By Island County Extension Agent, Donald B. Meehan
HOUSE PLANT SOILS NEED SPECIAL CARE
One of the most common problems associated with growing plants in containers, particularly indoors, is the buildup of soluble salts in the growing medium. The result of this situation is often manifested by the tips and margins of leaves turning brown or black and dying. In severe cases it looks as though the leaves have been scorched.
Most folks tend to blame diseases or insects, or even humidity, for this malady but mostly the cause can be traced to the accumulation of soluble salts and other toxic substances, like fluoride, in the container soil. Sometimes under-watering or the tendency to allow the medium to dry out too much or too frequently is thought to be at fault. Indeed, this latter case may often be part of the problem, but it's not actually the direct cause for the leaves of the plants burning. Often associated with the dieback of the leaf tip and margins is plant wilting. This occurs even after the plant is watered and the soil is moist. Sometimes a whitish crust on the soil surface, on the inside and even the outside of the pot rim, as well as on plant stems may be evident.
Well, if it's not insects, disease or low humidity, but rather soluble salts, what are these salts and where do they come from? And, perhaps even more importantly, what, if anything, can be done about it? Most water supplies naturally contain a certain amount of dissolved salts in them and the use of such water consistently can lead to a buildup of these salts in the growing medium. Fertilizers which must be applied to provide the necessary mineral nutrients to plants also consists of salts. In addition, many water softeners use sodium chloride in the softening process, and when such water is applied to plants, the salts are included.
Fertilizing is frequently at fault, especially over-fertilizing. Plants in low light levels in homes just don't need as much in the way of nutrients as do those growing in high-light intensities. Most indoor plants in Puget Sound houses are, by definition, growing under low-light conditions, especially from November to March. In many cases where a plant is growing in less than 300-foot candles of light through the year it may only need fertilizing twice a year with, say a fertilizer containing 20% nitrogen. A 10% nitrogen or less fertilizer will probably be needed more frequently.
Recently-purchased, larger-foliage plants that have been grown very rapidly in California or Florida with large amounts of fertilizers and lots of water, also often high in salts, boron, or fluoride, will begin to burn once they are brought home. The usual reduction in the amount of water regularly applied leads to the concentration of salts in the soil becoming very high and what water is in the soil cannot move into the roots. Even if the roots are not killed by the salts, the greater concentration of salt outside the roots prevents the water from moving into the plant. Sometimes the salt difference between the medium and the root cells is so great, water actually moves out of the plant into the medium. This, of course, can lead to root death, wilting, yellow leaves, burned foliage and, if nothing is done about it, to the death of the whole plant.
Poorly drained potting soils are often at fault because they tend to prevent movement of water and dissolved salts through the pot and out. Often, even when excess salts are leached out with proper watering, if the bottom of the pot is allowed to sit in this water, it's pulled back up, salts and all, by capillary action and the salt concentration goes way up. Folks who don't put enough water on a plant each time it is watered so that excess salts can leach out often cause a salt layer to accumulate midway in the soil or wherever that point is where the water gradually stops. This almost always happens when plants are in pots with no drain holes to allow excess water and salt to get out.
CONSIDER THE FOLLOWING TO HELP DEAL WITH SALT PROBLEMS
1. Leach all newly purchased plants (especially larger ones) with plenty of warm water. Do this three times with half-hour intervals between water applications; all water must be allowed to drain away. Re-potting such plants in fresh soil is not a bad idea.
2. This leaching process should be carried out two to three times a year, especially on salt-sensitive plants showing damage. Many palms, spider plants, dracaenas and avocados seem to be salt -sensitive.
3. Except for cacti and succulents, keep the soil of most plants evenly moist. Don't allow them to dry to the point where the concentration of salts in the medium goes up.
4. Do not over-fertilize.
5. Do not allow excess water to be pulled back up into the medium. Pour this away, or elevate the bottom of the pot from the saucer so it never actually sits in water.
6. Do not use softened water to irrigate house plants, or any plants for that mater. Sodium can be gotten rid of by the use of one tablespoon of gypsum per six-inch pot before leaching.
7. Fluoride, which also burns the leaves of many containerized plants, is not very leachable nor will it settle out or dissipate if the irrigation water is allowed to stand before use. It will not go off as a gas. Use rainwater or distilled water for very sensitive plants, or keep the pH of the soil up at 6.5 to help tie up the fluoride.
My thanks to George Pinyuh, King County WSU-Cooperative Extension Agent, for the use of this material. Several publications are available for a small fee from our office on care of house plants. Please call for copies.
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