular level.


Leaf Anatomy

Before we go any further, lets define a few terms:


Macronutrients

Macronutrients are elements that plants need in large quantities. These nutrients have a direct effect on critical plant functions, like amino acid (protein) formation, photosynthesis, and cellular respiration.


Nitrogen, phosphorus, potassium, calcium, magnesium, and sulphur all fall into this category.


Micronutrients

Micronutrients play an indirect role in plant biology and must be present for certain chemical reactions to take place; such as protein synthesis and regulating osmotic pressure.


Micronutrient deficiencies often occur simultaneously.


Iron, boron, manganese, copper, molybdenum, chlorine, zinc, cobalt, vanadium, silicon, and sodium are all micronutrients.


Mobile vs. Immobile

In addition to being micro or macro, nutrients can also be categorized as Mobile or Immobile.


Mobility affects where nutrient deficiencies first appear so by understanding which nutrients fall into which categories, we are able to diagnose nutrient issues more accurately.



The NPK Ratio

The NPK ratio is prominently featured on the front of most nutrient products and soils. The NPK ratio represents the percentage (by volume) of Nitrogen, Phosphorus, and Potassium that the product contains.


The NPK ratio 18-24-12 is easy to see
The NPK ratio 18-24-12 is easy to see

Why is pH Important?

The plant's ability to absorb nutrients is heavily dependant on the pH of the soil in which the plant resides. Macronutrients tend to be more available at higher pH levels whereas micronutrients tend to be more available at lower pH levels.


Conversely, plants can lock out nutrients if the pH is imbalanced.


The most common cause of nutrient deficiencies in plants is not a lack of nutrients, but rather a pH imbalance that is preventing the plant from being able to absorb the nutrients in the first place.

Before you augment your nutrient regiment, check the pH of your soil with a soil pH tester. You can also collect the runoff water that drains out of the bottom of the pot and test the runoff with a water-based pH meter.


Ideally, your pH should always be between 6.0 and 7.0.


Always measure the pH of your water with a good pH meter right before you feed your plants to ensure that your plants receive the maximum benefit from the nutrients.


Adjust the pH as necessary with pH Up & Down additives.


The Plant Nutrient Deficiency Chart


Diagnosing a Nutrient Deficiency

Use the chart below to quickly identify nutrient deficiencies.


If you recognize a particular pattern of symptoms and suspect that your plants may be exhibiting symptoms of a particular nutrient, you can click on the list of nutrients beneath the list to learn more about how to correct the problem.


Summary of Nutrient Deficiency Symptoms
Summary of Nutrient Deficiency Symptoms

Get to Know Your Nutrients

Here is a list of the 17 nutrients that plants require. Click to learn more about the function of each individual nutrient, as well as symptoms of toxicity and deficiency; and how to treat both.


Macronutrients


Micronutrients


 

Nitrogen Deficiency in Plants

Symbol: N (Macro/Mobile)


Function

Nitrogen plays a role in forming amino acids (proteins), nucleic acids (RNA & DNA), enzymes, coenzymes, cell membranes, and chlorophyll.


Symptoms of Deficiency

A nitrogen deficiency is the most common ailment among cannabis plants. Early symptoms include chlorosis (yellowing) of the lower leaves. The yellowing typically starts in the midrib (the spinal cord of the leaf blade) and expands outward as the symptoms progress.


The yellowing will work it's way up toward the top of the plant if the deficiency is left untreated.


In a late stage nitrogen deficiency, the entire plant may turn pale yellowish-green with small, sparse leaves. Stems may turn red; but red stems may also indicate a phosphorus deficiency.


Nitrogen deficiency is more common in loose, sandy soils.


Nitrogen deficient plant
Only the lower leaves are yellow

How to Fix a Nitrogen Deficiency

There are a number of different ways to fix a nitrogen deficiency; and choosing the proper course of action for your particular situation depends largely on what is causing the deficiency in the first place.


If you're already using fertilizer, the problem is likely a pH imbalance in your soil or water.


Make sure that your pH meter is calibrated properly to avoid misdiagnosing the problem.


Proper Fertilization

Use a water-soluble, high-nitrogen fertilizer when watering your plants. You should see the green color start to return after about 4 days.


Store bought nutrient fertilizers are a good option for treating deficiencies in the short-term because they are shelf-stable and ionized to allow for faster absorption. Almost every store bought fertilizer will contain nitrogen, but it's still a good idea to verify to amount of nitrogen by checking the N-P-K ratio.


For a long-term fix, consider augmenting your soil with organic nutrients that will help fix nitrogen over time.


High-nitrogen soil amendments include:

  • Bat & seabird guano

  • Earthworm castings

  • Blood meal

  • Urine (diluted with 10 times as much water)


Symptoms of Toxicity

It's also possible to add too much nitrogen. Although rare, nitrogen toxicity is still something you'll need to manage and avoid.


