Recognize & remedy CalMag deficiency: Causes & tips

Is your cannabis plant not growing properly, are the leaves spotting or curling? This could be a CalMag deficiency - one of the most common but often overlooked nutrient problems in growing. It is becoming increasingly common, especially with LED grows. If you don't react in time, growth can stagnate, flowering suffers and your yield shrinks. But don't panic! In this guide, you will learn how to recognize a CalMag deficiency, what causes it and how you can remedy it quickly. 🔎💡

What is a CalMag deficiency? 🤔

If cannabis plants don't grow properly, have discolored leaves or suddenly weaken, this is often due to a CalMag deficiency. 🌱 But what exactly is behind it?

CalMag stands for calcium (Ca) and magnesium (Mg) - two essential nutrients that your plants urgently need. Without them, the metabolism does not run smoothly, growth stagnates and visible damage occurs to the leaves. This problem is particularly common in indoor grows with LED lamps, but more on that later. First, let's take a look at why these two nutrients are so important.

Why calcium and magnesium are essential 🧐

🔹 Calcium (Ca) is the "building material" of your plants. It strengthens the cell walls, promotes root formation and ensures resilient stems. A stable calcium balance prevents leaves from becoming brittle or deformed.

🔹 Magnesium (Mg) is the fuel of photosynthesis. Without magnesium, the plant cannot form chlorophyll - and without chlorophyll there is no energy production. The result? The leaves turn yellow, the plant can no longer grow properly and becomes increasingly weak.

What does this mean in practice? Quite simply: without calcium and magnesium, the entire plant suffers. First there are small symptoms such as yellow spots or slow growth, later the tissue dies and the yield can plummet.

Why cannabis reacts particularly sensitively to a CalMag deficiency 🌿

Cannabis is one of the plants with a high nutrient requirement - and this also makes it more susceptible to deficiencies. Especially in the flowering phase, they need sufficient calcium and magnesium to develop large, strong flowers. But there is another factor that can exacerbate the problem: LED lighting.

Modern LED grow lights have many advantages, but can also promote a CalMag deficiency. The reason? LED light often has a different light spectrum, which increases the plant's magnesium requirements. This means that anyone growing with LEDs must pay particular attention to a sufficient supply of CalMag!

But a deficiency is not only caused by the light - the pH value of the substrate, the water quality and the fertilization scheme used also play a major role. More on this in the next section! 😉

Causes of a CalMag deficiency 🔍

A CalMag deficiency doesn't just appear out of nowhere - there is always a cause. And this is where the key to the solution lies: if you know why the deficiency occurs, you can prevent it before the plant suffers seriously. 🌿 But why does cannabis suddenly show symptoms of CalMag deficiency?

CalMag deficiency due to LED lighting 💡

LED grow lights are efficient, save electricity and emit less heat than classic NDL lamps. But that can be a problem! 🤯

🔹 Why is that? LEDs emit less infrared light, which keeps the leaves cooler. The plant evaporates less water - and therefore absorbs fewer nutrients.
🔹 The result: less calcium and magnesium end up in the plant sap, even though there could be enough in the medium. This can lead to growth disorders and deficiency symptoms.

Hydroponic growing media or soil with a weakly buffered pH value are particularly affected, as nutrient uptake here depends heavily on the plant's evaporation rate.

Incorrect pH value in the substrate ⚖️

Even if there is enough CalMag in the soil or water - if the pH value is not right, the plant cannot absorb the nutrients. An incorrect pH value therefore blocks the supply, even though the minerals are right under the "nose" of the roots.

🔹 Optimum pH value:

• Soil: 6.2 - 7.0
• Hydro / Coco: 5.5 - 6.2

If the pH value is outside this range, calcium and magnesium may no longer be soluble and therefore cannot be absorbed by the roots.

Water that is too soft or filtered 🚰

Many growers use osmosis water or very soft tap water - which is not bad in itself. But this is precisely where the risk lies: water with low hardness contains hardly any calcium and magnesium!

🔹 Tap water with too little hardness (<100 mg/l CaCO₃): Naturally lacks the necessary amounts of calcium and magnesium.
🔹 Osmosis water: Is almost completely freed of minerals by the filter - perfect for some plants, but problematic for cannabis.

Without an additional supply of CalMag, the plant cannot absorb enough of these important minerals, even if the pH value is optimal.

Recognize CalMag deficiency: Symptoms on the leaves 🔎

A CalMag deficiency is not immediately apparent. Initially, the plant may only appear a little slower in growth or look slightly weakened. But over time, the symptoms become more and more obvious - especially on the leaves.

It often starts with small, light-colored spots on the older leaves. These spots gradually become larger and turn brownish in color. The leaves feel brittle, curl or develop cracked edges. It is particularly noticeable that the leaf structure suffers - the surface can feel rough or uneven, almost as if someone has pressed tiny dents into it.

