Are the leaf tips curlling up, do the leaves have yellow margins or tips with green veins

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Magnesium (Mg) deficiency.

Magnesium deficiency will exhibit a yellowing (which may turn brown) and interveinal chlorosis beginning in the older leaves. The older leaves will be the first to develop interveinal chlorosis. Starting at leaf margin or tip and progressing inward between the veins. Notice how the veins remain somewhat green though as can be seen in figure 15.
Notice how in Figure 16 and 17 the leaves curl upwards like they're praying? They're praying for Mg! The tips may also twist.
This can be quickly resolved by watering 1 teaspoon Epsom salts/gallon of water. Until you can correct nutrient lockout, try foliar feeding. That way the plants get all the nitrogen and Mg they need. The plants can be foliar feed at teaspoon/quart of Epsom salts (first powdered and dissolved in some hot water). When mixing up soil, use 2 teaspoon dolomite lime per gallon of soil.
If the starting water is above 200 ppm, that is pretty hard water, that will lock out mg with all of the calcium in the water. Either add a 1/4 teaspoon per gallon of epsom salts or lime (both will effectively reduce the lockout or invest into a reverse osmosis water filter.
Mg can get locked-up by too much Ca, Cl or ammonium nitrogen. Don't overdo Mg or you'll lock up other nutrients.

Figure 14.

Figure 15.

Figure 16.

Magnesium (Mg) Toxicity

Magnesium toxicity is rare and not generally exhibited visibly. Extreme high levels will antagonize other ions in the nutrient solution.
  Last modified: 21:40 - Jun 22, 2001  

faq:1029 "Are the leaf tips curlling up, do the leaves have yellow margins or tips with green veins"