Nutrient Deficiencies in Cannabis

Nutrient Deficiencies in Marijuana Plants

Luckily, even novice growers can usually identify damage to their plants caused by pests and disease with a little help. Identifying nutrient deficiencies is equally important, though. Read on to find out what growers need to know about how to identify nutrient deficiencies and what steps to take towards solving them.

What Is a Nutrient Deficiency?

Marijuana plants require a healthy mix of macronutrients like nitrogen (N), phosphorus (P), and potassium (K) and micronutrients like magnesium, calcium, and iron to grow and thrive. Organic soil naturally contains most of these essential ingredients, but unless growers are buying high-quality soil every year, they won’t be perfectly balanced to sustain optimal growth. When the nutrients in the soil get out of balance, it causes nutrient deficiencies and the related issue of nutrient lockouts.

Nutrient deficiencies can slow plant growth and reduce yields. If left unchecked, they can lead to substantial crop losses. Growers should do everything they can to prevent nutrient deficiencies, but they should also know the early warning signs to look out for throughout the season.

Nutrient deficiencies in outdoor and container grows typically originate in the soil. Soil that has been used repeatedly without amendment may not have sufficient nutrients, while poorly drained soil can cause nutrient deficiencies in marijuana even if it’s packed with N, P, K, and various micronutrients. This is because waterlogged roots growing in compacted soil won’t be able to absorb these nutrients efficiently.

Macronutrient Deficiencies

The first step toward solving any nutrient deficiency is to identify which nutrients, in particular, are absent or not being properly absorbed. The symptoms of different deficiencies may appear similar to the untrained eye, but all growers should have an idea of what to look for. Nitrogen, phosphorous, and potassium are the best-known of the macronutrients, but they’re not the only ones plants need to thrive, so let’s take a look at five of the most common macronutrient deficiencies.

Nitrogen Deficiency

Cannabis plants require a lot of nitrogen. They use this macronutrient to facilitate photosynthesis, produce amino acids, and process chlorophyll. Since plants use photosynthesis to feed themselves, nitrogen deficiencies can cause substantial reductions in overall plant growth and yield.

The early signs of nitrogen deficiency are most obvious in a plant’s older leaves. If the leaves in the base and middle of the plant are turning yellow and dying, a lack of nitrogen might be to blame. Discoloration and leaf death typically begin at the bottom of the plant and move upward.

Nitrogen deficiencies are most common during the flowering phase, when cannabis plants start directing their stored nutrients to the buds. Don’t worry about yellowing right before harvest. It’s only when nitrogen deficiencies occur during the vegetative phase that growers really need to amend their soil.

Use amendments like blood meal, bat guano, fish emulsion, or worm castings. The yellow leaves will still fall off, but the plant should recover within a week and start producing new leaves.

Phosphorus Deficiency

Phosphorus deficiencies aren’t as common as nitrogen deficiencies, at least in soil-based grows. Since they primarily impact the plant’s roots, they’re more common in hydroponic systems. Adequate levels of phosphorous are most important during the germination phase and the flowering phase.

Symptoms of phosphorous deficiency include slow growth and low plant hardiness. It also causes the foliage to darken, losing its vibrant color around the edges, or even turning brown and curling inward. The plant’s petioles may also darken to blue or red.

Marijuana plants have more trouble absorbing phosphorous when it’s cold out. Soil alkalinity and drainage problems can also contribute to phosphorous deficiency by causing nutrient lockouts.

Most growers amend soil using a phosphorous-heavy fertilizer developed specifically for cannabis plants in the flowering phase, although organic growers tend to prefer bat guano, bone meal, or worm castings, all of which are also high in phosphorous. Apply them liberally and expect to see results within a week.

Potassium Deficiency

Potassium deficiencies are common in part because commercial fertilizers typically only include minimal amounts of this macronutrient. Although marijuana plants need less potassium than nitrogen and phosphorous, it’s still vital for plant function. More specifically, potassium is partially responsible for water respiration and circulation, photosynthesis, and disease resistance.

The symptoms of potassium deficiencies take a while to spot. Plants grow taller than normal and look healthy at first, but the lower foliage eventually turns brown and dies, slowing growth and development. The lower leaves will also be slow-growing and may appear burnt around the edges or at their tips. Mature leaves might start to look mottled and yellow between the veins, then eventually turn yellow or brown and die.

