Gardening With Allen: Indoor plants need light feeding

I was about to fertilize our indoor plants with lawn fertilizer. My wife stopped me and said, “You will burn them.” Isn’t it safe to use lawn fertilizer on indoor plants?

Lawn fertilizer and other outdoor fertilizers can be used on indoor plants if applied sparingly. The typical lawn fertilizer is about five times more concentrated than the typical indoor plant fertilizer. So you would want to apply about one-fifth as much as the directions for indoor fertilizer.

Your wife is right about “burning” when too much fertilizer is applied. All fertilizers are salts. When there is too high a concentration of salts in the soil, it pulls the water out of plants resulting in a burned look. Brown leaf edges are typically the first symptom.

Indoor fertilizers come in three types. The first two are either liquid concentrates or soluble powders, which are applied in teaspoons per gallon of water. Rates for application every time plants are watered are lower than rates for monthly application. The third type is a slow-release form which is typically applied once every three to four months. Slow-release types such as Osmocote have individual pellets that are coated with a porous plastic. Water flows through the tiny pores and dissolves a little fertilizer and then flows out again. This supplies a small amount of fertilizer every time the plants are watered.

Although regular fertilization according to directions will usually be adequate most of the time, I pay attention to symptoms that may indicate some adjustment in fertilization is needed. I already mentioned the brown leaf edges as a possible indication of too much fertilizer. This same symptom may also be caused by letting plants get too dry between waterings. In this case it is also accompanied by leaf wilting.

It is normal for a few older leaves to turn yellow as they mature and fall off. If a larger number of the oldest leaves turn yellow, it may be a sign of inadequate fertilization. Nitrogen is the fertilizer element used in largest quantity by plants. It is also the most soluble and therefore the quickest to be leached from the soil. When plants cannot get enough nitrogen from the soil for normal growth, they transfer it from the older leaves to the new growth.

When the newest leaf growth is yellow but retains dark green veins, it is usually a sign of iron deficiency. Not all fertilizers contain iron. Iron and other micro nutrients are used in very small quantities, but are absolutely required. Many natural soils contain adequate iron, but some potting soils do not. So if you do notice iron deficiency, make sure you use a fertilizer that contains iron.

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