Magnesium (Mg) is vital for photosynthesis since it is a part of the chlorophyll molecule. When you add Mg the plants will become a dark green. Magnesium is important also because it facilitates the use of nitrogen, phosphorus and sulfur. Also, Mg is needed in the formation of some plant proteins.
Signs of a Magnesium Deficiency -- For most plants a Mg deficiency first shows up in the lower leaves as discoloring in the veins. They first turn yellow, then orange and finally brown. Plant leaves will feel thin, brittle and sometimes cup upward.
Causes of a Magnesium Deficiency – soils that are too wet, (if growing in soil the top of the soil in the pot should be dry before watering.) Low pH (acidic) or soils high in peat or sand. Moreover, soils given a high concentration of potash fertilizers or calcium can also show signs of an Mg deficiency. Mg can get locked-up by too much chlorine or ammonium nitrogen. Keep in mind you do not want to overdo Mg or you'll lock up other nutrients.
Cures for a Magnesium Deficiency -- Epsom salt is a cheap way to add Mg, using one table spoon per gallon once a month should suffice. Also, dolomite lime will add Mg, Calcium and keep the pH in a good range for most plant growth. When mixing up soil, use at least 2 teaspoon dolomite lime per gallon of soil. I use twice that personally.
For a quick correction of an Mg deficiency you can foliar fed the plants with ½ teaspoon/quart of Epsom salts (first dissolved in some hot water).
If your water is above 200 ppm, the hard water may lock out Mg with all of the calcium in the water. Either add a 1/4 teaspoon per gallon of Epsom salts or lime or invest in a water filter from HTGSupply.com (LINK to water filters).
Dr. E.R. Myers