Nutrient deficiencies: How to prevent and fix them.

Contents:

Just like us, cannabis plants require a varied and healthy diet to survive and thrive. They need the right amount of nutrients to fulfil important physiological functions, and if they lack just one piece of the puzzle, growth will slow down and yields may be affected.

Luckily, the cannabis plant does a pretty good job of communicating what it needs. If nutrient deficiency strikes, it often sends out a signal—wilting, discolouration, curled leaves—to inform the cultivator of what it requires.

Before we dive into how to prevent and fix each nutrient deficiency, they are a few important things you should know.

MACRO VS MICRONUTRIENTS

Macronutrients are minerals that cannabis plants require in large amounts. These include nitrogen, phosphorus, and potassium. Just like fats, carbs, and protein form the cornerstone of the human diet, cannabis needs these important minerals in large quantities to carry out key processes.

Micronutrients are minerals and elements required in much smaller amounts. However, they are just as vital to plant health. These include iron, zinc, sulphur, boron, and others. Think of them like minerals and vitamins in the human diet. We don’t need much of them, but without them, it wouldn’t take long to fall ill.

MOBILE VS IMMOBILE NUTRIENTS

Learning the difference between mobile and immobile nutrients can help growers diagnose deficiencies more accurately.

Mobile nutrients are minerals that can be shuttled throughout the plant to areas that need them the most. For example, phosphorus stored in older fan leaves can be directed towards newer growth if a deficiency occurs. Therefore, deficiency of a mobile nutrient will first become noticeable in older growth.

Immobile nutrients remain locked in place and plants cannot redistribute them. For example, if a zinc deficiency takes hold, the signs will first show in the newer growth as the plant can’t relocate its mineral stash.

ELEMENTS (Symbols)MOBILITY IN PLANT

NUTRIENT DEFICIENCY
(Old leaves)

NUTRIENT DEFICIENCY
(New leaves)
Nitrogen
(N)
Mobile X
Phosphorous
(P)
Mobile X
Potassium
(K)
Mobile X
Calcium
(Ca)
Immobile X
Sulfur
(S)
Immobile X
Magnesium
(Mg)
Mobile X
Iron
(Fe)
Immobile X
Manganese
(Mn)
Immobile X
Boron
(B)
Immobile X
Molybdenum
(Mo)
Mobile X
Zinc
(Zn)
Immobile X


PH MATTERS

Your soil can hold all of the nutrients your plant needs, but it won’t be able to access them if the pH is off. Cannabis plants thrive in a soil pH of between 6.0–6.5. Any lower or higher and plant roots will struggle to absorb key nutrients—a phenomenon known as nutrient lockout. Routine flushes can prevent this from happening. Keep an eye on your values using a pH meter. You can also change the pH of your soil using these techniques.

GOING ORGANIC

Advances in soil science have shown that the rhizosphere (root zone) is beaming with life. Here, a complex network of microorganisms works synergistically with the root system. The soil needs an optimal balance of bacteria and fungi to break down organic matter and free up nutrients for plants to use.

Focusing on composting and building living soil provides long-term prevention of nutrient deficiencies and supports thriving biodiversity within the rhizosphere. Such beneficial life will help your yields soar.

However, growers can take more direct action in the short-term. Foliar spraying can be administered as a quick fix when plants are missing out on some nutrients. These feeds bypass the roots and are absorbed straight through the leaves.

HOW TO PREVENT AND SOLVE NUTRIENT DEFICIENCIES

Below is a list of key nutrient deficiencies that may arise, as well as how to prevent them and fix them if they strike.

NITROGEN

A mobile macronutrient, nitrogen plays a major role in photosynthesis and the formation of vital plant proteins. Nitrogen deficiency can result in yellowing older leaves, older leaves dropping off, eventual discolouration of the entire plant, and reduced yields.

Nitrogen

Prevention

• Keep pH within an optimal range (6.0–6.5).
• Start off with a nutrient-dense potting mix.
• Start composting to ensure a nutrient-dense medium in the future.
Mycorrhizae are associated with nitrogen-fixing bacteria. Add them to your soil to boost nitrogen levels.

