Question: Some of my indoor plants have yellow leaves. Is that a sign that they need fertilizer?

Answer: Yes, yellow older leaves are a sign that plants need nitrogen fertilizer. When the youngest leaves are yellow, it means they need iron fertilizer. All indoor plant fertilizers contain nitrogen, but some do not contain iron.

I like liquid or soluble plant fertilizers that are applied when you water your plants. Most indoor plant fertilizers give a rate for use every time you water. The amount of fertilizer applied is in direct proportion to the amount and frequency of watering. Plants that use more water are also getting more fertilizer.

If you applied gradual release, coated fertilizer several months ago, it may be almost completely gone. If there are a few coated fertilizer granules on top of the soil, pinch one between your fingers. If it is hollow or almost hollow, it is time to add more.

Indoor plants respond to increasing light by increased growth rate this time of year. Light intensity is increasing as the sun moves north and gets higher in the sky. Days are getting longer too. Both of these factors provide more light energy that can be turned into growth for indoor plants near windows.

Increased growth rate means water and fertilizer are being used more rapidly than two months ago. If you have gotten into a routine of watering plants once a week or some other regular interval, it is time to check the soil to see if it is drying out more quickly.

The best way to tell when indoor plants need water is to feel the top of the soil with your fingers. When it is dry on top, there is still some moisture at a deeper level. That is a good time to water before the soil dries completely and leaves start to wilt.

Watering frequency depends upon several factors. Most important is the relative size of the pot to the size of the plant. If the plant foliage is considerably larger than the volume of soil, it will dry out more quickly. Small plants in large pots dry out much more slowly.

Temperature and relative humidity play an important part. Winter heating causes air to become quite dry, which increases the amount of water used by plants. Plants that are in direct sunlight will use more water and fertilizer than those some distance from a window. Some potting soils hold more water than others.

Dry soil is lighter in color than wet soil. Once you learn the relative color of the soils your plants are growing in, you may be able to determine dryness just by looking.

Treat your indoor plants as individuals when you water and fertilize and they will be healthier and more attractive.

Allen Wilson can be contacted at allenw98663@yahoo.com.

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