While a calcium deficiency is mainly visible on the lower leaves, a magnesium deficiency is more apparent on the middle to upper leaves. The plant draws magnesium from the older leaves to put it into the new shoots. As a result, the older leaves slowly turn yellow, while the leaf veins often remain green - a classic sign of magnesium deficiency.

If you don't react in time, you risk the deficiency spreading. The spots become larger, some of the leaves die and eventually fall off. At the same time, growth suffers: new shoots remain small, the flowers develop more slowly and the plant appears weaker overall. During the flowering phase, this can lead to considerable losses in yield.

Many growers confuse the symptoms with a lack of nitrogen or over-fertilization. However, while nitrogen deficiency tends to cause uniform yellowing, the spots and uneven dying of the leaves are a clear sign of a CalMag deficiency. Action should be taken at the latest when growth stagnates and the leaves become brittle - but how? We'll find out in the next section! 🌱💡

Correcting CalMag deficiency: What helps? 🛠️

As soon as the first symptoms of a CalMag deficiency appear, action should be taken quickly. The sooner the plant is supplied with calcium and magnesium again, the better it can recover. However, simply fertilizing blindly is not a solution - the cause must first be found and remedied.

An important first step is to check the pH value. If it is outside the optimum range, the plant cannot absorb the nutrients properly, even if they are present. In soil, the pH value should be between 6.2 and 7.0, in hydro and coco setups between 5.5 and 6.2. If the value is too low or too high, it helps to adjust the irrigation water accordingly. A pH value that is too high can be lowered with slightly acidic solutions, while one that is too low can be raised with a little lime or dolomite rock.

Water hardness also plays a decisive role. Very soft water - whether due to natural conditions or the use of osmosis filters - often contains too little calcium and magnesium. In this case, it can be useful to enrich the water with special CalMag additives. Alternatively, osmosis water can also be mixed with a small amount of tap water to add the required minerals.

If LED lamps are used, it is worth keeping an eye on the light intensity and temperature. Leaves that are too cold evaporate less water, which reduces nutrient uptake. A slight increase in the ambient temperature or a gentle increase in air circulation can help to promote evaporation and thus improve the absorption of calcium and magnesium.

If the plant is already showing clear signs of deficiency, rapid foliar fertilization with a light CalMag solution can help. The nutrients are absorbed directly through the leaves so that the plant can react more quickly. However, this is more of an emergency solution - in the long term, the cause of the deficiency must be eliminated so that it does not recur.

As soon as the plant is supplied with sufficient calcium and magnesium again, the symptoms stop relatively quickly. The already damaged leaves will not recover, but new leaves will grow back healthy and growth will return to normal. To avoid future problems, it is worth taking some preventative measures - and this is exactly what the next section is about! 🌿✨

Preventing CalMag deficiency: tips for a healthy grow 🌿

A CalMag deficiency can not only slow down growth, but also have a negative impact on the flowering phase and yield. The good news: with the right measures, the problem can be avoided in the first place. Here are the best tips to keep your plants healthy and strong.

1️⃣ Ensure optimal nutrient supply

Cannabis requires more calcium and magnesium than many other plants. If you work with pre-fertilized soil, you should make sure that it contains sufficient CalMag. Regular additions are often necessary in hydro or coco grows.

✅ Advantage: The plant receives all the important nutrients directly.
❌ Disadvantage: Too much CalMag can displace other nutrients, so the dosage should be correct.

2️⃣ Check the water quality regularly

Not all water contains enough calcium and magnesium. Soft or filtered water can be problematic as it often does not contain sufficient amounts of these minerals. If you use osmosis water, you should supplement it with a CalMag additive or some tap water.

✅ Advantage: Controlled nutrient supply and stable water quality.
❌ Disadvantage: Additional effort due to regular tests and adjustments.

3️⃣ Keep an eye on the pH value

The pH value determines whether the plant can absorb the nutrients or not. An incorrect pH value blocks the absorption of calcium and magnesium, even if there is enough available.

✅ Advantage: Stable nutrient uptake and healthy growth.
❌ Disadvantage: Requires regular measurements and adjustments.

4️⃣ Using LED grow lights correctly

LEDs save energy and produce less heat, but can reduce the plant's evaporation rate, which affects nutrient uptake. A slightly higher ambient temperature or better air circulation can help to support the natural flow of water and nutrients.

✅ Advantage: Efficient lighting with less heat development.
❌ Disadvantage: Increased magnesium requirement and potentially poorer nutrient uptake.

5️⃣ Look out for symptoms early on

If you inspect your plants regularly, you can recognize signs of deficiency at the earliest stages. Pale spots, brittle leaves or slowed growth are warning signs that should be taken seriously.

✅ Advantage: Problems can be solved early before they cause major damage.
❌ Disadvantage: Requires attention and experience to interpret symptoms correctly.

These measures can effectively prevent a CalMag deficiency. Paying attention to the right conditions from the outset not only saves you stress, but also ensures strong, healthy plants with maximum yield. 🌱💪