Most rapidly developing plants quickly deplete the nutrient reserves available in the growing medium. The first sign of trouble is a general yellowing of the entire plant. Do not apply only nitrogen; use a complete water-soluble fertilizer. Plants with too little fertilizer tend to turn pale green while new top leaves ascend. Few side branches develop as the oldest leaves turn yellow, dry, and drop. Nutrients from older leaves are transferred by the plant to newer growth in the plant’s struggle to survive.
Excessive fertilizing generally boosts growth initially by favoring the development of large, deep green foliage. But it produces problems later. Leaf tips on newer growth begin to brown or burn, after which older foliage begins to turn yellow, burn and fall prematurely. Salt damage to the root system renders water unavailable to the developing roots. These dangers are especially applicable to peat-based potting mixes and rockwool. Plants may rot and die rapidly. Reducing fertilizer applications, removing the upper crust of the growing medium, and leaching excess salts correct the condition.
The optimum pH for most interior plants is 6.0 to 6.5. In this slightly acid range, most nutrients are available for uptake by the roots. Excess fertilizer will cause a dramatic drop in pH, locking up nutrients before they can be used by the plant. Raise pH in soil mixes by watering with a suspension of hydrated lime at 2 teaspoons per gallon of water. Hydroponic solutions can be adjusted with dilute solutions of phosphoric acid to lower pH and potassium hydroxide to raise the pH level. Do not use fish chemicals to adjust pH. Toxic buildups can occur.