Houseplant Care: Fertilizing Houseplants

Over-fertilization burns roots and may cause an accumulation of fertilizer nutrients, causing a highly soluble salt condition. If this happens, plants should be repotted, or salts flushed out of the soil by running a fairly large amount of water through it. Under-fertilization is a much less serious problem and one that is easily corrected. Select a good house plant fertilizer and follow directions carefully.

Remember that fertilizer is not a cure-all for sick plants. It is only a cure for a nutrient deficiency, but lack of nutrients is not a common houseplant problem. More often poor growth results from improper environmental conditions: light, water or humidity. Fertilizing a damaged plant when it does not need it can do more harm than no fertilizer at all.

Nutrient deficiencies are difficult to diagnose, because they are often the same as symptoms arising from other causes. Evaluate environmental conditions first. If environmental conditions are satisfactory and there is no evidence of insect or disease damage, then consider the fertility situation. If the plant has not been fertilized recently, look for specific symptoms of fertilizer deficiency.

Nitrogen causes quick plant response and is one of the first elements to show symptoms when it is lacking. Low nitrogen causes plants to be pale green, and new growth may be stunted. Pale coloration often begins on older leaves which may turn yellow and drop off. Without treatment, the damage gradually progresses upward. It is most evident at times when a plant is producing new terminal growth.

Phosphorus deficiency also causes stunted plants, and also may be accompanied by a reddish-purple coloration on the edges of older leaves. This is more often evident in young plants or seedlings, but seldom in large, more mature plants. Potassium joins nitrogen and phosphorus as the other major plant nutrient. Lack of potassium shows as scorch and yellowing at the tips and margin of leaves.

As in the case of nitrogen deficiency, it usually starts on lower, older leaves and progresses upward on the plant. Iron deficiency is a common problem in such plants as gardenia, azalea and citrus. Lack of iron in the leaves appears as a yellowing of the leaf blade between the veins. It is especially hard on the youngest leaves of a plant, but may progress into older leaves. This deficiency often is a sign of other problems and not usually a simple lack of iron in the soil. It develops when there has been excessive root damage, as when plants have been kept too wet or dry.

Constant watering with hard water causes the soil pH to rise, which in turn causes the development of iron deficiency in these acid-loving plants. Excesses of certain elements may sometimes be a problem. Excess fluoride is believed to tip burn in some plants of the lily family such as the dracaena and spider plant. When the damage occurs, repotting and watering with rain water may be helpful. Fluoride comes in many materials, and in addition to being found naturally in soil, it may be in water, perlite and some types of peat moss.

Houseplants for Dummies, by Larry Hodgson.