How to use Cal-Mag Plus with marijuana plants

Botanicare Cal-Mag Plus is a calcium, magnesium, and iron nutrient supplement used for marijuana plants. It is typically used to correct calcium and magnesium deficiencies in marijuana plants.  In this tutorial we’ll walk you through how to use Cal-Mag Plus with marijuana plants, including dosage, frequency, usage with coco coir, ingredients, and alternatives.

Cal-Mag Plus directions and dosage

The recommended dosage for Cal-Mag Plus is 3 to 5 mL per gallon of water. 5mL equals one teaspoon. So if you were using a 5 gallon bucket to fill up, you would use between 15 and 25 mL of Cal-Mag supplement. This would be the correct dosage of Cal Mag to use in soil if you have an existing magnesium or calcium nutrient deficiency in your plants.

While the directions of 3-5 mL are recommended Cal mag dosage on the label, if you’re using it as a preventative measure (rather than correcting an existing problem) then you can go with a weaker ratio such as 1 mL per gallon in soil and coco grows.

If using a soiless medium such as coco coir, it’s a good idea to load up on the Cal-Mag prior to transplanting your established clone or seed. Start with 5 mL/gallon for the first few waterings, then scale back to 1-1.5 mL/gallon thereafter.  Watch your vegetating plants’ leaves closely to check for early signs of a calcium or magnesium deficiency. Really get in there and inspect them, it’s the best way to catch any nutrient or pest issues.  The quicker you catch them, the easier it is to recover.

Using the standard strength dilution of 5 mL of Cal-Mag per gallon will achieve approximately:
  • N 53
  • Ca 84
  • Mg 31
  • Fe 2.6

Cal-Mag Plus feeding schedule and frequency

The frequency you should use Cal Mag plus may change depending upon your growing medium, the amount of Mg and Ca in your primary nutrient line, and the specific needs of the marijuana strain you are growing.

For marijuana grows in coco coir using tap water, initially during the first few weeks of veg you can use the recommended feeding schedule recommendation 5 mL per gallon. Then you could try using around 1 mL per gallon or 1.5 mL per gallon of Cal-Mag Plus every time you water your plants thereafter as a preventative measure. Some coco growers prefer to skip every other time if the plants are looking good.

Your tap water has varying degrees of minerals. Therefore the amount of Calcium and Magnesium supplement you’ll need may vary. After a while you can get dialed in for a particular strain. Taking detailed notes will help you establish your own Cal-Mag feeding schedule that’s best for the strains you are growing.

If you’re using Cal-Mag in hydro systems with RO water, you’ll need to use the full dosage of 5 mL of Cal-Mag per gallon and do it time you water since the reverse osmosis process will leave your water devoid of Ca and Mg.

The correct dosage for Cal Mag in soil is 5 mL per gallon each time you water per the Botanicare label instructions.

When to use Cal Mag Plus in coco coir

One factor to help determine the best Cal-Mag Plus frequency and dosage is how much calcium and magnesium is already  in your main nutrient source. If you run the Canna Coco line of nutrients, for example, Canna A and B’s formula already provides some level magnesium and calcium as part of their nutrient mix. Canna indicates that their Canna A and B nutrients should contain sufficient Ca and Mg when used with tap water, but many growers in coco have deficiencies despite this. If you are using Cal-Mag with RO water in coco then you should plan to use more than if you’re using tap water.

Not using Cal-Mag in coco grows is a risk as you may experience some magnesium or calcium deficiency symptoms after a few weeks. These can be quickly corrected – if caught early – by using the recommended bottle dosage of 3 mL per gallon of water for the next few waterings, and then a week or two after the new growth stopped showing symptoms I would use 1.5 mL per gallon thereafter.

Repeated runs are a helpful way to really get to know how to use Cal-Mag Plus with the marijuana strain you’re growing. To prevent the need for emergency correction for a calcium or magnesium deficiency down the line, get in the habit of using a little bit of CalMag throughout the grow.

Plan to typically back this off towards the last few weeks of flowering though, because you don’t want to risk having extra nutrients negatively affecting the taste of your weed. The last week of flowering always flush for at least seven days with clean pH’d water. This water for flushing should contain no Cal-Mag or other nutrients (just Cannaboost or similar flowering enhancers that allows buds to live up to the max potential).

