⚠ MinorYellowing between distinctly green veins on older leaves — classic interveinal chlorosis from magnesium deficiency.
Magnesium Deficiency in Tomatoes
Commonly affects: Solanum lycopersicum (Tomato)
Yellowing appears between the leaf veins while the veins themselves stay distinctly green, starting on older, lower leaves and progressing upward if untreated. Leaf edges may curl slightly upward.
The short answer
Yellowing between the veins on the lower leaves, with the veins themselves staying distinctly green and often a V-shaped or marbled look, is magnesium deficiency. It is extremely common on tomatoes in containers, especially heavily fed plants, because high potassium feeds compete with magnesium uptake. A dilute Epsom salt solution corrects it within one to two weeks.
Confirm it before you treat it
- 1Look at the veins. Magnesium deficiency leaves a clear green vein network against a yellow background — the single most reliable sign.
- 2Check which leaves. It starts on the oldest, lowest leaves, because magnesium is mobile and the plant moves it to new growth.
- 3Note the shape of the yellowing. A V or an inverted arrow from the leaf edge toward the midrib is typical on tomatoes.
- 4Review the feeding. Heavy tomato feed is high in potassium, and potassium competes directly with magnesium at the root.
- 5Rule out iron deficiency by leaf age. Iron looks similar but appears on the NEWEST leaves, not the oldest.
Potential Causes
- 1Magnesium-poor soil, common in sandy or heavily leached growing media
- 2High potassium fertilisation competing with magnesium uptake
- 3Cool soil temperatures reducing root uptake efficiency even when magnesium is present
Solution Steps
- 1Apply a diluted Epsom salt solution (1 tablespoon per litre of water) as a foliar spray or soil drench.
- 2Repeat every 2 weeks until new growth shows normal green colour.
- 3Reduce potassium-heavy fertiliser applications if magnesium deficiency recurs.
- 4Incorporate dolomitic lime into soil before planting in known magnesium-poor beds.
What else looks like this
| Could be | How to tell it apart |
|---|---|
| Iron deficiency | Nearly identical interveinal pattern but on new growth at the top of the plant. Magnesium affects old leaves at the bottom. |
| Nitrogen deficiency | Also on old leaves, but the yellowing is uniform with no green veins remaining. |
| Early blight | Distinct brown lesions with concentric rings and a yellow surround, also on lower leaves. Magnesium produces no lesions. |
| Natural lower-leaf senescence | A couple of the very lowest leaves yellowing on a heavily cropping plant is normal. Deficiency climbs steadily up the plant. |
| Tomato mosaic virus | Irregular mottling with distorted growth, not a clean interveinal pattern respecting the veins. |
Recovery timeline
- Days 1–2Apply Epsom salts at about one tablespoon per 4–5 litres of water as a soil drench. A foliar spray at half that strength acts faster.
- Week 1A foliar application can green up treated leaves noticeably within days. Soil application takes longer to show.
- Weeks 1–2New growth comes through green and the yellowing stops climbing. Badly affected old leaves will not fully recover.
- Weeks 3–4Repeat the drench if the pattern reappears. Container plants often need it monthly through the season.
- OngoingIn containers, expect this to recur — the limited soil volume and heavy potassium feeding make it close to routine on tomatoes.
If you do nothing
Magnesium sits at the centre of the chlorophyll molecule, so a deficient leaf physically cannot photosynthesise properly. Yellowing progresses upward, affected leaves brown between the veins and drop, and the plant loses leaf area exactly when it needs it to fill fruit. Yield falls and fruit ripens unevenly. The plant rarely dies, but on a container crop the difference between a treated and untreated plant across a season is substantial.
Common mistakes
- Confusing it with iron deficiency and treating for the wrong nutrient. The difference is which leaves are affected — old for magnesium, new for iron.
- Increasing the tomato feed. High potassium is frequently what caused it, so more feed makes it worse.
- Using Epsom salts as a general tonic on plants with no symptoms. Excess magnesium interferes with calcium uptake and can contribute to blossom end rot.
- Applying it dry to the soil surface. It needs dissolving in water to reach the roots.
- Expecting the affected old leaves to regreen. Severely yellowed tissue does not recover; the new growth is the measure.
Prevention Tip
Epsom salt (magnesium sulfate) is a fast, cheap, and reliable fix — many experienced tomato growers apply it preventively once fruit begins forming.
Questions people actually ask
- How much Epsom salt should I use?
- About one tablespoon per 4–5 litres of water as a drench, or half that as a foliar spray. More is not better.
- How do I tell it from iron deficiency?
- Leaf age. Magnesium hits the oldest lower leaves first; iron hits the newest top growth first. The pattern on the leaf itself is very similar.
- Why does it keep coming back in pots?
- Limited soil volume plus high-potassium tomato feed. Potassium competes with magnesium at the root, so heavy feeding creates the deficiency.
- Can I prevent it?
- A magnesium-containing feed, or a monthly Epsom drench through the season for container tomatoes, prevents most cases.
- Should I remove the yellow leaves?
- Remove them once mostly yellow — they contribute little and removing the lowest leaves improves airflow around the plant.
