The short version: Before you reach for fertiliser, look at where the yellow sits. Trouble that starts on the oldest, lowest leaves points to a nutrient the plant can move, such as nitrogen, potassium or magnesium. Trouble that starts on the newest growth points to one it cannot move, such as iron, sulphur or calcium. Then look at the shape: the whole leaf, between the veins, or the edges. That gives you a likely suspect, not a verdict, so check the roots, the water and the soil pH before you feed.
Plants cannot tell you what they are short of, but they do leave clues, and most of them are written in yellow. The silverbeet fades from the bottom up. The lower leaves on a tomato go yellow between green veins.
Reading them comes down to two things you can check in a minute: which leaves, and what pattern. Bear in mind that a leaf is a late signal: by the time it shows a shortage, the plant has usually been short for a while.
Start with two questions
Which leaves showed it first? Some nutrients can be moved inside a plant. When supply runs low, the plant strips them from its older leaves to feed the new growth, so the shortage shows low down first. Anyone who has shared a fridge with teenagers will recognise the arrangement. Nitrogen, phosphorus, potassium and magnesium behave this way. Iron, calcium, sulphur, boron and zinc stay where they were first laid down, so the newest leaves go short while the old ones look fine. Manganese tends to show on the middle and upper leaves.
What shape is the yellow? Hold a leaf to the light. Has the whole blade faded evenly, does the yellow sit between green veins, or does it start at the edges? Business Queensland's page on huanglongbing, a citrus disease, adds a useful test: nutrient yellowing is usually the same on both sides of the midrib, while some diseases make lopsided blotches. Ringed spots, fuzz or one-sided yellow point to disease or pests, covered in our visual guide to tomato leaf problems.
Old leaves first: the nutrients a plant can move
- Nitrogen. The oldest leaves fade evenly to pale green, then yellow, and the yellowing climbs the plant. New leaves stay green but small. Hungry leaf crops such as lettuce, silverbeet and Asian greens are the ones to watch.
- Phosphorus. Leaves turn dull, dark grey-green or purplish and growth stalls. It is seen most on seedlings set out into cold soil, young tomatoes especially, and often fades as the soil warms.
- Potassium. Yellowing, then a brown, papery scorch, starts at the edges of older leaves and works inward between the veins. It can develop fast in hot weather, and cucumbers and tomatoes both show it.
- Magnesium. Older leaves go yellow between the veins, often keeping a narrow green border along each main vein. Tomatoes show it often. It is most likely on light, sandy, acid soils, and heavy potassium feeding can bring it on.
For a fuller key, NSW DPIRD, the state's primary industries department, has a leaf symptom guide for greenhouse cucumbers, sorted by which leaves show trouble first. It notes the descriptions would be similar for pumpkins, melons and gherkins.
New leaves first: the nutrients that stay put
- Iron. The newest leaves turn pale yellow with a fine net of green veins, while older leaves stay dark green. In bad cases young leaves go almost white. The cause is usually access rather than an empty soil: it is most likely in alkaline, limestone-based or over-limed soil and in poorly drained beds.
- Manganese. A mottled, pale green to yellow blade with green veins on the middle and upper leaves, also tied to alkaline or over-limed soil.
- Sulphur. The youngest leaves go pale, with no green veins, then yellow. It passes for nitrogen shortage, except that it starts at the top.
- Zinc. New leaves come out small, narrow and bunched, mottled yellow between the veins.
- Calcium. New leaves emerge scorched, distorted or cupped and growing tips can die. It is usually a delivery problem rather than a soil short of calcium: uneven water, hot wind, waterlogging and heavy potassium feeding get in the way. Blossom end rot is the fruit version, covered in our guide to cracking, blossom end rot and other tomato fruit troubles.
- Boron. New leaves come out distorted and the growing point can die. Cabbage and cauliflower may show hollow stems, and celery cracked stalks.
Magnesium or iron?
Both give yellow between green veins, and leaf age decides it: magnesium starts on the old leaves at the bottom, iron on the new leaves at the top. It matters because one magnesium fix makes iron trouble worse: dolomite, a common magnesium source on acid soil, also raises pH, the last thing an iron-starved plant on alkaline soil needs.
When it is not a deficiency at all
Plenty of yellow leaves have nothing to do with feeding. Rule these out before you buy anything.