Fixes

• The majority of organic fertilisers contain enough nitrogen to fix the deficiency: Try fish meal, manure, alfalfa, or feather meal.
• Adjust pH accordingly.
• Apply compost tea as a foliar spray for a fast-acting solution.
• Increase the amount of nitrogen in your compost using kitchen scraps, fresh prunings, and grass clippings.

PHOSPHORUS

Phosphorus also acts as a macronutrient in the cannabis plant. Being a mobile nutrient, plants can direct the mineral to the areas that need it most. Phosphorus plays an essential role in photosynthesis and protein synthesis, and it's a crucial component of DNA. Phosphorus deficiency can manifest as red/purple stems, brown spots on leaves, and dry leaves.

Phosphorus

Prevention

• Utilise soil high in organic matter.
• Increase the absorption rate by using well-aerated soil.
• Use mycorrhizal fungi in your soil to improve phosphorus uptake. These microbes help to turn insoluble phosphates into available molecules.
• Add more manure to your compost.

Fixes

• Nudge pH up to the higher end of the spectrum—your plant will have an easier time absorbing it.
• Add worm castings and fish meal to your soil.
• Apply an organic fertiliser high in phosphate.
• You may be overwatering. Only water when the top 3cm of soil is dry to avoid making the medium overly compact.
• Move your plants to a warmer location or erect a tarp to trap heat. Plants find it harder to uptake phosphorus in temperatures below 15°C.

POTASSIUM

Potassium: the third and final macronutrient. It helps to regulate CO₂ uptake and plays a role in photosynthesis. The mobile nutrient also helps in the production of ATP (the cellular unit of energy). Potassium deficiency appears as brown and yellow leaf tips and edges, curled-up leaves, and stretching.

Potassium

Prevention

• Be careful when using fertilisers. Feeding your plant too often can cause salt to build up and interrupt potassium uptake.
• Bolster your compost with hardwood ash and kelp meal.
• Don’t overwater.

Fixes

• Flush the medium.
• Measure and adjust pH to correct possible nutrient lockout.
• Add chicken manure to the soil.
• Apply organic seaweed as a foliar spray.

CALCIUM

Critical to plant health, this immobile micronutrient helps to hold plant cell walls together. Calcium deficiency can lead to new growth—root tips and young leaves—forming incorrectly and becoming warped.

Calcium

Prevention

• Add dolomitic lime/garden lime to the growing medium.
• A pH of 6.2 provides the best environment for calcium uptake.
• Add plenty of eggshells to your organic compost.
• Keep a worm farm! Worm castings provide loads of nutrients, including calcium.

Fixes

• Apply a cal-mag supplement.
Increase or decrease pH towards 6.2.
• Add one teaspoon of hydrated lime to 4l of water and use the solution to water your plants.

SULPHUR

Although required in very small amounts, this key immobile nutrient contributes to forming vital enzymes and proteins. A sulphur deficiency will lead to yellowing of new growth and discoloration on the undersides of leaves.

Sulphur

Prevention

• Bolster your compost pile with manure.
• Fungi and bacteria are key for releasing sulfur in the soil. Employ techniques such as no-till to encourage them and add some mycorrhizae if you’re growing in pots.

Fixes

• Epsom salts are rich in sulphur. Add 1–2 teaspoons of Epsom salts to about 4l of water and supplement until symptoms disappear.
• Adjust pH to the optimal range if needed.

MAGNESIUM

Without this immobile micronutrient, photosynthesis would not be possible. The mineral sits at the heart of the chlorophyll molecule and enables it to absorb light. Magnesium deficiency will result in lower growth looking worse for wear. Leaves will become yellow, dry out, and eventually turn brown.

Magnesium

Prevention

• Include dolomitic limestone in the growing medium.
• Use compost rich in manure.
• Maintain good pH balance.

Fixes

• Flush the medium with 6.0 pH water if pH is out of balance.
• Epsom salts also provide magnesium. Add 1–2 teaspoons of Epsom salts to about 4l of water and apply until symptoms resolve.

