Because hydroponic gardeners must regularly add fertilizer to their plants' watering solution, fertilizer burn--dehydration damage caused by too strong a concentration of various nutrients--is always a concern. Even organic fertilizers can burn your plants, marketing claims to the contrary; as R.C. Harris points out, what won't burn, won't feed. Recognizing visible signs of fertilizer burn is key to reversing the damage before it's too late.
General Signs of Fertilizer Burn
A common symptom of fertilizer burn is a browning of the leaves as once-green plant cells die from lack of water. This may be occur all over the plant at once or in spots and dapples. Leaves may also develop sooty, blackened tips.
Though green is generally a healthy color for plants, the appearance of a noticeably darker green color can indicate too high a concentration of nitrogen in the nutrient solution. Too much nitrogen weakens the plant, causing long, floppy stems and soft leaves. Keep an eye on the root systems, too, as underdevelopment is another indication of nitrogen overdose.
Visible signs of too strong a concentration of phosphorus include undersized, curling leaves and overdeveloped root systems.
Potassium is a salt, and too much of it will affect a plant the way drinking sea water affects a shipwrecked sailor. The saline environment causes water to pass out of cell walls rather than in. Leaves will appear burned, wilted and dry because hydration isn't reaching them in the quantities required.
You probably won't have to worry about too much calcium, as overdose conditions are rare. But if you see abnormal wilting along with signs associated with of high alkalinity and deficiency of iron and potassium, an overly high level of calcium may be the culprit. Symptoms of iron deficiency include "yellow or white chlorosis between veins of younger leaves" and "stunted new growth with spindly stems," according to Hydroponics B.C. The same source describes indicators of potassium deficiency as follows: "Brown, necrotic (dead) tips and edge margins on older (lower) leaves followed by yellowing of the entire leaf. Dead brown spots on older leaves. Slender, weak stems and small seeds."