Feeding problems

Potting soil holds only a few months of nutrients. After that, a plant depends on you, but more fertiliser is not better: excess salts burn roots faster than hunger harms them.

Too little: how to tell

• Older leaves turning evenly pale or yellow: usually nitrogen. The plant moves it from old leaves to new ones.
• Yellow between green veins: on older leaves often magnesium; on new leaves often iron, which hard, alkaline tap water can lock up even when it's in the soil.
• Small new leaves and slow growth in spring and summer, in soil that hasn't been changed for a year or more.

Too much: how to tell

• Brown, crispy leaf tips and edges.
• A white crust on the soil surface or pot rim.
• Wilting or leaf drop right after feeding, although the soil is moist.

How to fix it

Underfeeding: feed with a balanced liquid houseplant fertiliser at half the label dose every two to four weeks from spring to late summer. Repot plants that have been in the same soil for more than two years. For iron yellowing, water with rainwater or filtered water and use a fertiliser that contains iron.

Overfeeding: flush the soil: run plenty of water through the pot for a couple of minutes and let it drain. Skip feeding for four to six weeks. If the crust is heavy, repot in fresh soil.

How to feed without problems

Water first and feed onto moist soil, never dry. Don't feed newly repotted plants for four to six weeks, or sick or dormant ones at all. Cacti and succulents need a quarter of the dose; flowering plants like orchids do best with their own fertiliser.