IRON

Iron plays an essential role in chlorophyll formation. The element also forms part of several enzymes and some important pigments. Overall, this immobile micronutrient helps plants carry out metabolic and energy-forming processes. If your plant experiences iron deficiency, you’ll notice young growth at the top of the plant becoming bright yellow.

Iron

Prevention

• Help your plants absorb existing iron by adding mycorrhizae to the soil. These synergistic organisms help to shuttle the element into the root system and beyond.
• Test your soil pH to rule out nutrient lockout.
• Add chicken manure, kitchen scraps, and seaweed to your compost pile.

Fixes

• Hit the pH sweet spot.
• Flush the growing medium and add an iron supplement afterwards.
• Use a small amount of nitrogen fertiliser to lower pH and make iron more accessible.

MANGANESE

Manganese doesn’t receive too much attention in the world of cannabis growing. However, this immobile micronutrient plays a fundamental role in cannabis physiology. It aids photosynthesis, respiration, nitrogen assimilation, and root cell elongation, and protects roots from bad microbes.
Manganese deficiency will show up as light green discolouration near the base of new growth. This eventually spans out to the tips, and brown spots begin to emerge.

Manganese

Prevention

• Imbalanced pH often underpins manganese deficiency. Frequently measure your soil pH and keep it within the optimal range to ensure your plants can access this mineral.
• Make a manganese-rich compost by adding pineapple, tomatoes, cranberries, and carrots to your pile.

Fixes

• Flush your containers.
• Prune back any affected growth that doesn’t recover.
• Hit the canopy with a seaweed foliar spray.

BORON

Boron works alongside calcium to give integrity to plant cell walls and help with cell division. Breeders also have a soft spot for this immobile nutrient as it helps with the pollination process. A lack of boron will lead to deficiency symptoms such as a lack of turgor, reduced fertility, poor vegetative growth, and terminated meristems. New growth will become twisted, sugar leaves will wilt, and leaves will show a yellowish-brown discolouration.

Boron

Prevention

• Don’t let plants dry out often.
• Avoid nutrient lockout by NOT overfeeding.
• Don’t let humidity levels drop below 25%.
• Use well-draining, aerated soil.
• Add generous amounts of apples, bananas, broccoli, and chickpeas to your compost pile.

Fixes

• Flush the medium and aim for an ideal pH range.
• Mix one teaspoon of boric acid in 4l of water and apply to affected plants.

MOLYBDENUM

Another lesser-known nutrient, molybdenum helps to form two essential enzymes that convert nitrate into nitrite and then into ammonia. Plants use the latter to make amino acids, which eventually become proteins. If your plants become deficient in molybdenum, they’ll start to display red and pink discolouration at the edges of new growth. Leaves will also start to become yellow and spotted. Thankfully, molybdenum deficiency is quite rare.

Molybdenum

Prevention

• Keep pH between 6.0–6.5.
• Start your grow with good-quality, living compost.
• Throw the occasional bean, pea, grain, and raw nut onto your compost.

Fixes

• Flush and adjust pH.
• Spray affected plants with a seaweed foliar spray.
• Water plants with a worm casting compost tea.

ZINC

Plants don’t need much zinc at all, but illness will strike if they miss out. Zinc forms parts of proteins, membranes, and growth hormones. The immobile micronutrient also regulates enzyme function and stabilises DNA and RNA. What happens when zinc goes missing? Deficiency symptoms manifest in slowed new growth; the distance between nodes lessens, and leaves will look wrinkly and yellow. Eventually, new leaves will display yellowing and rust-coloured tips.

Zinc

Prevention

• An excessively alkaline pH causes most zinc deficiencies. Maintain proper pH.
• Boost zinc levels in your compost using pumpkin and squash scraps.
• Good microbes play a large role in zinc uptake—add beneficial fungi to your growing medium

Fixes

• Reduce alkaline pH to the ideal range.
• Stop overwatering.
• Use a fish or seaweed foliar spray to swiftly boost zinc levels.

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