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Nitrogen | Plant deficiencies & toxicities.

A nitrogen deficiency is a very common deficiency because plants use a lot of nitrogen. If there is a general issue with a lack of fertilizer, a nitrogen deficiency will be one of the first symptoms to appear.

During the vegetative phase, plants take up nitrogen in high quantities to support growth, and also to store for use later during the bloom phase.

Plants in vegetative growth that are deficient in nitrogen will experience small leaves, stunted growth, and sparse branching.

A nitrogen deficiency shows as lower leaves turning pale green and then yellow. The yellowing starts at leaf tips and moves inward.

Plants will take nitrogen from older growth to protect younger growth.

Yellowing will start on the old growth at the bottom of the plant first and then travel up to the the new growth as the nitrogen in the lower leaves gets sucked dry.

A deficiency will likely first show up during a phase of rapid vegetative growth when nitrogen is getting used up quickly. To prevent nitrogen deficiency, add extra nitrogen to the reservoir before periods of rapid vegetative growth.

If extra nitrogen is added to solution after a deficiency has already begun, it will take about a week for plants to recover. Leaves will turn from pale green and slightly yellow back to a healthy green. Very damaged, badly yellowed leaves won’t recover and will die.

An application of a foliar spray with fertilizer containing nitrogen will help speed up recovery and prevent further leaf damage. Only use a foliar spray during vegetative growth.

Cannabis plants that are grown in hotter than average environments use approximately 10%-20% less nitrogen. If the grow room is warmer than average, consider reducing nitrogen. If the grow room is colder than average, consider raising nitrogen.

If nitrogen is toxic, leaves will become dark green and droop, starting from the bottom of the plant.

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Phosphorus | Plant deficiencies & toxicities.

Phosphorus supplies the energy that powers root development and budding. When phosphorus is deficient root and bud development will be slow and stunted. Plants grow slowly, with small leaves.

A plant with a phosphorus deficiency will show it in their older leaves first. Leaves become dark green and weak. Then they develop brown and yellow patches and limp downward. Younger leaves will pull phosphorus from the older leaves, showing symptoms last.

Extra phosphorus is beneficial during the first week of growth to aid in root development. Phosphorus can also be increased from the last week of vegetative growth to the 2nd last week of bloom to support flower development.

Cold weather below 10°C, 50°F slows the absorption of phosphorus. Increase phosphorus when the grow room is cold.

Corrective measures will not fix existing damage, but will restore plant health. Symptoms will clear up on the new growth.

If phosphorus is toxic, copper and zinc get locked out. Symptoms of a phosphorus toxicity look like a deficiency of these other minerals. This can make a toxicity diagnosis difficult.

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Potassium | Plant deficiencies & toxicities.

Potassium plays a role in many aspects of plant growth and water transportation throughout a plant’s lifecycle, and is particularly important for bud growth.

If a plant is deficient in potassium it might not be immediately obvious because minor deficiencies can make plants grow taller than plants that aren’t deficient.

Symptoms of a minor deficiency begin to show as necrotic spots or browning on the leaf margins.

If the deficiency is more severe, the veins of the leaves will turn red or yellow, and the leaves will develop large chlorotic spots. Plant stems turn red.

To correct a potassium deficiency add kelp extract to the solution because kelp is high in potassium. Plant will recover in under a week.

Potassium deficiency is the least common macro nutrient deficiency for hydroponics.

If potassium is toxic, magnesium, iron, zinc, and manganese get locked out. Symptoms of a potassium toxicity can look like a deficiency of these other minerals making a toxicity diagnosis difficult.

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Calcium | Plant deficiencies & toxicities.

A calcium deficiency is common because many hydroponic fertilizer products contain very little calcium, plus in some regions there isn’t very much of it native in the tap water either. If calcium is deficient in both the tap water and the fertilizer a calcium deficiency is probable.

The first signs are yellowing on the new shoots, followed by brown spots appearing on younger leaves.

Older leaves start to show symptoms after damage has already appeared on the new growth. They turn dark green while stems weaken. Browning starts along the leaf edges before advancing inward and killing the leaf.

Cal-Mag can be used as a foliar spray and in solution to correct a calcium deficiency. It can be used in solution during both flowering and vegetative growth. Only use Cal-Mag as a foliar spray during vegetative growth.

A calcium toxicity blocks out potassium and magnesium, causing symptoms that look similar to a deficiency of either of these two minerals.

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Magnesium | Plant deficiencies & toxicities.

Magnesium deficiency is very distinctive and easy to diagnose. The lower leaves turn very light yellow, except for the veins which remain green. The sick leaf edges also feel crispy.

Magnesium is a mobile element. Plants can take it from older growth to save new growth. Symptoms will show up in the old growth first.

Symptoms appearing on the old growth first is the way to distinguish a magnesium deficiency from an iron deficiency which has the exact same symptoms except they appear on the new growth first.

As the deficiency gets worse, symptoms travel up the plant, finally affecting the leaf shoots which change from healthy green to golden.

Magnesium deficient plants respond very quickly to Cal-Mag added to the reservoir, or applied as a foliar spray. It’s the easiest deficiency to fix.

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Sulphur | Plant deficiencies & toxicities.

Sulphur deficiencies are uncommon in hydroponics because most fertilizers contain enough that it’s rarely ever an issue.

If sulphur is deficient the plant will experience overall stunted growth. Leaves grow slowly and are smaller and narrower than usual. Stems grow tall and lanky instead of bushy. Buds are slow to develop and can die.

Sulphur is an immobile nutrient. Plants cannot relocate sulphur to support new shoots and leaves. This causes symptoms to show up on the new growth first.

A clear indicator of sulphur deficiency is yellowing of the new growth that starts at the base of the leaf and moves out toward the tips.  

Sulphur deficiency also causes stems to turn purple. 

A sulphur toxicity stunts the plant. Foliage turns dark green.  The leaf tips turn brown.