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How to Prevent Nitrogen, Calcium & Magnesium Deficiencies (N & Cal-Mag)

Here’s the short-and-sweet, cheat-sheet version:

  • Bottom leaves are wilting, turning yellow, and falling off on their own
  • Plant is in the flowering stage
  • Leaves are dark green
  • Leaf tips are clawed

You may need a Cal-Mag supplement if…

  • Using reverse osmosis (RO) or soft water
  • Growing in coco coir
  • Your plant is showing a deficiency in Calcium (brown spots), Magnesium (lower leaves turn yellow between the veins) or Iron (bright yellow new growth) even though the pH is correct
  • Some growers always include a Cal-Mag supplement

Cannabis plants grow best when they’re getting the nutrients they need at the right time. Humans are the same way; if we’re missing certain vitamins it limits our growth when we’re young, and as we get older a lack of vitamins causes us to become sick in strange ways. Conversely, humans can also run into problems when they get an overabundance of vitamins.

Just like humans with vitamins, giving cannabis too high levels of nutrients can also cause problems. Like many things in life, cannabis plants (and plants in general) need a sort of balance.

Luckily, for the most part, cannabis plants are not too picky about nutrients despite what nutrient companies might have you believe.

Cannabis is generally not that picky about nutrients, except for a few…

Your cannabis plants can tolerate a wide range of environments, but there are a few nutrients which tend to cause problems more often than others. If these nutrients aren’t present in the correct amounts, it will affect your growth rates, as well as the quality and quantity of your buds.

That being said, most of the time, nutrient deficiencies are caused by the wrong root pH, not from needing more nutrients.

Most nutrient deficiencies are caused by the wrong root pH, but sometimes you actually need to add more or less of a particular nutrient for plants to grow their best.

A few major nutrients often cause problems that are not related to pH. Although pH can be the culprit, these nutrients often need to be provided in higher or lower doses to prevent deficiencies/toxicities.

Many growers need more or less of these nutrients for proper cannabis growth and bud formation:

Nitrogen (N)

First, there’s nitrogen…Nitrogen makes up more of your plant (as a percentage of dry mass) than pretty much any other nutrient or mineral, and it plays a hand in many plant processes.

Without an external source of nitrogen, your plant cannot grow past its first or second set of leaves. Nitrogen is that important to growth and photosynthesis.

With nitrogen, you can get both deficiencies and toxicities.

Here’s what to look out for…

Symptoms of Nitrogen Deficiency

Lower growth on the plant turns yellow and sometimes the yellow leaves get brown splotches.

Affected leaves become soft and start wilting.

Once the leaf has wilted, it sometimes dries up and turns crispy.

Leaves fall off the plant on their own; a gentle tug will remove affected leaves.

If left unchecked, a nitrogen deficiency will climb up the plant.

There are other nutrient problems that cause the lower leaves of a cannabis plant to turn yellow, but only a nitrogen deficiency causes leaves to become soft enough to fall off on their own.

It is normal to lose a few leaves to nitrogen deficiencies here and there, especially if you have a lot of lower leaves that are receiving low amounts of light. It’s also normal to see nitrogen deficiencies in the second half of the flowering stage as the plants are focusing on buds instead of leaves.

However, if you see a nitrogen deficiency early in the plant’s life, or if you’re losing tons of leaves at once, a nitrogen deficiency needs to be addressed immediately.

But it’s not just too little nitrogen that causes problems. Often, you’ll see problems caused by too much nitrogen; a nitrogen toxicity.

Symptoms of Nitrogen Toxicity

Leaves can become dark green and sometimes shiny looking

Tips of leaves are bent sharply down in what’s known as “the Claw.”

Buds grow slowly and final taste/smell is not as good as normal.

Leaf tips pointing down from nitrogen toxicity

Here’s what a nitrogen toxicity looks like during flowering. This much nitrogen during the budding stage will cause buds to develop more slowly and reduce overall yields. Plus, nitrogen-burnt buds tend to carry a “hay” or “fresh-cut grass” taste/smell even after being harvested and cured.

