First time, small-scale indoor growers often run into magnesium and calcium deficiencies when using garden store potting soil and fertilizer, because cannabis requires so much of these nutrients to grow as quick and strong as it wants to. A soil mix designed for cannabis avoids this problem, but for now you need to make this crop worth your while.
If after about a month of growth the bottom leaves on your pot plants start to turn yellow in between the veins, this is a sign of a magnesium deficiency, along with slowed growth and a droopy look overall. The deficiency progresses from the bottom leaves, upwards and the leaves that first started to show yellowing will become more yellow at the fringes.
You may also see parts of the most affected leaves get brown, dead spots. This indicates an added calcium deficiency. Both magnesium and calcium deficiencies often appear together, but don’t go dumping Cal-Mag supplements in there just yet. First you should check the pH of your water (and/or nutrient solution), runoff water, and most importantly: the soil. Specialized techniques allow use a regular pH meter as opposed to a dedicated soil pH meter to measure soil pH.
If your pH is below 6, to correct your magnesium deficiency you should make an effort to bring it up to 6.5, but not any higher. If the pH of the water/ nutrient solution you add to your plants is too low, bring it up to 6.5 using any commerical product that raises pH. You may also need to correct the pH of your soil; adding dolomite lime (buy it from a garden supply store and add the recommended amount) should raise pH and stabilize it, all while supplementing calcium and magnesium.
For quick results, and additional magnesium supplementation at any stage of growth, use an Epsom salt foliar feed. Just add one tablespoon of Epsom salts, which is actually magnesium sulfate, to a gallon of water and spray every three or four days, but replace with pure water every so often to clean off the leaves. You’ll find Epsom salts can also be added directly to soil and is part of Subcool’s Super Soil.
During bloom cannabis plants require even more calcium and magnesium because of the heavy bud production. Don’t leave your plants starving for nutrients during their most productive stage in life, so pay close attention to their health. While nutrients made for cannabis will have different bloom preparations that keep this in mind, standard fertilizers may not. Even so, keep an eye on your plants’ leaves. If you start to notice the symptoms of a Cal-Mag deficiencies during early flowering you’ll need to act very fast. Use Epsom salt or special Cal-Mag foliar feed first, but you must also supplement the soil without causing any major fluctuations in pH, which can cause other deficiencies.
Cal-Mag supplements are great, but pay attention to the labels. You want something that provides just what you need. If it also has nitrogen or other nutrients, you run the risk of overfeeding, which isn’t good either. There isn’t any magical calcium to magnesium ratio you need to keep, just make sure your plants get enough of them.