Reading your plants' signals: recognising deficiencies and knowing when to feed

Daniel Platteau Articles gardening tips

Before reaching for the fertiliser: rule out the false culprits

This is the most useful habit to acquire: most yellow leaves are not deficiencies. Before blaming the soil, go through this list; it will save you many unnecessary feeds.

Watering. Too much water and too little produce surprisingly similar symptoms — limp leaves, yellowing, leaf drop. Waterlogged compost suffocates the roots, which stop absorbing: the plant then shows deficiency symptoms even though the soil is full of nutrients. It is the leading cause of houseplant decline, far ahead of everything else. Push a finger three centimetres into the compost before each watering.

Light. A plant short of light turns pale, becomes leggy and loses its lower leaves. No fertiliser will make up for a dark corridor.

Soil pH. A crucial and widely underestimated point: an element can be present in sufficient quantity but chemically locked up, and therefore out of the roots' reach. Iron becomes unavailable above pH 7, which explains yellowing hydrangeas and camellias on chalky ground. Phosphorus is locked up in very acid soil as well as in very chalky soil. In these cases, adding more fertiliser is pointless: it is the pH that needs correcting, or the plant that needs changing.

Normal ageing. A lower leaf that yellows and drops from time to time, on an otherwise vigorous plant, is senescence, not a problem.

Pests and diseases. Scale insects, red spider mites and thrips cause speckled discolouration that is easily mistaken for deficiencies. Always look under the leaves, with a magnifying glass if necessary.

Salt build-up. In pots, fertilisers and tap water leave mineral salts that accumulate. A whitish crust on the surface, brown and dry leaf tips: that is an excess, not a shortage. The solution is to flush the compost thoroughly, not to feed.

The golden rule of diagnosis: young leaves or old leaves?

Here is the trick that turns diagnosis from a guessing game into logical reasoning. Some elements are mobile in the plant: when they run short, the plant withdraws them from its old leaves to supply new growth first. Symptoms therefore appear at the bottom, on the older leaves.

Others are immobile: once fixed in a tissue, they stay there. The deficiency then hits the young leaves at the top, while the older ones stay perfectly green.

Mobile elements → symptoms on old leaves: nitrogen, phosphorus, potassium, magnesium.

Immobile elements → symptoms on young leaves: iron, calcium, sulphur, boron, manganese, zinc.

This criterion alone already rules out half the possibilities. Then look at the pattern of discolouration: even, between the veins, at the margins, in spots.

The catalogue of deficiencies, symptom by symptom

Nitrogen (N) — older leaves, even yellowing

The most common and easiest to recognise. The lower leaves yellow evenly, veins included, before browning and dropping. The whole plant stays small and puny, with stems sometimes tinged red or purple. Growth stalls.

Common on: leafy vegetables, lawns, plants that have been in the same compost for a long time, sandy soils after heavy rain.

Quick fix: nettle tea diluted to 10%, dried blood, or a nitrogen-rich liquid fertiliser. Visible effect within one to two weeks. Beware: it is also the easiest element to overdo.

Phosphorus (P) — older leaves, purplish tints

A discreet symptom that is often missed. Foliage dark green, dull, sometimes bluish, with purple or violet tinges on the stems and the undersides of older leaves. Slow growth, poor flowering, weak roots, fruit slow to ripen.

Watch out for the trap: cold spring weather temporarily blocks phosphorus uptake and produces exactly the same purple mottling on young tomato or maize plants. It disappears by itself when it warms up. Add nothing until you have waited a fortnight of mild weather.

Potassium (K) — older leaves, scorched edges

Very characteristic: the lower leaves show yellow, then brown and dried-out margins, as if burnt around the edge, while the centre stays green. The leaf may curl downwards. Soft stems, greater sensitivity to drought and disease, small fruit with little sweetness.

Common on: tomatoes, potatoes, roses, sandy or heavily watered soils.

Fix: wood ash in very small amounts (it raises the pH sharply — never more than a handful per m² per year, and not at all on soil that is already chalky), comfrey tea, sulphate of potash with magnesium (Patentkali), or a "tomato and fruit" fertiliser.

