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Nutrient deficiency

A plant short of a nutrient shows it in its leaves, and the pattern names the nutrient: old leaves or new, all over or between the veins. Many shortages are really lock-outs, where the nutrient is in the soil but the pH or cold, wet roots keep it out.

Soil & nutrients
Leaves showing yellowing between green veins
Leaves showing yellowing between green veins

What it looks like

Start with where on the plant the colour changes.

  • Nitrogen: the plant moves it to new growth, so old leaves yellow first.
    Oldest leaves pale all over Nitrogen: the plant moves it to new growth, so old leaves yellow first.
  • Magnesium: veins stay green, the tissue between yellows, lower leaves first.
    Yellow between veins, old leaves Magnesium: veins stay green, the tissue between yellows, lower leaves first.
  • Iron or manganese: the youngest leaves go pale with sharp green veins.
    Yellow between veins, new leaves Iron or manganese: the youngest leaves go pale with sharp green veins.
  • Potassium: leaf margins brown and curl, and fruit ripens unevenly.
    Scorched edges on old leaves Potassium: leaf margins brown and curl, and fruit ripens unevenly.

How to confirm it

Rule out the common impostors before you feed.

  1. Check water and roots

    Waterlogged or dry, damaged roots cause the same yellowing. Fix those first.

  2. Test the pH

    A cheap kit tells you if the soil is too alkaline or acid for the plant to take nutrients up.

  3. Think about the compost

    Plants in the same compost for more than two months have usually used up its feed.

What to do

Feed for what the leaves say, and no more.

  1. Nitrogen

    A balanced or high-nitrogen liquid feed greens leafy plants within a fortnight.

  2. Magnesium

    Epsom salts as a foliar spray, 20 g per litre of water, gives a quick fix; add dolomitic lime or a Mg feed to the soil.

  3. Iron

    Sequestered iron, and rainwater for acid-loving plants, rather than hard tap water.

  4. Potassium

    A tomato feed high in potash for fruiting plants from first flowers.

Prevent it next time

  • Start liquid feeding pot plants 6 to 8 weeks after potting.
  • Mulch beds with compost or manure every year.
  • Grow acid-loving plants in ericaceous compost and water with rainwater.
  • Test soil every few years rather than feeding by habit.

What not to do

Do not add more fertiliser to a yellowing plant in wet, cold soil. The roots cannot use it, and salts build up.

Sources

  1. Nutrient deficiencies, Royal Horticultural Society
  2. Fertilisers, Royal Horticultural Society