How to use Cal Mag for weed plants

Botanicare Cal-Mag Plus is a calcium, magnesium, and iron nutrient supplement for weed plants. It is typically used to correct a cal mag deficiency in cannabis.

Cal Mag for weed plants is used to correct Calcium and Magnesium deficiencies.
Cal Mag is used to correct Calcium and Magnesium deficiencies in weed plants. It comes in quart or gallon sizes.

In this tutorial we’ll walk you through how to use Cal Mag for weed plants, including dosage, frequency, usage with coco coir, ingredients, and alternatives. This will help resolve problems with Cal Mag deficiency during flowering.

Cal Mag Plus directions and dosage

The recommended dosage for Cal Mag for weed is 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 Cal Mag nutrient deficiency in your plants.

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 as soon as roots are established.

It’s common to have a Cal Mag deficiency for cannabis grown in coco coir. Watch your vegetating plants’ leaves closely to check for early signs of a calcium 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. For the first few weeks of vegetation in grows using either coco coir or using reverse osmosis (RO) water, Botanicare recommends increasing the amount to 7 ml per gallon.

Here's the guaranteed analysis when using the 5ml/gallon feeding schedule of Cal-Mag for plants

The best way to avoid a Cal Mag deficiency during flowering is to make sure you’re in good shape before you flip your lights to 12-12. If you see signs of a problem, ramp up your Cal Mag dosage to 7-10 ml per gallon for a few waterings and scale back to 5ml.

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

Best Price on Cal Mag Plus

The best prices for Botanicare Cal-Mag Plus can usually be found on Amazon.

Looking for alternatives to Cal Mag for cannabis plants? Here’s some other Cal Mag fertilizer options used by weed growers:

Knowing how to use Cal-Mag Plus will prevent calcium and magnesium deficiencies in your cannabis plants.

Cal Mag Plus feeding schedule and frequency

The best Cal Mag feeding schedule may change depending upon your growing medium, the amount of Mg and Ca in your primary nutrient line, the quality of your tap water, and the specific needs of the marijuana strain you are growing.

The correct feeding schedule to correct Cal Mag deficiencies during flowering or vegetation for weed plants is 5ml per gallon of water.If you’re growing cannabis plants in coco coir using tap water you risk cal mag deficiencies. The Cal Mag feeding schedule for coco is 5 mL per gallon, each time you water. You can try backing that off to 3 mL per gallon but watch plants closely for signs of deficiencies.

The mineral content of your tap water varies depending on where you live. 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 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, Mg, and other minerals.

The Cal Mag feeding schedule for soil is 5 mL per gallon each time you water per the Botanicare label instructions. Use a double-strength application of 10 mL/gallon to correct a severe Cal Mag deficiency in plants during vegetation. To correct a Cal Mag deficiency during flowering, avoid using strong doses during the later weeks up to harvest, and stop using entirely when you flush with plain water the final week.

Cal Mag feeding schedule for plants will prevent or correct a CalMag deficiency.

The best Cal Mag alternatives

Looking for a Cal Mag alternative? There’s a bunch of commercially available options for fertilizers to correct Cal Mag deficiencies in weed plants:

Cal Mag Plus vs. CaliMAGic

Calmag vs. CALiMAGic: CALiMAGic is a Cal Mag supplement for plants that is a good Cal Mag alternative.
CALiMAGic is a Cal Mag supplement for plants that is a good Cal Mag alternative.

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.

Find lowest price on Amazon

Organic Alternative to Cal-Mag Plus

TPS Nutrients makes an organic Cal-Mag supplement. At $15/pint it’s a bit more than Cal-Mag Plus, but if you’re looking to grow fully organic and need to correct a Cal Mag deficiency then this is the one for you.

Find lowest price on Amazon

FoxFarm Bush Doctor Cal-Mag

FoxFarm Bush Doctor Cal-Mag is a Cal Mag cannabis alternative that’s a bit more pricey than Botanicare’s version of Cal-Mag Plus. It runs about $28/quart.