An excess of nitrogen can cause lush, dark green leaves that are particularly susceptible to pests and diseases. Stalks become weak and become prone to damage.


Plants may turn a golden copper color before wilting and dying.


Treat nitrogen toxicity with frequent, heavy irrigation.

 

Phosphorus Deficiency in Plants

Symbol: P (Macro/Mobile)


Function

Phosphorus is a vital component of sugar phosphates (ATP), nucleic acids (RNA & DNA), lipids, and coenzymes. Phosphorus also promotes healthy root formation and flowering.


Symptoms of Deficiency

Phosphorus deficiency will lead to stunted growth and small, dark, blueish green leaves. Branches, petioles, and main leaf veins may develop a reddish-purple tint.


The tips of lower leaves may turn brown and curl downward. Lower leaves may even become stiff and brittle.


Flowering may be delayed and when flowers eventually develop they will be prone to fungal diseases.


Phosphorus deficiencies are also more common in colder weather.


How to Fix a Phosphorus Deficiency

Make sure that you're using nutrients that are suitable for flowering as opposed to growth and that your growing environment is warmer than 59 degrees Fahrenheit (15 degrees Celsius).


Every commercially available cannabis nutrient regimen will contain phosphorus, so if you're confident that your soil already contains sufficient phosphorus, the culprit is likely a pH imbalance.


To fix a phosphorus deficiency, use a pH meter to make sure that your soil and water have a pH between 6.0 and 7.0. Adjust your pH as necessary with pH Up & Down and be sure to use a water-soluble phosphorus supplement if you feel that supplementation is appropriate.


Symptoms of Toxicity

Plants use heavy amount of phosphorus for just about everything so it's difficult to give them too much. However, toxic levels of phosphorus can reduce the availability of zinc, iron, calcium, and magnesium in newer leaves.


Simply reducing the amount of phosphorus you give your plants and flushing the substrate with water should be enough to cure toxicity.

 

Potassium Deficiency in Plants

Symbol: K (Macro/Mobile)


Function

Potassium is the primary intracellular cation and a major enzyme catalyst. Potassium also drives stomatal movement and fuels the "hydrogen pump" that powers the cell.


Symptoms of Deficiency

Potassium deficiencies manifest differently depending on the age of the plant. Look for chlorotic mottling on younger leaves.


Older leaves may develop brown "burn" at the margins. Stalks weaken and fall off easily. Small branches and petioles may turn red. Flowers are small and sparse.


Potassium deficiencies are more common in cold & dry weather. Insufficient light and sandy soil can also be to blame.



How to Treat a Potassium Deficiency

Apply potash or potassium sulfate fertilizer directly to the soil and wait a few weeks.


Unlike nitrogen and phosphorus, potassium is available to the plant over a much broader pH range so if you're experiencing symptoms of a potassium deficiency, there likely isn't enough potassium in the soil.


Symptoms of Toxicity

Excessive amount of potassium can scorch plants and lead to wilting. Toxicity can also cause the plant to lock out calcium and/or magnesium.


Treat toxicity by flushing your soil with water.

 

Calcium Deficiency in Plants

Symbol: Ca (Macro/Immobile)


Function

Calcium acts as cement within the cell wall and regulates Nitrogen metabolism. Calcium also helps promote healthy root and stem development.



Symptoms of Deficiency

Leaves that are deficient in calcium may become distorted and wither. Their margins curl and their tips hook backward.


Growth becomes stunted, branches become brittle, and roots become discolored.


Flowers at the apex may wither and die.


Calcium deficiency leaves plants exposed to stem and root pathogens including:

  • Botrytis

  • Fusarium

  • Rhizoctonia fungi

  • Ditylenchus nematodes

Deficiencies are most common in acidic, sandy soil; and in overly humid environments.



How to Fix a Calcium Deficiency

Cannabis requires large amount of calcium. However, most store-bought nutrients omit calcium from their products because of the varying amounts of calcium that exist in municipal water supplies.


Regions with particularly hard water may not need to supplement their nutrient regimen with calcium at all but growers in places with softer water will need to supplement with Cal-Mag or similar products.


In any case, your water should have around 120 ppm (parts per million) of calcium.


Cal-Mag is an excellent way to address a calcium deficiency quickly. Adding 1/2 teaspoon per gallon of hydrated lime to your water is also a good idea.


Organic sources of calcium make good additions to a compost heap. However it takes calcium about 18 months to start to decompose and around 50 years to break down entirely; so they offer little benefit in correcting a serious deficiency in time for harvest.


Organic sources of calcium include:

  • Bones

  • Egg shells

  • Sea shells

Symptoms of Toxicity

Calcium toxicity is extremely rare, however excess calcium can lead to wilting and cause the plant to lock out magnesium, potassium, iron, and manganese.

 
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