how is the best way to treat magnesium deficiency?

The quickest easiest way to treat mag deficiency is with Epsom salt. You can grab it at the grocery store and add 2tsp per gallon to your nutrient solution. It is a great amendment for soil and organic while most calmag is derived from chemicals. Good luck with your grow

Wont repeat the advice already given so will offer a different solution. For minor Calcium or Magnesium deficiencies I prefer to foliar spray. It bypasses problems in the roots (PH lockouts etc) it just needs to be done well. Use a wetting agent (coco-wet etc) this helps absorption, use full or near full strength cal-mag (diluted down for your spray container), and pure water, RO or distilled best but not by a lot, tap will do. If you have a one quart spray bottle (mist setting preferred) that would be @0.5-1ml/quart. Get a sprayer that will spray mist upwards, makes coating the bottom the leaves (your target) much easier. As always this is a "Lights Off" procedure. Good luck

Hey! If you are going organic, you can use Epsom Salts on your growing medium. Personally, I prefer using CalMag (from the fertilizing company of your choice). You can use this one in 2 ways: - Water solution: Mix around 1-2ml of CalMag (it depends between companies) with your solution. - Sprayed: Mix 1-5ml (again, it's depends on what product you are using) with water and level the PH around 7.1 (for better absorbing). Any of these three options should fix the magnesium problem. PD: You can use Mg too but you need to mix it with proper quantities of Calcium in other to prevent other problems.