Epsom salt magnesium sulphate – magnesium source for plants in aquaponics and hydroponics

Epsom salt, soluble magnesium sulphate

3,90 €

Weight	Price	Price per kg	vs 50 g
50 g	3,90 €	78,00 €	—
200 g	5,80 €	29,00 €	−63 %
500 g	7,00 €	14,00 €	−82 %
1 kg	8,50 €	8,50 €	−89 %
2 kg	12,30 €	6,15 €	−92 %

This magnesium sulphate heptahydrate contains 16% MgO and 32.5% SO3, both water-soluble. It is used as a foliar spray, at 50 g per litre of solution, and comes in five pack sizes.

Magnesium sulphate

Weight : 50 g

200 g

500 g

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The summary

What it provides: 16% MgO and 32.5% SO3, soluble in water.
Use in solution: 5 kg in 100 litres of water, i.e. 50 g per litre of solution.
How it gets in: through the leaf, as a foliar spray.
What dissolves: up to 100 kg per 100 litres of water at 20 degrees.
The symptom that calls for it: yellowing between the veins of older leaves.
Shipping: by tracked parcel from France, from the 50 g sachet to the 2 kg bag.

Soluble magnesium sulphate, for a foliar feed of magnesium and sulphur

Epsom salt, a soluble magnesium sulphate, supplies magnesium and sulphur through the leaf, as a foliar spray. It contains 16% MgO and 32.5% SO3, both soluble in water. It is used at 50 g per litre of solution, that is 5 kg in 100 litres of water, when the older leaves turn yellow between the veins.

Epsom salt is magnesium sulphate heptahydrate, with the formula MgSO4·7H2O. It supplies 16% MgO and 32.5% SO3, which works out at around 9.6% magnesium and 13% sulphur as pure elements, and both are soluble in water. Magnesium sits at the centre of the chlorophyll molecule, which is why a deficiency shows first as a loss of colour in the foliage.

🍃 Feeding through the leaf

This salt is used as a foliar fertiliser: 5 kg dissolved in 100 litres of water, i.e. 50 grams per litre of solution, sprayed onto the foliage. On vegetables grown in a greenhouse, or treated on the same day against a disease, the manufacturer lowers this concentration to 2 to 3%, i.e. 20 to 30 g per litre. This way you bypass the soil and its blockages, and you act as quickly as possible when the deficiency is already visible on your plants. A plant whose older leaves turn yellow between the veins while the veins stay green is the typical situation that brings you here. Check for this symptom on two or three leaves before preparing your solution. For a fundamental correction over the whole season, an application of potash and magnesia to the soil acts more slowly, but for longer.

Salt goes into the water, never the other way round

Always add the salt to the water, stirring, and never pour the water onto the salt. This avoids undissolved lumps at the bottom of the sprayer, which block the nozzle and leave marks on the foliage. Store the bag in a cool, dry place between uses. If the yellowing you are seeing does not match that of magnesium, reading the deficiency signs will spare you an unnecessary addition, and chelated iron addresses a similar but distinct symptom.

🐟 Magnesium in aquaponics

In an aquaponic system, magnesium comes from the source water and from the fish feed, and its level often remains far below that of a hydroponic solution: comparative readings give around 1.6 mg/L in aquaponic water against 40 mg/L in a hydroponic solution. An excess of potassium, supplied for example by the potassium bicarbonate used to raise the pH, also limits its uptake. Magnesium sulphate is the usual correction for this deficiency in aquaponics, because it is well tolerated and does not alter the pH. Make only one addition at a time, never two different products on the same day, so that you know which one produced the observed effect. The guide to deficiencies in aquaponics describes the other symptoms to tell apart from this one.

🧮 What actually dissolves

Magnesium sulphate heptahydrate dissolves very easily: about 100 kg in 100 litres of water at 20 °C, and still around 75 kg at 5 °C. The foliar solution, at 5 kg per 100 litres, therefore stays 15 to 20 times below saturation, even when prepared with cold well water. A deposit of crystals at the bottom of the sprayer therefore does not mean the water is overloaded, but that the salt was added too quickly or stirred too briefly: you avoid it by sprinkling the salt into the water, stirring all the while, until the solution becomes clear again.

