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What Are the White Deposits on the Surface of Houseplant Pots?

Angelo (admin)

Salt accumulation or build-up in plant pots appears as a white-to-yellow-coloured, hard, crusty mineral deposit along the edges of the growing medium.

What Is Salt and Fertiliser Accumulation in Houseplants and Is It Harmful?

Water naturally contains dissolved minerals, also known as soluble salts. Water that’s high in dissolved calcium and magnesium salts is described as hard water and characteristically doesn’t produce suds, foam or lather up with soap. By comparison, water that’s low in dissolved minerals is termed soft water.

When water containing dissolved salts evaporates away, the minerals are left behind and increase in concentration. We see this phenomenon in the form of the white-coloured lime (calcium) deposits (known as scale) at the bottom of kettles that are used for boiling water. As the water boils away, the lime is deposited and increases over time.

When we water houseplants in pots, with each watering we add more minerals to the growing medium (potting mix). As the water evaporates away, it leaves behind residual salts that accumulate within the growing medium.

Additionally, all plants need food to grow, and fertiliser is applied to plants to provide nutrients for their growth. The fertiliser we add, whether it’s in the form of slow release fertiliser pellets, or fertiliser that’s dissolved in water, is an additional source of soluble salts that also builds up in the potting mix over time.

As the salt levels in the potting mix become increasingly concentrated, plants find it more difficult to take up water. When the salts build up in the potting mix to extremely high levels, water is pulled out from the roots through the process of osmosis, where water moves from an area of low salt concentration (within the roots) to a area of high salt concentration (the growing medium) through a semi-permeable membrane (the cell wall of the roots that allow water but not salts to pass through). This results in water loss from the roots, causing them to dry out and die off, and rot away, leading to root rot

When root tips die, a plant loses its ability to take up water, leading to water shortage, regardless of how wet the growing medium is. Plants suffering root loss due to excessive salts will show the same symptoms as an underwatered plant, with dry, brown leaf tips and edges, and in this weakened state, become more susceptible to attack from pests and diseases. Plants may eventually wilt and die if left in this state.

If too much fertiliser is applied to a plant, whether in a single application or many smaller applications, this can cause root burn in the manner described above, which will cause houseplants to develop brown leaf tips.

How to Identify Excessive Salt Build-Up in Pots

As water evaporates from the growing medium, the soluble mineral salts contained in the water stay behind, often accumulating near the surface of the growing medium, forming a ring of salt deposits that appear as white to yellow crust along the level of the growing medium.

On unglazed terracotta pots, salt accumulation also appears as a white build up on the outside surface of the pots, as the clay is porous, allowing the salts to leach through.

How to Prevent Salt Build-Up in Houseplants

The best way to prevent soluble salt build-up and root injury is by watering correctly and not overfertilizing. This can be done in several ways:

  1. Don’t water plants with hard water as it’s high in mineral salts. Water softener just adds sodium salts which doesn’t help the problem. Use rainwater where possible, as rainwater doesn’t contain dissolved minerals. Another option is to water plants with filtered water.
  2. When watering plants, water thoroughly until water flows out though the drainage holes at the bottom of the pot. This will help wash excess soluble salts out from the growing medium.
  3. Avoid watering from the bottom, such as with self-watering pots, or using pots with insufficient drainage. If a plant pot is sitting in a saucer, empty the water from the saucer under the plant. The pot should never be left to sit in the water that has flowed through it, as it will just reabsorb the drained water containing the washed out soluble salts straight back into the growing medium through the drainage holes or the porous walls of unglazed terracotta clay pots.
  4. One preventative method is to periodically leach out some of the soluble salts from the potting mix by taking the plant off its saucer and placing it over the sink, and watering thoroughly until water pours out of the drainage holes to an amount that’s equal to around 10% of the pot volume.
  5. Do not over-fertilise houseplants, follow the application rates indicated on the label. Over-fertilising is just as bad as under-fertilising and does not benefit the plant.
  6. Remove salt crusts and deposits from the potting mix surface, scrape away some of that potting mix showing salt accumulation and add fresh potting mix to the pot.
  7. Potting mix doesn’t last forever, so it’s important to repot houseplants every couple of years using fresh, premium quality potting mix.

How to Leach Out Accumulated Salt Build-Up in Houseplants

In addition to watering correctly and not overfertilizing, another way to decrease salt buildup in pots is by leaching the potting mix periodically. This can be done by pouring plenty of water through the potting mix and allowing it to drain out completely, to leach out all the excess soluble salts.

Leaching can be carried out every few months, though doing this once a year during the warmer seasons is a good way to maintain plant health. In warmer weather, plants take up more water, so the growing medium won’t remain waterlogged for an extended period of time.

To leach the growing medium in a pot of soluble salts, the amount of water to be poured through it should equal twice the volume of the pot.

For more information on what the volume of standard pots is, and how to calculate pot volume, please see article – How Much Potting Mix Does A Garden Pot Hold?

For example, a standard sized plastic pot measuring 15cm (6”) across the top is referred to as a 15cm or 6” pot, which holds a volume of 1.9 L (0.50 gal), so we would need to pour twice that amount, around 3.8L (1 gal) through the growing medium (potting mix) to wash the salts out. That’s a fair amount of water, so it’s best to add the water slowly to ensure a constant flow out of the bottom of the pot, and it’s much easier using a watering can for the purpose. I like to do this outside over the lawn or a garden bed.

If a ring of mineral salts has formed a crust on the top edges of the growing medium, remove it first before commencing the leaching. Don’t remove any more than 6mm (1/4″) of the growing medium from the surface when doing this, as going deeper may disturb and damage the surface roots of the plant. Replace the removed growing medium with fresh growing medium if necessary so that roots are not left exposed.

It’s possible to leach out some self-watering pots that have a watering port on the lower side of the pot, by allowing the excess water to pour out and then tipping them on an angle to allow the water in the reservoir at the bottom of the pot to drain out.

When soluble salts levels are extremely high, or if the pot has no drainage holes (they all should!), then the plant should be repotted using fresh, premium quality potting mix.

How much salt accumulation can plants tolerate? In general, houseplants may be injured by salt concentrations of 200 ppm and higher, but this will vary depending on the type, its age and how it is being grown.

References

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