just an idea but have a look at the thread in plant probs may help,the postman.
Potassium (K) Toxicity

Usually not absorbed excessively by plants. Excess potassium can aggravate the uptake of magnesium, manganese, zinc and iron and effect the availability of calcium.

Magnesium
Magnesium is a component of the chlorophyll molecule and serves as a cofactor in most enzymes.

Magnesium (Mg) deficiency.
pic2
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 pic2


This can be quickly resolved by watering with 1 tablespoon 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 ½ 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