There is a white crust on the rim of my pots. How do I flush fertiliser salts out of the soil safely?

That crust is accumulated soluble salts — mostly nitrates, phosphates and the sodium and chloride that come along with tap water and synthetic feed. They build up because water leaves as vapour but the dissolved minerals stay behind. Past a certain concentration the soil solution becomes more concentrated than the root cells, water moves out of the roots instead of in, and you get brown crispy leaf tips that look exactly like underwatering.

To flush, take the pot somewhere it can drain freely and pour water slowly through the surface until you have run roughly twice the pot's volume through it. A two-litre pot needs about four litres. Pour in stages rather than all at once, letting each pass drain, so the water actually moves through the whole root ball instead of channelling down the gap at the edge.

Use room-temperature water. Rainwater or distilled water works best because it starts with nothing dissolved in it and has the most capacity to pick salts up; if you only have hard tap water it will still help, just less. Never flush with fertiliser solution, and do not feed for at least three weeks afterwards.

Scrape the visible crust off the rim and the top centimetre of soil first, otherwise you wash it back down into the root zone.

Flushing treats the symptom. The cause is usually feeding at full label strength every watering. Most houseplants do better at quarter strength with every second watering during active growth, and nothing at all from late autumn to early spring. If the crust returns within a few months, repot into fresh medium instead — at that point the substrate itself has broken down and holds salt in a way flushing cannot fix.