Magnesium (Mg) — older leaves, chlorosis between the veins

The pattern is spectacular and unambiguous: the veins stay clearly green while the areas between them turn yellow, drawing a herringbone network. It starts at the bottom of the plant. On some species, the yellow areas turn red or orange before dying.

Common on: tomatoes at the end of the season, roses, hydrangeas, citrus in pots, acid and sandy soils.

Fix: magnesium sulphate (Epsom salts), 15 to 20 g per litre of water as a foliar spray for a quick effect, or 100 g/m² on the soil.

Iron (Fe) — young leaves, chlorosis between the veins

The same pattern as magnesium — green veins on a yellow background — but at the top of the plant, on new shoots. In severe cases, young leaves become almost white, even cream.

This is the famous iron chlorosis, almost inevitable when you grow an ericaceous plant (azalea, camellia, rhododendron, hydrangea, blueberry, citrus) in chalky soil or with hard tap water. The iron is there, but locked up by the high pH.

Fix: iron chelate (EDDHA form in chalky soil, more effective), watering with rainwater, gradual acidification of the growing medium. The real cure is still to plant in the right place.

Calcium (Ca) — young leaves and deformed fruit

Young leaves are distorted, hooked, with dead edges, and the growing tip may die. On fruit, the symptom is famous: blossom-end rot in tomatoes, the black, leathery patch at the bottom of the fruit. It is also seen on peppers, courgettes and watermelons, and as "tipburn" or internal browning in lettuces and cabbages.

Key point: in the great majority of cases, the soil contains enough calcium. The problem comes from interrupted transport, because calcium only moves with the flow of water. Irregular watering, alternating drought and flooding, excess nitrogen pushing growth too fast: these are the real culprits. The answer is regular watering and mulching, not adding calcium.

Sulphur (S) — young leaves, even yellowing

It looks like nitrogen deficiency, but on young shoots instead of old ones. Rare in gardens, it is nevertheless becoming more common now that acid rain — which unintentionally supplied sulphur — has decreased thanks to regulations on industrial emissions.

Boron, manganese, zinc — young leaves, unusual symptoms

Boron: hollow or black heart in beetroot and celeriac, corky patches in apple flesh, aborted growing tips, hollow stems in cauliflowers. Handle with extreme care: the margin between deficiency and toxicity is very narrow.
Manganese: chlorosis between the veins of young leaves, with a finer, blurrier green network than for iron, often with small dead spots.
Zinc: markedly smaller leaves, shortened internodes giving a rosette look, mottled chlorosis.

And excess?

We think about it too little. Too much nitrogen: dark green, lush foliage but soft stems, a plant that does not flower, an invasion of aphids attracted by the tender tissue, bland fruit. General salt excess: brown, crispy leaf tips, a white crust on the compost, wilting despite moist soil. In pots, excess is as common as deficiency.

Reading the weeds: indicator plants

Here is a free, permanent diagnostic tool: weeds that grow spontaneously tell you about the state of the soil. Each species has its own preferences for pH, compaction, nitrogen and organic matter. A well-established population therefore reflects lasting conditions — far more reliable than a single plant brought in by a bird.

This approach, popularised in France by Gérard Ducerf, draws on the ecological concept of Ellenberg indicator values, a system established by the German botanist Heinz Ellenberg that scores each European species according to its requirements for light, moisture, pH and nitrogen. It is established science, but it needs to be used with nuance.

Soil rich in nitrogen, even too rich

Stinging nettle (Urtica dioica): the queen indicator of soils very rich in nitrogen and phosphorus, often near old manure heaps, hen runs or compost heaps. A dense patch means: whatever you do, don't add nitrogen.
Fat hen (Chenopodium album), redroot pigweed, black nightshade: cultivated soils rich in nitrates.
Cleavers, chickweed (Stellaria media), red dead-nettle: fertile soils, well supplied with nitrogen and humus. Chickweed is an excellent sign of fertility.