Many new growers accidentally give their plants too much Nitrogen, especially in the flowering stage

As a grower, you’re interested in how much nitrogen to give your plants at a specific time. The ratio of nitrogen to other nutrients has a huge effect on growth and bud formation.

Vegetative Stage – higher levels of nitrogen (pretty much any plant food will do)

Most complete plant foods that you get at a gardening store contain high levels of nitrogen (N). These nutrient systems tend to work well in the vegetative stage.

Some examples of cannabis-friendly one-part Vegetative nutrient systems…

Pretty much any complete plant food

Flowering Stage – lower levels of Nitrogen (use “Bloom” or Cactus nutrients)

It’s extra important to find a nutrient system with lower levels of nitrogen for the last part of your plant’s life. Many “Bloom” or “Flowering” style base nutrients are just the ticket.

Some examples of good one-part Flowering nutrient systems…

If you can’t order online and can’t find a good one-part base Bloom formula locally, you do have other choices. Though not an ideal choice, most Cactus plant foods will contain good nutrient ratios for growing cannabis during the budding stage. So in a pinch, you can use the cactus nutrients that can be found at most gardening stores.

Ok now that you’ve got a handle on nitrogen….

Cal-Mag: Calcium (Ca) & Magnesium (Mg)
and their cousin Iron (Fe)…

Calcium and magnesium often come together in a “Cal-Mag” supplement. These nutrients work together with iron in many plant processes, so iron is usually included in any Cal-Mag supplement.

Calcium, magnesium and iron are incredibly important to photosynthesis (making energy from light) and maintaining the structure of the plant. When things go wrong, you’ll often see multiple deficiencies at the same time while plant growth slows to a halt.

Why do these three specific nutrients almost always come packaged together? Calcium, magnesium and iron are closely related when it comes to nutrient uptake. If there’s a deficiency of one, it can trigger deficiencies of the others. So, iron is always provided in the right ratio of calcium-to- magnesium-to-iron, to help prevent an imbalance.

Here’s what to look out for…

Symptoms of Calcium Deficiency

Appears at the top of the plant – new growth on your plant is spotted and unhealthy

Growth slows to a crawl

New leaves develop small brown spots that never go away

Buds develop more slowly

With a Calcium deficiency, newer leaves look like this, with tons of little spots. This one was found near the top of the plant under the light

This lower fan leaf is mostly in the shade, but the calcium deficiency appears near the edges that are getting light. Calcium deficiencies often show up on parts of the leaves that are still actively growing.

Symptoms of Magnesium Deficiency

Appears at the bottom of the plant on older growth (instead of on top like a calcium deficiency)

Leaves become light green between the veins and may develop small brown spots

Leaves eventually turn yellow and photosynthesis stops on the affected leaves

If left unchecked, a magnesium deficiency slowly kills bottom leaves, with problems climbing up the plant as magnesium is transferred from older leaves to newer parts of the plant

Leaves with a magnesium deficiency do not usually fall off the plant on their own (as they do with a nitrogen deficiency)

Symptoms of Iron Deficiency

All new growth on your plant will be yellow, sometimes bright yellow in extreme cases like below.

Growth is severely stunted on affected leaves.

Relatively rare compared to calcium and magnesium deficiencies, as they usually “take over” the plant.

This iron deficiency may have been caused by not enough calcium in the coco coir growing medium.

Do you need a “Cal-Mag” supplement?

Just because you’re seeing related deficiencies DOES NOT mean that you necessarily need a Cal-Mag supplement for your cannabis plants.

In addition to being at the heart of many plant problems, Calcium and Magnesium deficiencies are also some of the most commonly misdiagnosed problems by indoor cannabis growers. I’ve seen growers prescribe Cal-Mag for almost everything, from slow growth to nitrogen deficiencies.

Cal-Mag is only needed for certain situations, and there’s no reason to add extra Cal-Mag if you’re not actually trying to fix something. This isn’t a “supplement” that will make your plants grow faster. It’s only useful in preventing nutrient deficiencies. That being said, a lot of cannabis growers include Cal-Mag in every grow and it

These things can trigger what appear to be Cal-Mag deficiencies – fix the problem and many deficiencies will go away…

PH is too high or low

Uneven moisture levels in the soil or growing medium

Too-high levels of N-P-K nutrients

Anything that negatively affects the ability of nutrients to travel through the plant can trigger Cal-Mag related deficiencies, even when the minerals are actually there near the roots.