- Old age. A tomato's lowest leaves yellow off and fall as it matures. A few at the bottom of a healthy plant is normal.
- Wet roots. Waterlogging kills roots and strips nitrogen, so leaves yellow from the bottom up in plainly wet soil. See what to do when the garden will not drain.
- Fresh woody mulch. Older leaves that go pale a few weeks after fresh chip or sawdust was dug in, or spread over soil already low in nitrogen, point to nitrogen drawdown.
- Root pests. Root-knot nematodes swell roots into galls, so the plant yellows and wilts in moist soil. Pull one up and look.
- Herbicide. Cupped, twisted, narrow new growth after nearby spraying, or after a load of manure or straw, points to herbicide residue.
- Overfeeding. Too much fertiliser gives dull, dark, leathery leaves with a thin yellow edge.
- Soil pH. Strongly acid soil tends to lock away phosphorus, calcium, magnesium and molybdenum; strongly alkaline soil iron, manganese, boron, copper and zinc. The pantry can be full and the door still locked.
What to do once you have a suspect
- Check water and roots first. Roots that are drowning or parched struggle to take up a meal.
- Test the pH. A garden kit tells you whether the soil is acid or alkaline. For lime or sulphur rates, or a problem that keeps coming back, a laboratory soil test is worth the money. Most vegetables do best at about pH 6.0 to 7.0 in water, or about 0.7 lower in a laboratory's calcium chloride test; see our guide to reading and adjusting soil pH.
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Feed for the suspect, at the label rate. Any leaf spray goes on late in the afternoon or in cloudy weather, to lower the risk of burn.
- Nitrogen: a liquid feed such as fish emulsion for a quick lift, then compost, worm castings or aged manure.
- Potassium: sulphate of potash.
- Magnesium: Epsom salts (magnesium sulphate) sprayed on the leaves at the label's spray rate, when the pattern fits light, sandy, acid soil or heavy potassium feeding, or a test agrees; dolomite worked in before the next planting on acid soil.
- Iron: iron chelate as a stopgap while you bring a high pH down or improve drainage.
- Phosphorus: in cold soil, patience. Otherwise a soluble phosphorus feed is a short-term prop; the lasting fix goes in before the next planting, guided by a soil test.
- Calcium: even watering and mulch, with lime only where a test shows acid soil.
- Sulphur: gypsum or another sulphate fertiliser.
- Trace elements: pH first, as it is often the cause, then a trace element mix at the label rate once a soil test confirms a shortage. Seaweed solution is more a tonic than a fertiliser.
- Judge it by the new growth. Brown, dead patches do not turn green again, and sprayed leaves can green up while the cause stays put, so watch the leaves that grow after the change.
A caution on boron and Epsom salts
The gap between too little boron and too much is narrow, and excess boron damages plants and is harder to correct than a shortage, so do not add borax on a guess: test first and follow the label exactly. Epsom salts do nothing useful in soil that already holds enough magnesium and sulphur, and too much can upset the uptake of calcium, potassium and several trace elements.
Frequently asked questions
Why are the bottom leaves on my tomato turning yellow?
An even fade suggests nitrogen, yellow between green veins magnesium, and scorched edges potassium. Check first that it is not old leaves dying off or roots sitting in wet soil.
Why are my new leaves yellow with green veins?
Iron is the usual suspect, especially in alkaline or recently limed soil. A light spray of iron chelate is a quick test: in warm weather, leaves short of iron start to green within about a week. The lasting fix is in the soil.
Will Epsom salts fix yellow leaves?
Only if the plant is short of magnesium, or of the sulphur it also supplies. Magnesium shortage is most likely on light, sandy, acid soil or after heavy potassium feeding; on other soils, test first.
Do purple leaves on seedlings mean phosphorus deficiency?
Sometimes, but cold, wet soil is a common reason, because it makes phosphorus hard for roots to take up. Seedlings often green up as the soil warms, so give them time.
Read the leaf, check the roots and the pH, and let the next flush of growth tell you how you went.
Related growing guides
- What the main plant nutrients do, and how pH affects them
- Choosing and applying fertiliser in the vegie patch
- Feeding crops once they are up and growing
Once the bed is back in balance, browse our vegetable seeds for the next crop, or our tomato seeds if the tomatoes started the whole investigation.