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Magnesium Deficiency in Tomato Plants: Symptoms, Causes & How to Fix It

Magnesium deficiency in tomato plants is a common nutrient problem that often appears mid-season, just as plants are flowering and setting fruit. Unlike calcium deficiency, which affects new growth, magnesium deficiency in tomatoes usually shows up on older leaves first and can quickly reduce photosynthesis, plant energy, and fruit production.

Because magnesium plays a key role in chlorophyll production, even a mild deficiency can cause yellowing leaves, reduced vigour, and poor fruit development if left untreated.

Magnesium Deficiency Symptoms in Tomato Plants

Magnesium deficiency symptoms in tomato plants typically begin on older leaves near the bottom of the plant and move upward as the deficiency worsens.

Magnesium Deficiency in Tomato Leaves

Yellowing between leaf veins while veins remain green
Older leaves affected first
Leaf edges may curl upward
Leaves may develop brown or rusty patches over time

This distinctive interveinal yellowing is one of the clearest signs of magnesium deficiency in tomatoes and is often confused with nitrogen deficiency or disease.

Magnesium Deficiency in Tomato Plants Overall

Reduced growth and plant vigour
Lower fruit yield and smaller tomatoes
Premature leaf drop in severe cases

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Why Magnesium Deficiency Happens in Tomatoes

Magnesium deficiency in tomatoes is rarely caused by an absolute lack of magnesium in the soil. Instead, it is usually the result of nutrient competition, soil conditions, or heavy fruiting demand.

Nutrient Competition

High levels of potassium or calcium can block magnesium uptake at the root level. This is common when tomatoes are fed heavily during flowering and fruiting, especially with potassium-rich fertilisers.

Sandy or Leached Soils

Magnesium is easily leached from sandy or free-draining soils. Heavy watering or rainfall can wash magnesium beyond the root zone, leaving plants deficient even when other nutrients are present.

High Fruit Load

Tomatoes with heavy fruit set have increased magnesium demand. If magnesium is not replenished, deficiency symptoms often appear during peak fruit development.

How to Fix Magnesium Deficiency in Tomato Plants

Step 1: Confirm the Deficiency

If yellowing starts on older leaves and veins remain green, magnesium deficiency is likely. New growth usually stays green until the deficiency becomes severe.

Step 2: Restore Magnesium Availability

The most reliable way to correct magnesium deficiency in tomato plants is to supply magnesium in a balanced form that does not disrupt calcium or potassium uptake.

Complete Grow Cal-Mag Hydroponic Nutrients Fertiliser provides readily available magnesium alongside calcium, helping restore leaf colour while maintaining nutrient balance during flowering and fruiting.

Step 3: Maintain Balanced Feeding

Avoid excessive potassium-heavy feeding without magnesium support. Balanced nutrition prevents magnesium deficiency from recurring as fruit load increases.

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Common Magnesium Deficiency Mistakes (and How to Avoid Them)

Confusing magnesium deficiency with nitrogen deficiency: Magnesium affects older leaves first, nitrogen affects the whole plant.
Overloading potassium: Excess potassium blocks magnesium uptake.
Ignoring early symptoms: Mild yellowing can quickly reduce yield if untreated.
Feeding calcium without magnesium balance: This worsens competition at the roots.

FAQs: Magnesium Deficiency in Tomatoes

What causes magnesium deficiency in tomato plants?
Most cases are caused by nutrient competition, leached soils, or high fruit demand rather than a true absence of magnesium.

How do I fix magnesium deficiency in tomatoes?
Restore magnesium with a balanced Cal-Mag supplement and avoid excessive potassium-heavy feeding.

Does magnesium deficiency affect fruit?
Yes. Reduced photosynthesis leads to smaller fruit and lower yields.

Is magnesium deficiency more common in pots?
Yes. Container-grown tomatoes lose magnesium faster through leaching.

Related Tomato Fertiliser Guide

Magnesium deficiency often occurs alongside poor overall feeding balance. For a complete breakdown of nutrient ratios, feeding schedules, and fertiliser selection, see our full guide:

Best Fertiliser for Tomato Plants: Complete Feeding Guide

Final Recommendation

Magnesium deficiency in tomato plants reduces photosynthesis, weakens growth, and limits fruit production if left untreated. Correcting the issue early with balanced nutrition is key.
Cal-Mag Nutrients Fertiliser
is ideal for restoring magnesium levels while maintaining calcium balance, helping tomato plants stay green, productive, and resilient throughout the growing season.