Feeding2 min read

White crust on soil? How to flush salts

What the crust is, why it hurts roots and how to wash it out

What the crust is

A white to gray, crusty layer on the soil or the pot rim is a buildup of salts. They come from too much fertilizer and from minerals in hard water, which stay behind as water evaporates. On clay pots, salts can seep through and form a white film on the outside.

A fuzzy white or yellow growth that looks like mold is different: it's usually a harmless soil fungus, not salt.

Why it matters

At high levels, salts pull water out of root tips and damage them. Look for brown leaf tips and edges, slow growth, dropping lower leaves and wilting. Salt on a pot rim can also dry out and damage stems that rest on it. Plants that keep showing these signs after a flush need fresh potting mix.

How to flush a pot

Flushing, also called leaching, washes the salts out.

Take the plant to a sink, tub or outdoors, where water can drain freely.
Remove any crust from the soil surface and top up with fresh mix.
Slowly run clear water through the pot, at least three times the pot's volume.
Let it drain completely, and don't let the plant sit in the salty runoff.
For a big pot, run water through several times, letting it drain fully between rounds.
Repeat every 4 to 6 months. If salts are severe, or the pot is small, repot into fresh mix.

Prevent buildup

Small habits keep salts from coming back.

Water until some runs out of the bottom, then empty the saucer so the plant can't soak the salts back up.
Don't overfeed. Follow the label rate, or use less.
Use rainwater or other low-mineral water if your tap water is very hard and crust keeps coming back.
Avoid water from salt-based softeners.

Clean crusty pots

Scrub mineral deposits off clay pots with a stiff wire brush or steel wool, and off plastic or glazed pots with a scouring pad, before you reuse them.

Sources

General guidance, not a diagnosis. Written by PlantWiz in our own words from the sources above.