If you notice signs of a calcium or magnesium deficiency on your plants, it is very important to address them as quickly as possible. This is why it is so vital to monitor your plants daily, even if you use an automated watering system.

The best way to resolve nutrient issues with cannabis is to catch them early. But it’s even better to prevent likely issues outright, that which is why you should use a light dose of Cal-Mag plus throughout your coco grows. It’s ok to do some waterings throughout the plant cycle without this calcium and magnesium supplement, but you may find better results using it at a minimum of 50-75% of the time just to be safe.

If you’re trying to determine how to use Cal-Mag for hydro with RO’d water remember that you’ll need more than if you use tap water.

Check for pH issues if you have nutrient deficiencies

If you suspect your marijuana plants have either a calcium or a magnesium deficiency, before you just run to apply a nutrient solution such as Cal-Mag to resolve what you think is the problem, it pays to make sure that your pH is in good shape.

Improper pH can lead to a number of symptoms in weed plants. If the pH in your water falls outside of the levels where marijuana can properly absorb nutrients those nutrients will be locked out.

The best pH range for hydroponic marijuana grows is between 5.5 – 6.5. For coco coir I usually try to keep it from 5.7 to 6.0. I like to put a range in there because some nutrients are better absorbed at different levels.

Check your pH meter to ensure there is a problem. Calibrate it regularly to ensure accurate readings. Store it correctly, keeping the bulb moist. Drying out your pH meter is a problem many novice growers experience. The device will stop reading accurately if you allow this to happen, so make sure you store it according to the meter’s instructions to ensure a long lifespan for it.

The best way to determine the best pH for marijuana growing in coco coir is to observe the pH of the runoff after you water, as this will be indicative of what’s happening with in the plants’ rooting system. With coco you want to water so about 15-20% of the nutrient solution passes through the plant container. Watering to runoff helps flush excess nutrient salts from the coco medium.


Collect this runoff and test the pH. If the runoff is over 6.2, you know that you need to try to back it down a bit by running the pH of your next few watering lower – around 5.6 or 5.7. Similarly if your run off pH is too low, you will want to go higher with the pH the next few times you water to bring it back up. Maybe around 6.2 or so for a few waterings. don’t forget to check PPMs of the runoff as well to check for other potential warning signs.

Cal-Mag Ingredients: what is Cal Mag Plus made of?

Cal mag is a calcium and magnesium booster that is made up of a number of ingredients. It has an NPK value of  2-0-0.

The ingredients in Cal-Mag Plus are:

  • calcium
  • magnesium
  • iron
  • boron
  • zinc
  • manganese
  • molybdenum
  • copper
  • cobalt
  • iodine
  • beneficial vitamins
  • twenty essential amino acids
  • other botanical plant extracts

The best alternatives to Cal mag: Cal-Mag Plus vs. Calimagic

There are several alternatives to Cal-Mag on the market. The most commonly used Cal-Mag alternative is CALiMAGic, made by General Hydroponics. The Cal-Mag Plus vs. Calimagic debate has opinions on both sides. Both have reviews from experienced marijuana growers stating they do a great job, so a lot of it comes down to personal opinion.

CALiMAGic is used for the same purposes as Cal-Mag to correct magnesium and calcium deficiencies, and often costs slightly less. Due to this difference in price, the Cal-Mag vs. Calimagic proponents often say one is a better value (Calimagic) or that one is a better manufacturer (Cal-Mag Plus). Opinions be what they may, you’ll likely find that either does a very suitable job.

In general a small or medium sized grow will not use a lot of either Cal-Mag or CALiMAGic if they know how to use Cal-Mag Plus correctly, so the extra dollar or two isn’t much of a consideration in the long run if you decide to use either of these calcium magnesium supplements for your marijuana garden.

How to use Calmag Plus with Marijuana plants
Article Name
How to use Calmag Plus with Marijuana plants
We'll show you the proper dosage and the frequency for using Calmag to fix calcium, magnesium, & iron deficiencies in your weed plants, and how to apply a low-strength dosage to prevent these deficiencies from happening.
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Happy Pot Farmer
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