How do I tell which nutrient my plant is short of?
Reading which leaves are affected — oldest or newest — to separate the mobile nutrients from the immobile ones, and what each pattern actually means.
- Difficulty
- intermediate
- Time
- 25 min
- Read
- 3 min
Short answer
Look at whether the symptoms are on the oldest lower leaves or on the newest growth. Mobile nutrients — nitrogen, magnesium, phosphorus, potassium — are moved into new growth, so shortages show on old leaves first. Immobile ones — iron, manganese, calcium — show on new growth first.
Before concluding anything is a deficiency, rule out drought, waterlogging and root damage. All three prevent uptake of nutrients that are perfectly well supplied in the soil, and all three are more common than a genuine shortage.
Step by step
- Check watering and roots first.A plant that is too dry, too wet or has damaged roots cannot take up nutrients regardless of what is in the soil. Fix that before feeding anything.
- Establish which leaves are affected.Oldest and lowest, or newest at the tips. This single observation halves the possibilities and is the basis of the whole diagnosis.
- Read the old-leaf patterns.Whole leaf evenly pale yellow is nitrogen. Yellow between the veins with green veins is magnesium. Purple or bronze cast with stunted growth is phosphorus. Scorched brown margins is potassium.
- Read the new-leaf patterns.Yellow between sharply green veins on the newest leaves is iron, usually because the pH is too high. A duller, broader version of the same is manganese. Black distorted tips is calcium.
- Treat iron chlorosis as a pH problem.The soil almost always contains plenty of iron. Chelated iron relieves the symptom; the durable fix is ericaceous compost, rainwater and a container, or a different plant.
- Treat calcium problems as watering problems.Calcium moves in the transpiration stream. Blossom end rot on tomatoes and tip burn on lettuce are caused by irregular watering, not by a lack of calcium in the compost.
- Use Epsom salts for magnesium.Magnesium sulphate as a foliar spray gives a quick response, or applied to the soil for a slower, more durable one. Tomatoes and roses in pots are the classic cases.
- Feed with a balanced fertiliser for nitrogen.A general balanced feed or a nitrogen-rich one. Nitrogen deficiency is common in containers and in beds where uncomposted woody material has been dug in.
- Test the soil pH if the pattern repeats.Recurring iron or manganese symptoms across several plants points to alkalinity rather than to a shortage of any one nutrient.
- Change one thing and watch new growth.Damaged leaves do not recover. The evidence that a treatment worked is that the next leaves come out normal.
Tips
- Old leaves first or new leaves first is the whole diagnosis.
- Blossom end rot is a watering problem, not a calcium shortage. Feeding calcium rarely helps.
- Damaged leaves never green up again — judge by new growth.
Common mistakes
- Feeding a wilting or waterlogged plant — It cannot take nutrients up, and concentrated feed on damaged roots makes things worse. Sort the water first, then feed.
- Assuming yellow leaves always mean feeding — Overwatering, root rot, vine weevil and natural leaf shedding all produce yellowing. The pattern and position matter more than the colour.
If it doesn't work
Lower leaves uniformly pale yellow
Cause: Nitrogen shortage — Fix: Apply a balanced or nitrogen-rich feed and mulch with compost.
Yellow between veins on lower leaves
Cause: Magnesium shortage, common in tomatoes and pots — Fix: Epsom salts as a foliar spray, repeated fortnightly.
Yellow between sharp green veins on new leaves
Cause: Iron unavailable at high pH — Fix: Chelated iron short term; ericaceous compost and rainwater long term.
Scorched brown leaf edges on older leaves
Cause: Potassium shortage, or salt build-up — Fix: High-potash feed, and flush containers thoroughly to rule out salts.
Questions people ask
How do I tell a deficiency from overwatering?
Deficiencies follow a pattern by leaf age and by vein. Overwatering produces general yellowing, soft growth, and often a sour smell from the compost. Check the roots before feeding.
Are Epsom salts a general tonic?
No. They supply magnesium and sulphur specifically. They help magnesium deficiency, which is common in tomatoes and container plants, and do nothing for anything else.
Should I just use a general feed to cover everything?
For a mild, unclear problem, a balanced general feed and a compost mulch is a reasonable response. For a clear pattern — iron chlorosis in particular — a general feed will not help.