Frequency of fertilizer application varies somewhat with the vigor and age of each plant. Houseplants should only be fertilized during periods of active growth. If you fertilize according to label directions, you will probably over fertilize your plants. Plants in low light will not need as much fertilizer. Plants grown under less than 200 foot candles benefit little from fertilization and may actually be harmed. Houseplants outside or in bright light will need more fertilizer. A general guideline to use is to fertilize monthly in the spring and summer and only two or three times in the fall and winter. Never fertilize when the soil is dry.
Overfertilization can be a severe problem in houseplants. Symptoms include a decrease in rate of growth, stunted plants, burned or dried leaf margins, loss of lower leaves, browned roots, wilted, or even dead plants. Excess fertilizer can result in a buildup of soluble salts. These salts can accumulate on top of the soil forming a white crust. A ring of salt deposits may form around the drainage hole. Salts may also build up on the outside of clay pots.
Soluble salts are minerals dissolved in water. Fertilizer dissolved in water becomes a soluble salt. When water evaporates from the soil, the minerals or salts stay behind. As the salts in the soil become more and more concentrated, plants find it harder and harder to take up water. If salts build to an extremely high level, water can be taken out of the root-tips, causing them to die.
High soluble salts can damage the roots directly. Because the plant is weakened, it is more susceptible to attack from insects and diseases. One of the most common problems associated with high salt levels is root rot.
If a layer of salts has formed a crust on top of the soil, you should remove the salt crust. Do not remove more than 1/4 inch of soil. It is best not to add more soil to the top of the pot.
Pots should be leached every 4 to 6 months. Leaching is accomplished by pouring an amount of water equal to twice the volume of the pot on the soil and letting it drain completely. A 6 inch pot will hold 10 cups of water, so 20 cups of water are used for leaching. Water the plant, wait 30 to 60 minutes and water again.
The level of salts that will cause injury varies with the type of plant and how it is being grown. A plant grown in the home may be injured by salts at concentrations of 200 parts per million (ppm). The same plant growing in a greenhouse, where the light and drainage are good, will grow with salts at 10 times that level, or 2,000 ppm. Some nurseries and plant shops leach plants to remove excess salts before the plant is sold. If you are not sure that has been done, leach a newly purchased plant the first time you water it.
© Erv Evans, Consumer