That’s why it’s important to maintain correct humidity levels, and avoid letting some parts of the growing medium get wetter than others (best results are achieved by making sure all parts of the growing medium are equally wet).

More importantly, when you’re faced with a deficiency, check the pH levels of your rootzone. A surprising number of issues with pH are misdiagnosed and end up needlessly costing growers money. These charts will tell you what range is needed to uptake each nutrient: Soil and Hydro

Be careful not to overfeed plants in an attempt to keep away deficiencies. When plants are being bombarded with too-high levels of nutrients it can cause several strange leaf problems, with apparent signs of calcium, magnesium and iron deficiencies commonly showing up.

But sometimes you’ll see Cal-Mag deficiencies even when taking care of all the above problems. And there are several situations where a Cal-Mag supplement becomes necessary for healthy cannabis growth.

Add extra Cal-Mag when…

Growing in coco coir (coco coir tends to lock out calcium for cannabis, so extra Cal-Mag should be provided for at least the first two weeks of cannabis growth, and possibly throughout the grow)

Using RO water (Reverse Osmosis water is completely pure, and doesn’t contain any extra calcium, magnesium or iron)

Using “soft” or heavily purified water

Cal-Mag deficiencies are still appearing even after taking care of all the environmental triggers listed above

That’s it! You are now versed on some of the most important cannabis nutrients! Following the tips in this article will help you achieve the fastest growth and biggest yields possible!

Nitrogen, calcium and magnesium cause the most common cannabis nutrient deficiencies. Learn what to look out for and how to protect your cannabis plants!

Why & How to Use CalMag in Coco

Why You Need Cal Mag in Coco

Calcium and magnesium deficiencies are the most common nutrient problem with coco grows. To avoid these problems and effectively manage your fertigation, it is important to understand the relationship that coco has with calcium and magnesium. In this article, I explain the basics of cation exchange in coco to help you understand why you need calmag supplement when growing in coco. I also provide specific instructions for when to use calmag in coco and how much calmag is needed at different times during the grow.

All cannabis fertilizers come with Calcium (Ca) and Magnesium (Mg). This leads some growers to wonder if Cal Mag supplement is really needed. However, when you are growing in coco you have to be aware that the coco itself requires a certain dose of calcium and magnesium. The cation exchange sites in coco will lock on to the Ca and Mg and render them unavailable to the plant. We must provide larger doses to ensure that there is some Ca and Mg available to the plant!

Buffering the cation exchange sites with calmag prior to use is critical. See our tutorials, “How to Prepare and Buffer Coco Coir” and “How to Reuse/Recycle Coco Coir”. However, even if you buffer the coco thoroughly, or use a pre-buffered coco product, such as Canna or Roots Organics, you should still add additional Cal Mag supplement to all irrigation water during the grow.

Understanding Cation Exchange in Coco Coir

Coco is known as a neutral grow medium, but there are cation exchange sites in coco. These sites on the surface of the coco fibers will form bonds with particular nutrient cations. The cation exchange capacity of coco is unlike soil or other grow media. There are far fewer exchanges and the exchange sites will become stable (buffered) once they attach to Ca or Mg.

The cation exchange sites in coco naturally come loaded with sodium (Na) and potassium (K) cations. However, the sites have a weak hold on the Na and K cations. In the presence of calcium (Ca) or magnesium (Mg), the sites will release their Na or K cations and lock onto the Ca or Mg. Once the cation exchange sites in coco have locked on to Ca or Mg, they are “buffered” and stay stable.

The cation exchange sites in unbuffered coco can pull nearly all the Ca and Mg out of a nutrient solution. If you try to grow cannabis plants in unbuffered coco, they will experience Ca deficiency. This is tough to correct while plants are growing because Cal Mag supplements have a strong impact on electrical conductivity (EC). As a result, the total dose of CalMag that you can provide is limited, even though it all may be going to the coco and none to the plant.

