Do my Tomato Plants Really Need Cal-Mag?


The short answer: Yes, all plants need calcium and magnesium but whether or not the gardener needs to add it to their fertilizer routine it may depend on….

  • What type of tomato you’re growing

If you’re growing a cherry-type, salad-type or other small fruited tomato, the cal-mag in your soil and other fertilizers will likely be enough. If you’re growing larger “all-purpose”, “beefsteak” or “heirloom” type of tomato, you will benefit from applying Cal-Mag fertilizer when these tomatoes begin to flower.

  • If you’re growing in pots or in the ground/large raised beds

If you’re growing your tomatoes in a container garden (aka pots), they will be relying solely on you to provide their nutrition.  My favorite way to do this is to apply a dry “slow release” product at planting time like Down to Earth Rose & Flower Food or Lilly Miller Tomato & Vegetable Fertilizer and reapply once per month through the growing season.  Then, I water “weakly” weekly with a compost tea product like FoxFarm BigBloom + Cal-Mag upon flower formation.  I treat my raised beds/in ground plantings in a similar manner; however, these beds are fertilized according to a soil report that is custom to my soil in my yard.  This soil report often tells me to apply Gypsum (Calcium Sulfate), Langbeinite (Sulfur, Potassium, Magnesium) and other NPK containing fertilizers so some of my calcium and magnesium demand is already being added to build my soil.  I still water in Cal-Mag bi-weekly instead of weekly, again, at the time of flower formation to ensure that my tomatoes do not have Blossom End Rot.

  • What kind of fertilizers or soil builders you’ve already applied/ going to apply 

Again, check the label of the products that you are already applying. If they already have a calcium and magnesium content, water with additional cal-mag bi-weekly.

  • If you’re providing ample water to your tomatoes

Providing liquid Cal-Mag to your garden tomatoes will be in vain and a waste of time and money if your tomatoes aren’t getting enough water.  The lack of calcium is a culprit to Blossom End Rot, but so is inconsistent watering (which leads to a lack of calcium in the plant).  


The long(er) answer…


Understanding the Role of Calcium and Magnesium

Many food crops are calcium loving and tomatoes are no exception. Calcium is crucial for tomato plants as it aids in the formation of cell walls, preventing issues like blossom end rot.  This nutrient ensures strong, healthy plants that can support the weight of numerous fruits.

Magnesium is a key component of chlorophyll, the molecule that enables plants to photosynthesize and create energy. It also aids in the uptake of other essential nutrients, promoting overall plant health and growth.

Most products come pre-formulated as “Calcium + Magnesium” and many times nitrogen and iron are in the mix.  This is due to the source of calcium and magnesium and has a good bit of chemistry to it, but to simplify, calcium works better when magnesium is in the mix as well.

Why Use Calcium-Magnesium Fertilizer?

The primary reason most gardeners use Cal-Mag fertilizer is to prevent Blossom End Rot.  Blossom End Rot is a common issue in tomatoes where the bottom of the tomato fruit becomes dark, sunken and rotted looking.  While Blossom End Rot does not render your tomato completely unusable, it is unsightly and does cause yield loss.  Large fruited tomatoes are very prone to Blossom End Rot and will see the most benefit from using Cal+Mag.  Don’t forget that calcium and magnesium are still crucial to plant growth as they are on the list of 17 Essential Elements for plants. 

How to Use Calcium-Magnesium Fertilizer?

Calcium and Magnesium can both be added to the soil via “amendments” such as:

  • Gypsum (Calcium Sulfate, may help lower soil pH over time)
  • Oyster Shell Meal (Calcium)
  • Crab Meal (Nitrogen, Phosphorus, Calcium, Magnesium)
  • Bone Meal (Nitrogen, Phosphorus, Calcium)
  • Fish Bone Meal (Nitrogen, Phosphorus, Calcium)
  • Dolomite Lime (Calcium, Magnesium, raises soil pH)
  • Langbeinite (Sulfur, Potassium, Magnesium)
  • Epsom Salts (Magnesium)
  • Lime (Calcium, raises soil pH)
  • Seabird Guano (Phosphorus, Calcium)

Add these amendments to your soil per a soil test’s recommendations or follow the label on the product.  In most soils in the Gallatin Valley, gypsum, bone meal and langbeinite are the most common additives.  Add these amendments to your garden before planting.  Always consult a soil scientist before applying amendments to your soil–especially if this is a new concept to you.  I would recommend reading The Intelligent Gardener by Steve Solomon if soil health and amending is of interest to you.  

These amendments can be added to pots and container gardens as well but be sure to check the calcium and magnesium content of other fertilizers you plan to use.   Most container gardens are filled with “Potting Mix” which tends to be more acidic than our native soils in Gallatin Valley.  A simple pH test kit can be used (kits are around $10) to check your pH. If it is above 6.5, you may want to add Lime instead of Gypsum to your potting soil.

Begin using liquid Cal-Mag when your tomato plants begin to flower.  Do this weekly or bi-weekly depending on your fertilizer plan or soil report.  Follow the product label for how much to add per gallon of water.  When mixing any liquid fertilizer, fill your container about ⅓ full with water.  Then, add your fertilizer and fill the rest of the way with water.  This helps to mix the product with the water.  Popular liquid Cal-Mag fertilizers are:

  • FoxFarm Bush Doctor Cal-Mag (OMRI Listed Organic)
  • Botanicare Cal-Mag+ (Contains iron)
  • Roots Organics CalMag (Contains sulfur, CDFA Organic)

Cal-Mag is not a complete fertilizer and are not the only nutrients that your plants need.  However, by implementing Cal-Mag into your routine, you will be rewarded with stronger, happier plants that do not lose yield to Blossom End Rot.

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