Find lowest price on Amazon

Humboldts Secret Calcium, Magnesium and Iron Supplement

Humboldts Secret Calcium, Magnesium, and Iron Supplement is a bit cheaper than Cal-Mag Plus, it costs around $13/quart.

Find lowest price on Amazon

Bloom City Professional Ultra Pure Cal-Mag

Bloom City Professional Ultra Pure Cal-Mag is $35/gallon. We’ve never tried this Cal Mag fertilizer so we can’t offer any feedback on it.

Find lowest price on Amazon

If you grow marijuana plants in coco coir you'll need to consistently use a Cal-Mag supplement in order to avoid Calcium deficiencies.

How to use Cal Mag Plus for coco coir weed grows

One factor to help determine optimal Cal-Mag Plus dosage is how much calcium and magnesium is already  in your main nutrient source. Some nutrients account for this in their mix, so they’re less likely to run into a Cal Mag deficiency.

Cal Mag fertilizer prevents calcium and magnesium issues in weed plants.If you run the Canna Coco line of nutrients, for example, Canna A and B’s formula already provides some level of 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 (us included) have Cal Mag deficiencies in coco despite this.

Botanicare recommends using 5 ml of Cal Mag per gallon of water. 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 coir grows is a risk as you may experience some calcium deficiency symptoms after a few weeks. These can be quickly corrected – if caught early – by using the max recommended Cal Mag dosage of 5 mL per gallon of water for the next few waterings. Use 3-5 ml per gallon thereafter throughout your grow, ramping up if signs of a deficiency return.

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 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).

Cal mag can fix calcium deficiencies in weed plants.
A calcium deficiency in a weed leaf.

Cal Mag deficiency symptoms

The symptoms of calcium deficiency in cannabis plants are brown spots that appear on leaves. Yellow areas can appear around the brown parts and in other leaf areas as well. Calcium is an immobile nutrient so deficiencies will appear on new leaves.

Magnesium deficiency symptoms are a pale green, yellowish color in the outer edges of older, lower leaves. Magnesium is a mobile nutrient that can travel from old leaves to new ones, which is why these symptoms appear in older leaves.  A magnesium deficiency can occur when the pH level of your roots is too low, preventing magnesium uptake, and when that happens adding a calmag supplement won’t help. Correct by increasing pH levels for a few waterings to 6.0 in veg and 6.2 in flower to bring things back in line.

The best way to resolve Cal Mag deficiency symptoms and other nutrient issues with cannabis is to catch them early. This is why observing your plants daily is your greatest tool. But it’s even better to prevent likely calmag deficiency issues outright, which is why you should use a light dose of Cal-Mag plus throughout your coco grows.

If you’re trying to determine how to use Cal-Mag for weed grown with RO’d water, remember that you’ll need more than if you use tap water. Don’t wait for the Cal Mag deficiency to develop. Being proactive during vegetation means you won’t have to try and treat a Cal Mag deficiency during flowering.

Ideal pH for coco coir will allow your cannabis plants to properly absorb nutrients

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.

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 5.8 in veg, and 6.0 in flower.

A Bluelab pH pen can help you avoid Cal Mag deficiency symptoms in cannabis plants.
A Bluelab pH pen can help you avoid Cal Mag deficiency symptoms in cannabis plants.

Use pH Down to get your nutrient mix down to the right level.

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.

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.

One 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 within the plants’ root 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.

If you suspect your nutrient deficiencies are caused by incorrect pH, you can 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 waterings 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.

But don’t spend all your time chasing the runoff pH, if you’re putting the right levels in when you water the problems can be avoided.

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.

What does Cal-Mag do for plants? Cal-mag supplements are used to correct Calcium and Magnesium deficiencies in cannabis and other plants.
Cal Mag is a plant fertilizer that can fix a cal mag deficiency in weed plants. It comes in quart or gallon sizes.

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

What does Cal-Mag do for plants?

Cal-mag supplements are used to correct Calcium and Magnesium deficiencies in cannabis and other plants. Cal-Mag also contains other trace nutrients needed for proper plant health.

How to use Cal Mag for weed plants
Article Name
How to use Cal Mag for weed 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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