Soluble salts may accumulate on the top of the soil, forming a yellow or white crust. A ring of salt deposits may form around the pot at the soil line or around the drainage hole. Salts may also build up on the outside of clay pots. In house plants, signs of excess soluble salts include reduced growth, brown leaf tips, dropping of lower leaves, small new growth, dead root tips, and wilting.
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, it becomes more difficult for plants to take up water. If salts build up to an extremely high level, water can be taken out of the root tips, causing them to die. High levels of soluble salts damage the roots directly, weakening the plant and making it more susceptible to attack from insects and diseases. One of the most common problems associated with high salt levels is root rot.
The best way to prevent soluble salt injury is to stop the salts from building up. When watering, allow some water to drain through the container and then empty the saucer. Do not allow the pot to sit in water. If the drained water is absorbed by the soil, the salts that were washed out are reabsorbed through the drainage hole or directly through a clay pot.
House plants should be leached at least every 4 to 6 months. To leach plants, pour excess water on the soil and let it drain completely. The amount of water used for leaching should equal twice the volume of the pot. Keep the water running through the soil to wash out the salts. If a layer of salts has formed a crust at the soil surface, remove the salt crust before leaching. Do not remove more than 1/4 inch (6 mm) of soil. It is best not to add soil to the top of the pot. If the soluble salt level appears to be extremely high, repot the plant.
The level of salts that causes injury varies with the species of plant and how it is grown. A house plant may be injured by salts at a very low concentration, but the same plant growing in a greenhouse where watering is well managed may tolerate salts at high levels. Some nurseries and retail plant outlets leach plants to remove excess salts before the plant is sold. If you are not sure that a newly purchased plant has been leached, leach it the first time you water.