Poor, infertile soil

White clover and creeping clover: when abundant in a lawn, they generally signal soil low in nitrogen. Being legumes, they fix their own nitrogen and gain the upper hand where grasses struggle. It is the best indicator of a lawn that needs feeding — or of the case for accepting it as ground cover, which is not a bad idea.
Yarrow, cat's-ear: rather poor, dry soils.
Broom, heather, bracken: acid, poor soils.

Compacted, airless soil

Greater plantain (Plantago major), knotgrass, creeping cinquefoil: compaction, repeated trampling, a plough pan. Here the problem is not nutritional but structural: the soil needs loosening and organic matter, not fertiliser.
Annual meadow-grass, couch grass: compacted and often waterlogged soils.

Acid soil

Sheep's sorrel (Rumex acetosella), corn spurrey, field pansy: low pH. Liming can be considered if you grow demanding vegetables.

Chalky soil

Charlock (Sinapis arvensis, wild mustard), annual mercury, common poppy (very frequent on chalky ground, though not limited to it): chalky soils. Watch out for iron chlorosis on acid-loving plants.

Wet or poorly drained soil

Creeping buttercup (Ranunculus repens), field horsetail, rushes, water mint, marsh marigold: excess water, poor drainage (horsetail often signals a wet, compacted layer deeper down).

Three precautions

Judge by populations, not individuals. A single nettle tells you nothing; a mass of nettles tells you a lot.
Cross-check the clues. One species is never enough to draw a conclusion: look for consistency among several indicators.
It is not a soil analysis. Reading weeds points you in the right direction and saves time; it does not replace a laboratory analysis (€60 to €150) if you have to make a significant decision.

The special case of houseplants

In a pot, everything changes: the plant lives in a finite volume of compost, with no reserves and no soil life to recycle anything. The rules are therefore different.

New compost is already fertilised. Most commercial composts contain a starter fertiliser lasting about six to eight weeks. Feeding a freshly repotted plant risks burning it for nothing. Wait two months.

Respect the season. Feed during active growth, from spring to early autumn, when there is enough light. In winter, most houseplants slow down considerably: unused nutrients then build up as salts in the compost. Halve the feeding, or stop altogether from October to February, except for a plant under grow lights or a winter-growing species.

Always under-dose. The advice most widely shared by experienced growers is to dilute to half, or even a quarter, of the dose on the pack, but feed more often. Manufacturers dose for optimal conditions that our homes almost never provide.

Never feed dry compost. Water first with plain water, then apply the nutrient solution. On dehydrated roots, fertiliser burns.

Never feed a sick or stressed plant. It is an understandable but counterproductive reflex: a plant suffering from rotten roots, scale insects or lack of light cannot absorb more. Treat first, feed afterwards.

Flush the compost twice a year. Run plenty of lukewarm water through the pot, in the sink, for several minutes, to wash out accumulated salts. This is particularly useful if you see a white crust or brown tips appearing.

Spot the signs specific to pots:

Roots coming out of the drainage holes, water running straight through: the plant is pot-bound, and needs repotting, not fertiliser.
No growth at all in the growing season despite good light and correct watering: then, yes, a feed is justified.
New leaves much smaller than the old ones: a sign the compost is exhausted.

Light feeders: cacti, succulents, snake plants and ZZ plants need very little — two or three diluted feeds a year are enough. Epiphytic orchids need a specific low-strength fertiliser applied very regularly (the "weakly, weekly" rule).

Carnivorous plants are the exception: they grow naturally in extremely poor conditions and must never be given fertiliser in their compost, nor watered with tap water. Rainwater or demineralised water only.

A five-step protocol

Faced with a plant that is not doing well, work in this order:

Check water and light first of all. Finger in the compost, look at the position.
Inspect the roots and the undersides of the leaves: rot, pests, a root-bound pot.
Locate the symptom: young leaves or old leaves? This criterion alone halves the possibilities.
Identify the pattern: even yellowing, interveinal chlorosis, scorched margins, distortion.
Correct sparingly, starting with the minimum dose, and watch for two to three weeks before feeding again. Foliar spraying works faster for magnesium and iron, but it is only a quick fix: it is the soil that needs looking after in the long run.

And remember the principle that sums it all up: it is infinitely easier to rescue an under-fed plant than one burnt by too much fertiliser. When in doubt, wait.