Are Epsom salts good for plants?

Epsom salts have accumulated an extraordinary list of claimed garden uses, almost all of which trace back to gardening forums rather than to anything measurable. The one real use is genuinely useful, cheap and fast — which is probably why the rest attached themselves to it.

Safety

What you'll need

• Epsom salts (magnesium sulphate)
• A watering can or sprayer
• Kitchen scales

Step by step

1. Confirm you actually have a magnesium deficiency.Yellowing between the veins while the veins themselves stay green, starting on the oldest and lowest leaves and working upward. Magnesium is mobile, so the plant moves it out of old leaves into new growth, which is why the pattern starts at the bottom. If the newest leaves are affected first, it is iron or manganese and Epsom salts will not help.
2. Work out why it is short.Light sandy soils leach magnesium readily. Containers run out of it, especially in peat-free composts late in a season. Heavy potassium feeding competes with magnesium for uptake, which is why tomatoes on weekly high-potash feed are the classic case. And high pH reduces availability.
3. Spray the leaves for a fast response.Around 20g of Epsom salts per litre of water, sprayed on both surfaces of the leaves in the evening. Response is visible within a week. Repeat every two to three weeks while the deficiency persists.
4. Water it into the soil for a slower, more durable correction.Around 20 to 30g per square metre watered in, or a similar amount dissolved in a can for a large container. Slower to show but longer-lasting than a spray.
5. Fix the underlying cause rather than repeating the treatment.Recurring magnesium deficiency in a pot means the compost is exhausted or the feeding balance is wrong. A feed containing magnesium, or a compost change, is the real answer — and dolomitic limestone supplies magnesium along with lime where an acid soil is genuinely short.
6. Ignore the claims that have no basis.Epsom salts do not increase flowering, do not sweeten tomatoes, do not deter slugs, do not improve seed germination, do not green up a nitrogen-deficient plant, and do not substitute for fertiliser. They contain no nitrogen, no phosphorus and no potassium at all.
7. Do not use them routinely.Applying Epsom salts to a garden that is not short of magnesium builds up salts, and excess magnesium interferes with calcium and potassium uptake. It is a correction, not a tonic.

Tips

• Tomatoes in containers on weekly high-potash feed are the single most common case of genuine magnesium deficiency in British gardens. Potassium and magnesium compete for uptake, so heavy potash feeding creates the shortage.
• Roses in pots and long-established containers of shrubs are the second most common case.
• On acid soils that genuinely need liming, dolomitic limestone supplies magnesium as well as calcium and corrects the deficiency permanently while doing the liming job.
• Do not confuse magnesium deficiency with a virus. Virus produces mottling, mosaic patterns and distortion; magnesium deficiency produces a clean geometric pattern of yellow between green veins.
• Epsom salts are cheap in bulk from agricultural and equestrian suppliers, at a fraction of the price of the small garden centre packs.

Questions people ask