Buffering the coco in advance is a critical step to creating an ideal growing media. However, even in fully and properly buffered coco, you should continue to provide additional Cal Mag supplement.

Why do you need Cal Mag Supplement If the Coco is Buffered?

The simple answer is that the coco is constantly breaking down. As large fibers of coco break down in the pots they expose new surface areas of the coco fiber. This also exposes new cation exchange sites that will take Ca and Mg from your nutrient solution and reduce the amount available to the plant.

When to use CalMag in Coco

You should add CalMag supplement to all the irrigation water that you provide to the plant throughout the grow. When adding Cal Mag supplement, you are not adding it for your plants! You are adding it to satisfy the coco. The amount that you need to add reflects the needs of the coco, not the plant.

Coco breaks down due to a variety of factors, but you can expect it to need the most Cal Mag supplement early in the grow. The fresh coco will have the greatest potential to break apart and this is hastened by the roots of growing plants. Young plants spend much of the energy pushing their roots through the coco. As they do so, they contribute to breaking it down and create an ongoing need for additional Ca and Mg to be added. Later in the grow, as plants are flowering, the coco will have become more stable and as a result it will need less Cal Mag supplement. This is helpful, as Bloom Boosters can step in and fill the EC quota that CalMag can surrender.

How to select a CalMag supplement

I use and recommend General Hydroponics CaliMagic. It is formulated to deliver a high dose of Ca and Mg without significantly contributing to the amount of nitrogen, phosphorous or potassium. When selecting a CalMag product, be aware of the quantity of nitrogen (N) in particular. N is the first number in the three number set listed on all fertilizers, for Calimagic it is 1-0-0. Many CalMag products also deliver high doses of N, which can create problems with your nutrient element ratio.

How Much CalMag to Use

The specific dose required will depend on the CalMag product that you are using. Cal/Mag supplements have a strong impact on Electrical Conductivity (EC). As a result, they must be dosed carefully. The amount of Calmag that you use will limit the doses of the other nutrients that will be added. To understand more about dosing nutrients and the importance of EC, read our articles, “Understanding EC, PPM & TDS for Growing Cannabis” and “EC/PPM Targets for Fertigating Cannabis”.

Because CalMag has a strong impact on EC, it takes up a lot of your “EC budget”. For example, I recommend providing nutrient solution at an EC of 1100-1200 during vegetative growth. However, a full-strength dose (5ml/gal) of Calimagic accounts for about and EC of 800. That leaves only 300 EC left in the budget for all the other nutrients. If you are confident in your buffer, you may lower the dose to make room for other nutrients, especially late in flowering. However, some Cal/Mag supplement should be given in all nutrient solution.

CalMag doses during Vegetative Growth

It is a good practice to begin with a full dose or a nearly full dose. With Calimagic this is 5ml/gal and accounts for about 800 EC. This can be impossible during the seedling stage when the plants require a very low inflow EC. See our article, “How to Grow Cannabis Seedlings in Coco” for specific advice about dosing CalMag for seedlings. If you start out with the full dose and the plants avoid CalMag problems, then you can reduce the dose as you enter late vegetative growth.

If you are using a prebuffered product like Canna or Roots Organics, then you may start out with a half dose. However, even with these pre-buffered products, Ca deficiency can occur if the dose of CalMag provided is too low.

See our article, “How to Mix Nutrients for Cannabis” for a complete nutrient recipe schedule that will allow you to mix nutrients with the right dose of CalMag and everything else!

CalMag doses during Flowering

If the plants are not having issues with Ca or Mg deficiency, then the dose can be lowered by half during flowering. This can be important, especially in mid to late bloom when you want to be able to provide a higher dose of phosphorous and potassium for flowering.

By mid-bloom, plants should do fine with half the dose of CalMag that they required during vegetative growth. However, always watch your plants and be prepared to increase the dose again if symptoms of deficiency emerge. In mature plants, the first symptom is usually brown spots on lower leaves.

Learn why you need additional Cal/Mag when growing in coco coir. Covers the role of cation exchange & offers simple instructions to avoid Cal/Mag problems