Since potassium is essential for water and nutrient transfer, substantial deficiencies can impact the entire plant. They may start in localized areas, but eventually, the symptoms described above will move across the entire plant.

Thankfully, plants absorb potassium quickly and easily. Combat potassium by adjusting fertilizer mixes or adding water-soluble potassium additives. Organic growers can use wood ashes, granite dust, sulfate of potash, or kelp meal instead. Either way, the plants should start to recover within a week.

Magnesium Deficiency

Magnesium aids in chlorophyll production and breaking down enzymes. It also stimulates the production of healthy leaves. While magnesium deficiencies are relatively rare in outdoor grows, they’re common in indoor container and hydroponics systems.

The symptoms of magnesium deficiencies begin at the bottom of the plant. The lowest leaves will grow yellow and weaken, turning inward and eventually falling off. The outer margins become brittle and new shoots become blanched. Petioles and stems also change color, usually turning purple or red. Red stems are one of the clearest signs of a magnesium deficiency. If left unchecked, the symptoms will progress through the plant, working its way upward as the leaves mature.

The easiest way to get extra magnesium to plants is to apply Epsom salts. Dolomite lime, magnesium sulfate, garden lime, and worm castings also contain plenty of magnesium, along with other micro- and macronutrients. Test the pH levels of the soil before applying amendments, because this solution will only be effective if the pH is balanced.

Calcium Deficiency

Like magnesium deficiencies, calcium deficiencies are more common in indoor grows. The symptoms of calcium deficiency begin in older growth. The branches will become weak and may break off under stress, and eventually, the plant’s roots will sustain damage.

Treating calcium deficiency is easy. Apply lime to the soil or hydroponic substrate. The plants will uptake the calcium and start healing their branches and stems almost immediately.

Micronutrient Deficiencies

Marijuana plants need lower levels of micronutrients than macronutrients, but that doesn’t mean growers can just ignore things like iron, manganese, zinc, and other minerals and micronutrients. Educating yourself on what to look for in regards to micronutrient deficiencies can aid in maintaining healthy, bountiful crops.

Iron Deficiency

Unfortunately, iron deficiencies are common in both outdoor and indoor grows. The symptoms of iron deficiency are similar to those of magnesium deficiency because this mineral also plays a role in chlorophyll production. Unlike the effects of magnesium deficiency, when plants are deficient in iron, the new growth begins to suffer first. The yellowing of the leaves occurs from the top down, not from the bottom up.

It’s common for iron deficiencies to occur in soil-based grows with improper pH levels, so growers should always check their pH before adding amendments. Once the pH is balanced, apply compost, fertilizer, or elemental sulfur and spay the affected leaves with iron chelate while these amendments take effect.

Manganese Deficiency

Manganese deficiency often occurs in conjunction with iron deficiency, although they’re less common. This nutrient deficiency primarily affects the plant’s new leaves, which can develop necrosis in spots and turn yellow. Like iron and magnesium, manganese plays a role in chlorophyll production, which explains the discoloration.

Be careful when applying water-soluble manganese or greensand to remedy symptoms of this deficiency. Too much magnesium can exacerbate iron deficiencies by causing a nutrient lockout. It’s safer to apply a mineral-heavy compost.

Zinc Deficiency

Marijuana plants only require a minute amount of zinc, but that doesn’t make this mineral any less essential. Zinc aids in plant development and vital enzyme production and unfortunately, zinc deficiencies are quite common in both indoor and outdoor grows. Like magnesium, zinc is also linked to iron, so most growers supplement all three of these micronutrients at once.

Symptoms of zinc deficiency include yellowing of the veins in mature leaves and twisted foliage. Zinc-deficient plants may also lose their color, and in the flowering stage, the buds may curl up or deform. In severely deficient plants, the stems may break, and the leaves may wilt. Growers can apply chelated zinc directly to leaves to tide their plants over and reduce damage while other soil amendments take effect.

The Bottom Line

This article has covered the most common macro- and micronutrient deficiencies, but growers may also want to keep an eye out for boron, copper, molybdenum, silicon, and sulfur deficiencies in their plants. Like the other micronutrients and minerals described above, these substances occur naturally in most healthy soil, but if the soil becomes depleted, growers may have to amend it. After applying amendments, pay careful attention to the plant’s growth. If they are producing new, healthy leaves, prune the severely discolored leaves to encourage a quicker recovery.

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