Why Do Leaves Turn Yellow?

LittleGardenBeauty
Published on June 2nd 2019
31
A close up of a tree leaf slowly turning yellow
Leaves turning yellow is something we usually associate with autumn colour. Plants stop producing chlorophyll in preparation for winter, and as this green pigment dissipates from the leaves, the other colours become visible. So seeing yellow leaves in late spring is unexpected and several community members have recently contacted us for advice.

Symptoms

Different nutrient shortages can result in similar but slightly different effects on plant foliage. For example:
  • Spindly stunted growth (Nitrogen)
  • Yellow margins with pink tinges can appear around the edges of leaves (Nitrogen)
  • Yellow or purple leaf-tints with browning at the leaf edge (Potassium)
  • Yellow (as well as red and brown) areas developing between the veins of leaves, which stay green. (Magnesium)
  • Lower and older leaves of a plant turn yellow and start to shrivel up. (Magnesium)
  • Slow growth with dull yellow foliage (Phosphorus)
  • Yellowing areas developing between the leaf veins of acid-loving plants and the leaf edges turning brown. (Manganese and Iron)
A close up of a geranium leaf showing signs of Potassium shortage

Causes

The most common cause is over-watering and heavy rainfall, causing nutrients to be washed (leached) away.
  • Light sandy soils have naturally low levels of water and nutrients; this reduced level of water in the soil makes it even harder for the plants to access the nutrients.
  • Clay soils hold on to nutrients, effectively locking them away from plants.
  • When growing ericaceous (lime-hating plants) in alkaline soils, plants may struggle to access the nutrients they need.
  • Over application of high potash feeds such as tomato feed, can cause plants to absorb the Potassium in preference to all other nutrients.
  • Mulches that are not fully composted use up the available Nitrogen to continue breaking down woody lignin.
  • And finally the lack of nutrients in the potting compost to begin with!
For a more comprehensive explanation of how plants access nutrients, I found this webpage by Mosaic relatively easy to follow.
A potted plant showing yellowing leaves

Nitrogen (N)

Lack of Nitrogen is the most common cause of yellow leaves in Spring, and it can also stunt a plants growth.
To help the plant short term: apply a high nitrogen fertiliser such as sulphate of ammonia or pelleted poultry manure.
Long term: apply a mulch of well rotted organic matter regularly, which will provide a slow, steady source of Nitrogen. Or try growing legumes which will fix Nitrogen into the soil.

Potassium (K)

Controlling a plants ability to photosynthesise and their ability to take up water, a lack of K. can also reduce a plants flower or fruit production.
Apply a high potassium fertiliser such as sulphate of potash, but follow the dosage instructions as over-application can also cause problems.
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Phosphorus (P)

P shortage tends to only occur in areas with heavy clay soil and high rainfall amounts. If this is the case for you, then try a superphosphate or bone meal fertiliser.
A store filled with lots of different plant feed

Magnesium (Mg)

The main component in chlorophyll, plants will often strip this nutrient from older lower leaves when in short supply and move it to develop new leaves.
To help short term:
For a quick result, it is best to spray diluted Epson salts (hydrated magnesium sulfate) directly onto the leaves.
Dissolve 20g of Epsom Salts per litre of water (or a third of an ounce into a pint if you use Imperial) and a few drops of liquid detergent to help the spray stick to the leaves.
Apply this several times at fortnightly intervals on cloudy days to prevent sun scorch, and hopefully, the leaves will begin to green up.
Long term:
Dolomite limestone (calcium-magnesium carbonate) can be added to the soil around the roots of plants at the rate of 100g per squared metre (or 4oz per sq yard), however if your soil is acidic it is best to use Epsom Salts at 30g per sqm (10oz per sq yd) as ericaceous plants will not like the increased alkalinity that limestone creates.
A close up of a blueberry plant showing Yellowing of the leaves

Manganese (Mn) and Iron (Fe)

Acid-loving plants being grown in alkaline conditions will frequently show yellowing on their leaves, especially when growing in containers. Apply a chelated (bonded) iron and manganese treatment to the soil around the roots to help combat this.

Other possibilities

Nutrient deficiency is not the only reason why leaves turn yellow.
  • Onions and shallots are very susceptible to onion downy mildew, where the leaves start turning yellow and die off from the tip downwards. In wet weather white mould develops on dead parts often turning darker colour later. Remove the affected bulbs straightaway (do not compost), avoid planting onions in the same location for five years and grow cultivars that offer some resistance.
  • Evergreen plants shed their old leaves throughout the year, and the yellowing could just be part of the natural dying off process.
I hope this guide helps you work out why your plant baby is changing colour and how to restore them to lush green growth. I've tended to use Chempac products when I've needed to give plants a helping hand but there are plenty of other suppliers out there, just a quick search online or along the shelves of your local garden centre will give you options.
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