This summer my garden has seen day after day of persistent rain. It seemed like it was never going to stop! In the warmth and wet, my plants all grew lush and green – all, that is, except for my tomato plants, whose previously healthy foliage became mottled with an ominous yellow hue.
So what was causing this downturn in my tomatoes’ fortunes? The signs all pointed to one thing: a magnesium deficiency.
Magnesium Deficiency Symptoms in Tomato Plants
If your plants are displaying similar symptoms there is one surefire way to know if your soil is low in magnesium: perform a soil test. But if you’re not able to do this for any reason, you can carry out a little detective work to figure out what’s ailing your tomato plants.
The main sign of a magnesium deficiency is yellow leaves with distinct green veins – a phenomenon known as ‘interveinal chlorosis’. Magnesium is an important nutrient that powers chlorophyll production, so what you’re seeing is chlorophyll-less leaves. In autumn, trees delight us when the green chlorophyll fades from their leaves revealing a range of head-turning yellows and reds, but this is not what you want to see on your tomato plants in summer.
With a magnesium deficiency you may also see red, purple or brown tints on the leaves as time goes on and chlorophyll reduces further. It is mainly the older leaves that are affected. In severe cases the leaves can die off, plants become less vigorous and fruit poorly.
Deficits of other nutrients can also cause interveinal chlorosis, so it’s important to consider the factors that can lead to a magnesium deficiency before proceeding.
Causes of Magnesium Deficiency
Magnesium is a very common mineral and all but the poorest soils have plenty to spare. However, overuse of high-potassium fertilisers such as tomato feeds can result in plants being unable to take up magnesium efficiently.
Magnesium is easily washed out of sandy or other very free-draining soils after heavy rains or overwatering. My toms were planted in the greenhouse border so, considering how saturated the soil outside was, they were probably receiving plenty of moisture travelling horizontally through the soil. In retrospect, I should have eased off on the watering sooner!
How to Cure Tomato Magnesium Deficiency Using Epsom Salts
The fastest way to resolve a magnesium deficiency is by using Epsom salts (also known as magnesium sulphate), which are available from pharmacies and online sources.
Make up a solution of about a teaspoon of Epsom salts per litre (quarter gallon) of water in a spray bottle. Simply wet the foliage on your tomato plants every two weeks using a fine spray setting. It will quickly be absorbed by the leaves. Avoid spraying on hot, sunny days or when rain is imminent.
Don’t be tempted to overdose. It may seem like more is better, but a surfeit of one nutrient can make it harder for plants to take up others, leading to further problems. I like to alternate the Epsom salt treatment with a liquid seaweed feed or a tomato fertiliser containing seaweed. Liquid seaweed contains a range of nutrients that will boost the vigour of your plants, helping them to recover faster. Plus, this general plant tonic covers all bases in case your diagnosis proves incorrect. Nutrient deficiencies are notoriously hard to identify!
However, if the problem was brought on by overuse of high-potassium fertilisers avoid using tomato fertiliser until symptoms abate.
To enhance your tomato plants’ wellbeing further, mulch with well-rotted compost. Compost is rich in most nutrients, including magnesium. It will gradually be pulled down into the soil by earthworms and other soil life, where it will improve your soil’s structure and make it more capable of retaining magnesium – and other nutrients.
Compost is the ultimate plant food. Think of it as the basis to a healthy diet for your plants.