One aspect of houseplant care that is often neglected is leaching. This means pouring water through soil to dissolve and flush out undesirable minerals. It is often used outdoors to treat soil contaminated by chemicals, notably, in early spring in cold climates to flush road salt from lawns located near salt-treated roads, but even more commonly, on plants grown indoors. That’s because, since an indoor plant is watered in a closed environment (a saucer collects drainage water which is then reabsorbed by the plant, so salts never drain away), excess mineral salts (calcium, sodium, potassium, phosphorus, etc.) slowly accumulate in the soil as the months go by.
These salts are naturally present in tap water and fertilizers and, in small quantities, these salts are beneficial to plants. But when they accumulate in the soil, they start to “burn” the plant’s roots (kill them starting at the root tips). Hard water (and municipal tap water and well water are frequently hard) contains the greatest concentration of salts and plants watered with either will need fairly frequent leaching. Distilled water and rainwater are free of mineral salts… but plants watered with either will still eventually need leaching if, as is usually the case, you’ve been adding fertilizers to the soil.
When you see a white or yellowish crust form on the inner wall of the pot or on the stem of the plant, that’s a sign that leaching is overdue. At this point, the excess salts have probably already started killing roots. It’s better to being leaching before reaching that point.
The best treatment is to simply make a habit of leaching 2 or 3 times a year. If your tap water is very hard, with a pH of 7.5 or more (check with your municipality), leaching every two months is not excessive. If the plant spends the summer outside, rain will take care of leaching during that season. Some plants are less tolerant of accumulated mineral salts than others (corn plants, spider plants, azaleas, etc.) and need monthly leaching.
To leach a plant, simply remove its saucer and place it in the sink, the bathtub or outdoors. Water it normally, moistening the root ball as usual. Then wait 5 minutes (this will give the salts in the soil time to dissolve), then water again, as abundantly as the first time. Excess water, now rich in salts, will flow from the pot and disappear down the drain or into the ground. This will lower the concentration of salts in the pot, giving your plant a new lease on life!