Cal-Mag stands for Calcium and Magnesium, two of the macronutrients that are rarely discussed when we talk about plant fertilizer, but they are critical for plants. You can buy Cal-Mag fertilizer to add to your regular fertilizer, or you can buy fertilizer that contains calcium and magnesium. Which option is better and do you need either one?
If calcium and magnesium are so important to plants why are they not included in all fertilizer? It is more complicated than you think.
The Calcium Story
Calcium (Ca) is a mineral that is important for plant growth where it plays a major role in developing cell walls. It’s also critical for activating enzymes that carry out all kinds of functions and it plays an important role in fruit development. Plants use it in fairly large amounts making it a macronutrient. A lack of calcium shows up as localized rot and stunted growth at the tips of plants. Leaf tips, buds and fruit don’t develop properly, leaves are dark green and stems are weak. A common symptom of a deficiency is the black end on tomatoes, called blossom end rot.
Plant nutrients can be immobile meaning that the nutrient can move up the plant through the xylem, but once in the leaves it can’t move back down or to other parts of the plant.
A calcium deficiency in plants can happen because there is not enough calcium in soil, or because there is not enough water moving up the plant, which is usually the cause of blossom end rot in tomatoes. Except for sandy soil, most soil has enough calcium because it sticks to clay.
The Magnesium Story
Magnesium (Mg) is also an important mineral for plants. This macronutrient is the central atom in chlorophyll, it’s used to activate enzymes, it helps with the transportation of other nutrients and it stabilizes nucleic acids.
This nutrient is mobile, so if a plant has low levels it will move magnesium away from older leaves to where it’s needed. This results in a yellowing of older leaves.
Limestone rocks decompose in soil and slowly release magnesium. As a result most soil, except for sandy soil, has enough magnesium.
The Importance of The Calcium/Magnesium Ratio
Studies done in the 1940s showed that plants grow best with a particular calcium to magnesium ratio, however more recent research has shown that this is not true. The calcium to magnesium ratio is not important for plant growth, but this myth is still very prominent in agriculture and gardening circles.
It is much better to think of these as two independent nutrients and that plants need different amounts of each.
Commercial Fertilizer Lacks Cal-Mag
Why does most fertilizer not include calcium and magnesium?
As mentioned above, most soil contains enough of both nutrients so there is no reason to add more when you fertilize. Sandy soil or very acidic soil may lack these nutrients in which case adding some extra can be beneficial. People with acidic soil tend to lime it to raise the pH and if they use dolomitic lime they are adding calcium and magnesium as part of the liming process and don’t need to add more when they fertilize.
A second reason for leaving it out of fertilizer is that a lot of tap water is hard which means it contains calcium and/or magnesium.
Some products do contain Cal-Mag. Scotts 15-5-15 Cal-Mag fertilizer contains 5% calcium and 2% magnesium.
Calcium and Magnesium for Plants
How much calcium and magnesium should be used for plants growing in a soilless media? The above Scotts fertilizer suggests around 66 ppm Ca and 26 ppm Mg. The University of Massachusetts suggests an alkalinity range of 37 to 105 ppm, calcium in the range of 40 – 100 ppm, and magnesium in the range of 30 – 50 ppm. Suggestions for hydroponic solutions are 80-140 ppm Ca and 30-70 ppm Mg.
A recommendation for agricultural loam and clay soil is 200 ppm Ca and 50 ppm Mg.
Calcium and Magnesium in Tap Water
If you use city water, check with the water department and they should be able to provide three values for you: alkalinity, calcium levels and magnesium levels. If you are on a well, but near a town that uses well water, you may have similar water as the town.
The alkalinity value will give you a total hardness number that tells you how suitable the water is for plants. More on that in What is Alkalinity – It May Not Be What You Think.
The calcium and magnesium values will tell you if you need to add Cal-Mag to fertilizer. My tap water has 114 ppm Ca and 34 ppm Mg. The calcium is a bit high and the magnesium is on the low side for a soilless mix, but I don’t need to use a Cal-Mag fertilizer and I should not use a fertilizer that includes calcium or magnesium, at least not for anything growing in a soilless mix.
There are numerous DIY recipes online for Cal-Mag. Most are some combination of eggshells and Epsom salts. The Epsom salts will add a suitable form of magnesium. Since most potting media has a pH between 6 and 7, the eggshells will not decompose and therefore won’t add much calcium. Your tap water probably has more. Therefore these recipes do not make a good Cal-Mag mixture.
You can also make your own Cal-Mag using Epson salts and calcium nitrate, which is what commercial products use, and this will produce a suitable solution.
Do You Need to Add Cal-Mag to Garden Soil?
If the soil has a pH above 7.0, it probably contains enough calcium and magnesium and you should not add more. If the soil is acidic and you see symptoms of either calcium deficiency or magnesium deficiency it is a good idea to have the soil tested and then follow directions. Alternatively, you can lime with dolomitic lime and improve the pH and add both calcium and magnesium.
If you irrigate a lot with hard water, you may not need to add any more.
Do You Need to Add Cal-Mag to Potting Soil?
The discussion in this section applies to any container that holds a soilless mix, including houseplants, outdoor containers and even raised beds.
Soilless mixes are usually based on peat moss, coir or composted wood. None of these provide high levels of calcium or magnesium which means that they need to be added along with the fertilizer.
If you are using high alkalinity water, that may provide all the Cal-Mag you need. In that case select a fertilizer that does not contain calcium or magnesium.
If you are using rain water, RO water, or distilled water, you need to add Cal-Mag. This can be done using a separate product, or the calcium and magnesium could be part of your regular fertilizer.
As I was researching this material I came across the term “buffering coir”. The term is not used by reliable sources of information but it does seem to be popular with marijuana growers. The process itself does make sense.
Coir is made from coconut husk and this raw material contains a lot of sodium (from salt water) and potassium. The coir has a fairly high CEC, which means that it holds on to cations such as sodium, potassium, calcium, magnesium, etc. Washing coir in fresh water will wash off a lot of surface salt (i.e. sodium), but it won’t wash out the sodium that is attached to the coir due to the CEC.
When you use this material for plants and start to fertilize, the calcium and magnesium in the fertilizer, or in hard tap water, will cause the sodium and potassium to leave the coir and be replaced by calcium and magnesium. This process is exactly the same as the one that takes place in a water softener. The net effect of this is that plant roots are exposed to high levels of sodium and potassium, and they get no calcium or magnesium. That is not good for plants.
To prevent this problem you can pretreat the coir with Cal-Mag before using it. To do this, put the coir in a pail with fertilizer water that contains calcium and magnesium or hard tap water or a commercial Cal-Mag product. The calcium and magnesium will stick to the coir, and the excess sodium and potassium will be released into the water, which is then discarded.
Online Suggestions for Using Cal-Mag
Ignore online suggestions about what you should or should not do about adding calcium or magnesium. Instead, read and understand the above and then do what is needed for your specific situation. People online don’t know your details and can’t give you the right advice.