Magnesium (Mg) in crops: role, deficiency signs and how to correct it

Magnesium is a secondary nutrient at the centre of every chlorophyll molecule. The crop also needs it to make and use energy and to move sugars from the leaves to fruit, grain and tubers. A magnesium-short crop shows yellowing between the veins of older leaves, often with red or purple tints, and early leaf drop. Heavy potash use is one of the most common causes.

By Gafar Laiya and the Gujarat Bio Organics team. Updated November 2026.

What magnesium does in the plant

• Sits at the centre of the chlorophyll molecule.
• Activates enzymes that make and use energy (ATP).
• Moves sugars from the leaves to fruit, grain, tubers and roots.
• Needed for building proteins.
• Helps the plant take up and use phosphorus.

How magnesium moves in the plant

Magnesium moves easily from old leaves to new ones, so a shortage shows first on the oldest leaves while new leaves stay green.

Signs of magnesium deficiency

• Yellowing between the veins of older leaves while veins stay green, often in a fishbone or Christmas-tree pattern.
• Red, purple or bronze tints on older leaves in some crops.
• Leaves become brittle and may curl upward.
• Early drop of older leaves.
• Small fruit, poor sweetness and weak grain fill.

Crop by crop

More crops and look-alikes: Magnesium deficiency symptoms crop by crop.

Where and why it happens

• Heavy potash use, the most common cause.
• Sandy, acid, leached soils.
• Heavy ammonium nitrogen.
• High calcium in some soils.
• Heavy fruiting, which pulls magnesium out of the leaves.

How it works with other nutrients

• Potassium and magnesium compete: heavy potash brings on magnesium shortage.
• Ammonium and calcium also compete with magnesium.
• Magnesium helps the plant take up and use phosphorus.
• Magnesium and calcium must stay in balance; a field may look calcium-short when magnesium is part of the problem, and the reverse.
• Magnesium shortage cuts sugar movement, so fruit and tubers stay small even with good leaves.
• See nutrient interactions.

What happens if it is not managed

• Less chlorophyll, so less food and lower yield.
• Poor sugar movement: small fruit, low brix and poor grain or tuber fill.
• Early leaf drop that weakens the crop for the next season in tree crops.
• See the full picture of what unmanaged shortages cost.

How to confirm it

• Look at the oldest leaves: yellow between green veins, often with reddish tints.
• Check the potash dose: heavy potash often explains it.
• Test exchangeable magnesium, potassium and calcium together, to see the balance.
• A leaf test confirms.

How to correct it

• Magnesium sulphate as a foliar spray for quick correction, or to the soil.
• Dolomite in acid soils supplies magnesium and calcium and raises pH.
• Balance potash with magnesium rather than cutting potash the crop needs.
• Repeat foliar sprays during heavy fruiting if needed.
• Take the dose from your soil test report, the product label or your agriculture officer.

Too much magnesium

• Too much magnesium can lower calcium and potassium uptake.
• Soils very high in magnesium can have poor structure.

Doses depend on the crop, soil and product, so this page gives methods, not amounts. Take the dose from your soil test report, the product label or your local agriculture officer or KVK. Results differ from farm to farm with soil, weather, crop and management. Farmer experiences on this site are their own reports, not a promise of results.