Published: 12/12/2019 9:44:40 AM
Q: Some of my houseplants have a crusty white buildup on the surface of their soil. What is it and should I remove it? — D. M., Hadley
A: The short answer is soluble salts and yes. Soluble salts are minerals dissolved in water. Fertilizer is a soluble salt many of us use on our indoor plants, especially during the spring and summer growing season. Hard water is another source of them. I get them from the hard well water we use at my house.
Now that it is wintertime, these salts have had a good three quarters of a year at least to collect in our pots. When the water from either fertilizer or hard water evaporates, the salts stay behind. As they accumulate, they get more concentrated. If you do not remove the excess salts, your plants will have an increasingly hard time taking up water. When the salts build up to an extremely high level, they will damage your plant’s roots directly thereby weakening the plant.
Once salts accumulate to the point of forming a white or yellow crust on top of your plant’s potting soil, you know it is time to either repot the plants in fresh soil — the ideal — or leach them in the old soil. You may also see these salts as a ring around the pot at the soil line or around the drainage hole.
Leaching the pots sounds a bit medieval, but it really is rather simple. First, find a sink or tub where you can easily pour water on the plant and have it drain. Remember to cover the drain with a towel or cover of some kind so soil does not go down it. Next, remove your salty soil crust. Be careful to only remove the crust, just about ¼ inch deep. Now, it’s time to leach. Leaching is when you drain lots of water through the soil so it washes away the excess salts. Use twice the volume of your pot. For example, if you have a half-gallon pot, use a full gallon of water.
Pour all this water gradually through the soil and let it drain fully. Plants left sitting in water will simply draw the salt-laden water back up, defeating the purpose.Once all the water is used and drained completely, you are done. Houseplants should be leached every 4-6 months.
If you have evidence of a lot of excess salt, such as this crusting accompanied by browning leaf tips, dropping lower leaves, dead root tips, and wilting, it is best to repot the plant.
To help prevent future excess salt buildups, be sure to empty the saucers under your pots soon after you water them. I use 10 minutes as the maximum time for my houseplants to be in saucers with water. It typically takes a few minutes for them to drain fully after regular watering.
Hope this helps, D.M. Thanks for asking a (local) Master Gardener.
Have a gardening dilemma? Please send questions, along with your name/initials and community, to the Western Massachusetts Master Gardener Association at AskAMasterGardener@wmmga.org. One question will be selected and answered per week. wmmga.org