This is a problem where the plant looks like it has a virus with yellowing leaves but it is caused by lack of magnesium. If in doubt as to the cause, treat as if it is magnesium deficiency. It can do no harm to try!
Magnesium is an essential element for plants as it is required in the chlorophyll molecule. Chlorophyll is the chemical that enables the solar power plant to work – producing starches and sugars etc from other chemicals taken up by the roots.
Chlorophyll is what gives the leaves their green colour and yellowing of leaves is always a sign that things are going badly wrong.
It can happen to any plant and the cause need not be magnesium deficiency. Iron deficiency also causes yellowing leaves with deep green veins and nitrogen deficiency can cause yellowing leaves.
When it happens to tomatoes, especially greenhouse or container tomatoes you can be 99.9% sure of the cause. The use of high-potash tomato fertiliser has had the unwanted effect of locking up the soil or compost and the plant is failing to get the required nutrient balance.
The better and more expensive tomato fertilisers usually include trace elements and magnesium to prevent this problem from occurring
Curing Magnesium Deficiency in Tomatoes
Happily curing the problem is remarkably easy and just requires you to foliar feed the tomato plant with common or garden Epsom salts.
Epsom Salts Foliar Feed – Recipe
Dissolve ½ oz in a pint of water or 20g per litre of water. It’s easiest to dissolve the salts in warm or hot water but allow to cool to ambient temperature before using.
How to Foliar Feed Epsom Salts
Foliar feeds are best applied in the morning before the day gets too hot and the plant closes the pores in the leaves to conserve moisture.
Spray from top to bottom paying particular attention to the underside of the leaves where the pores that absorb the feed are concentrated.
Spray well so the leaves are wet and dripping every day for a week and the problem should be solved.
Preventing Magnesium Deficiency
Do not over apply high-potash tomato fertilisers. It’s always a temptation to use a little extra thinking this will result in more and better crops. Unfortunately the opposite can result.
Swap to a good quality tomato fertiliser that has magnesium added as a trace element.
For greenhouse borders or growing outdoors where there is a magnesium deficiency, increase the pH by using Dolomite lime that has magnesium rather than the usual ground limestone.
The addition of Keiserite which is the mineral that Epsom salts are made from will cure shortages
Further Information: Tomato Growing Guides
- Tomato Blight – Blight Resistant Tomato Varieties
- Tomato Blossom End Rot | Causes & Cures for Tomato Blossom End Rot
- Tomatoes Magnesium Deficiency – Yellow Leaves, Epsom Salts
- Types of Tomatoes – An Introduction to Tomato Growing Part 1
- Sowing and Starting off Tomatoes – Introduction to Tomato Growing Part 2
- Growing Tomatoes in a Tomato Grow-house (Mini-Greenhouse)
- Growing Tomatoes in a Greenhouse
- Growing Tomatoes in the Greenhouse Border
- Growing Tomatoes in Pots or Grow Bags in the Greenhouse
- Growing Tomatoes by Ring Culture
- Growing Tomatoes in Straw Bales
- Growing Tomatoes Outdoors
- Planting & Growing Tomatoes Outdoors
- Growing Patio Tomatoes – Dwarf Bush Variety Patio Tomatoes
- Water Requirements for Tomatoes
- Ideal Temperatures for Growing Tomatoes
- Removing Tomato Side Shoots (Suckers) & Stopping Tomatoes
- Best Tomato Varieties – My Top Tasty Tomato Picks
- Heirloom Tomatoes (Heritage Tomatoes)
- Best Tomatoes for Greenhouse Growing
- Tomato Troubles & Diseases | Causes & Cures of Tomato Problems
- Raising Tomato Plants from Seed