Technical specifications

Levels

Magnesium: 16 % MgOThat is 9.6% pure magnesium, soluble in water.
Sulphur: 32.5% SO3That is 13% pure sulphur, soluble in water.

Type of product

Formula: MgSO4·7H2OMagnesium sulphate heptahydrate.
Solubility: 100 kg for 100 L at 20 °CAround 75 kg for the same volume at 5 °C.

Use

Foliar solution: 5 kg in 100 L of water
Preparation: Salt added to the water, while stirring

To budget for separately

Sprayer: A clean garden sprayer
Other deficiencies: A chelated iron

In the box

1 pack of 50 g to 2 kg, depending on the size chosen

Contents, solubility and use as a foliar solution recorded on 20 September 2026 from the epsoTOP technical data sheet from K+S France; magnesium contents of aquaponic and hydroponic waters recorded on 11/10/2026.

Three fertilisers, three different deficiencies to make up

This is the salt that dissolves and can be sprayed. The other two are spread on the soil.

Correct quickly, or feed over the season A spray works within a few days. Granules in the soil work over weeks.

	Elements supplied	Supplies potassium and magnesium together	Provides nitrogen	Granules to spread on the ground	Adds sulphur	Dose given per square metre or per litre	Recommended dose	Price
Patentkali fertiliser granules, potash, magnesia and sulphurYour choice	Potassium, magnesium, sulphur	Yes	No	Yes	Yes	No	200 to 650 kg per hectare	6,50 €
Seabird guano, nitrogen and phosphorus fertiliserYour choice	Nitrogen, phosphorus, potassium	No	Yes	Yes	No	Yes	50 to 100 g per square metre	9,90 €
Recommended Epsom salt, soluble magnesium sulphateYour choice	Magnesium and sulphur	No	No	No	Yes	Yes	50 g per litre of solution	3,90 €

Patentkali fertiliser granules, potash, magnesia and sulphur

What the fertiliser provides

Elements suppliedPotassium, magnesium, sulphur

Supplies potassium and magnesium togetherYes

Provides nitrogenNo

The shape and the sulphur

Granules to spread on the groundYes

Adds sulphurYes

Use

Dose given per square metre or per litreNo

Recommended dose200 to 650 kg per hectare

In the shop

Seabird guano, nitrogen and phosphorus fertiliser

What the fertiliser provides

Elements suppliedNitrogen, phosphorus, potassium

Supplies potassium and magnesium togetherNo

Provides nitrogenYes

The shape and the sulphur

Granules to spread on the groundYes

Adds sulphurNo

Use

Dose given per square metre or per litreYes

Recommended dose50 to 100 g per square metre

In the shop

Epsom salt, soluble magnesium sulphateRecommended

What the fertiliser provides

Elements suppliedMagnesium and sulphur

Supplies potassium and magnesium togetherNo

Provides nitrogenNo

The shape and the sulphur

Granules to spread on the groundNo

Adds sulphurYes

Use

Dose given per square metre or per litreYes

Recommended dose50 g per litre of solution

In the shop

Contents, doses and certifications recorded on 20 September 2026 from the K+S France PatentKALI and epsoTOP data sheets, and from the Agro-Sens seabird guano data sheet.

Estimate your
delivery time

Shipped as a tracked parcel from France, which can be combined at no extra cost with the rest of your order. The exact charges and delivery times are shown in the basket once the address has been entered. Outside the European Union, import duties and taxes are payable by the recipient.

Frequently asked questions

6 questions

6 key questions

How much salt dissolves in a litre of cold water?

Does it need shaking before spraying?

How much for a 5-litre sprayer?

How do you recognise a magnesium deficiency?

What does it provide besides magnesium?

Can it be used on leaves in direct sunlight?

Your foliar solution

Enter the volume of solution you are preparing, in litres.

Solution volume

1 L100 L

of salt, at a concentration of 5 kg per 100 litres

Foliar solution use noted on 20 September 2026 from the epsoTOP technical data sheet by K+S France.

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Epsom salt, soluble magnesium sulphate

The summary