Reading a Yellow Leaf: Which Nutrient the Plant Is Actually Short Of

A yellow leaf is not yet a diagnosis. Eight different shortages can turn a leaf the wrong colour, and the way to separate them is to look at where on the plant the symptom started and which part of the leaf it sits in. The fertiliser shelf comes last, not first.

Old leaves or new leaves

Nitrogen, phosphorus, potassium and magnesium are mobile inside a plant: the plant can lift them out of an old leaf and move them to new growth, so a shortage shows up on the lower, older leaves first while the top carries on looking healthy. Calcium, iron, sulphur and boron are not mobile in the same way, so the plant cannot rescue the tip and the symptom appears on the youngest leaves and growing point.

That single observation is the useful part of the whole subject. The descriptions below then narrow it down.

Phosphorus, potassium and magnesium

Phosphorus shortage slows everything: growth stalls, flowering is late, the leaves take on a dull dark green with a sheen lost, and small purple or red blotches appear. Left alone the affected leaves die. A phosphorus fertiliser is the answer, though on a cold pot the roots simply cannot take phosphorus up, so warmth matters as much as the feed.

Potassium shows as brown spotting on the leaves with the older ones wrinkling and the margins looking scorched. Potassium is the element behind flower and fruit quality, so this one is worth correcting quickly.

Magnesium is the most misread of the three because it looks like iron shortage: yellowing between the veins while the veins themselves stay green. The difference is position. Magnesium starts low on the plant and climbs; if the pattern is on new growth instead, it is iron. Without correction the magnesium areas go brown and the leaf dies.

Iron, manganese, sulphur

Iron shortage prints on the youngest leaves first: they yellow between the green veins, then fade toward yellow-white as the veins pale too, and in bad cases small spots appear and the leaf dries off. Ferrous sulphate applied to the compost works slowly, and an iron-containing foliar feed is the faster route. The usual underlying cause is a compost that has gone too alkaline or too waterlogged for the roots to take iron up, which is why correcting the feed without correcting the pot often fails.

Manganese produces yellow-brown or reddish-brown flecks across the upper surface, the blade greys, and the spots enlarge until the leaf withers. A manganese-containing feed deals with it.

Sulphur is uncommon and easy to miss: yellowing starts along the veins and spreads, and untreated the whole plant declines.

Calcium and boron

Calcium shortage is a tip and edge problem. Margins yellow, then wither and curl, and the roots tell on it first: brown root tips, rotting young roots. Ground lime or another calcium feed worked into the potting mix corrects it, on acid mixes especially.

Boron stops the top of the plant growing. New leaves come out distorted and crinkled, their veins yellow, growth rate drops, and a badly affected plant will not flower at all. A boron-containing feed at the label rate is the whole treatment, and label rate matters here more than anywhere else on this page, because boron’s margin between useful and toxic is narrow.

Before you reach for a bottle

Two checks come first. Root and watering problems produce yellowing that looks exactly like a deficiency, and a pot that has been sitting wet will show it; second, a leaf or two of old yellowing on a plant that is growing normally is simply the plant cycling its foliage. Genuine deficiency is a pattern that repeats and spreads, not one leaf.

Feeding a plant that is short of one element is not the same as feeding a plant generally, and the schedule for that is in how much to feed and how often. The choice between a slow organic material and a measured mineral feed, which is where most of these single-element corrections are decided, is argued in organic versus inorganic fertiliser.