Nitrogen deficiency turns the old leaves uniformly yellow first, not the young ones. Start the treatment by separating it from iron and drought, then choose the nitrogen source and split the dose.
Nitrogen is the framework of green growth: chlorophyll, protein and new leaves are not built without it. When it is short, the plant spends the reserves of its old leaves, and those leaves lose colour uniformly. This pattern is seen again and again in the apple orchards of Faridan, the greenhouse tomatoes of Varamin, the urban lawns of Isfahan and cucumber transplants in the south — and it is mistaken for iron chlorosis. The usual result is buying an iron chelate for a tree that is short of nitrogen, or spreading heavy urea on a transplant that is merely thirsty. Farkesht reads a nitrogen deficiency from three things: which leaf is affected, what colour the veins are, and whether the soil at root depth is genuinely moist.
What nitrogen does in a plant and why its shortage shows early
A large share of the nitrogen taken up goes into the chlorophyll molecule. Without chlorophyll, photosynthesis falls, and the plant builds no new leaves, no firm stems and ultimately no fruit. Nitrogen is a mobile element: the plant can move it from an old leaf to a growing point. That is why the symptom appears first on the lower, older leaves while the shoot tip is still relatively green. If that movement continues, internodes shorten, stems stay thin, flowering is delayed, and in leafy vegetables the yield falls sharply. In a fruit tree, chronic nitrogen shortage makes weak buds and a thin crown; an acute shortage in spring stops shoot growth after flowering.
Read the symptoms on the leaf and on the whole plant
The classic face of nitrogen deficiency is uniform yellowing of the old leaf: both the blade and the veins go pale, and the leaf takes on a light green to straw-yellow cast. This differs from iron chlorosis, where the young leaf is yellow and the veins stay green. With nitrogen, the lower leaves are affected first, growth stops or slows, the stem is thin and brittle, and in cereals and maize the leaf sometimes becomes spindly and upright. In tomato and pepper, the plant stays sparsely leaved and the fruit is small and forced into early ripening. In turf, the colour moves from dark green to a uniform yellow and blade growth slows. Do not settle any of this from a photograph on a messaging app; look at the underside of the leaf for mite, and check the moisture at twenty to thirty centimetres with a spade.
- Uniform yellowing of old leaves at the bottom of the crown; the veins are pale too, not glossy green.
- Slow growth, short internodes, thin stems, and a plant or tree of small volume.
- Early loss of the lower leaves in mid-season, with no distinct fungal spotting.
- In leafy vegetables: pale colour, small leaves, poor flavour and marketability.
- In fruit trees: a thin crown, short current-season shoots, and small fruit where a crop has set.
Do not confuse nitrogen deficiency with iron, salinity and drought
In Iran's calcareous soils, yellowing of a young leaf with green veins is almost always closer to iron than to nitrogen. If only the upper crown is yellow while the old leaves stay green, the treatment route is not nitrogen. Midday drought makes leaves limp and dull but does not necessarily produce uniform yellowing of the old leaf; if the colour does not return after irrigation and the old-leaf pattern remains, then think about nitrogen. Salinity and marginal scorch look more like potassium or salt at the edge of the drip than like a nitrogen shortage. Spider mite makes the underside of the leaf silvery or dusty, and confusing that with nitrogen yellowing is a common greenhouse error. A leaf test taken outside a period of acute water stress separates diagnosis from guesswork. The article on drought stress against nutrient deficiency in this same blog sets out the order of spade and leaf separately; here just hold on to this: nitrogen equals a uniformly yellow old leaf.
Why nitrogen runs short in Iranian orchards and fields
A nitrogen deficiency does not always mean the soil has no nitrogen. In the light soils of Kerman and Yazd, urea and nitrate leach easily. In heavy soil low in organic matter, mineralisation is slow and the spring plant stays hungry. Flood irrigation on a long interval takes nitrate out of the root bulb. Unrotted manure temporarily locks nitrogen up in microbial bodies and yellows transplants. A high pH makes uptake harder, but nitrogen is not imprisoned in lime the way iron is; the problem is more about losses and wrong timing. Too much nitrogen too late is also a hazard: soft growth, pests, and in deciduous trees sensitivity to cold. Treatment means delivering the right source, at the right time, to the active root zone — not tipping another sack at the foot of the trunk.
Iron fertiliser is not the answer to yellowing lower leaves. First say whether the leaf is old or young; then say whether the soil is wet or dry. Open the nitrogen only after those two sentences.
Treating nitrogen deficiency: source, timing and splitting
If the diagnosis holds, treatment has two layers: an acute correction for the leaves still alive, and building a reserve for the rest of the season. For the acute correction in field crops and greenhouses, a soluble nitrogen fertiliser with irrigation — ammonium sulphate in Iran's alkaline soils is often more sensible than urea alone, because it has a slight local acidifying effect and loses less gas on a dry surface. Do not spread urea on dry, hot soil; it has to reach the root zone with water. Ammonium nitrate or complete fertilisers with a higher nitrogen ratio have their use in the vegetative growth of transplants and leafy vegetables, subject to the water's salinity. A dilute foliar urea on a tree is a supplement, not the pillar of treatment; leaf scorch on a hot midday is common. Take the dose from a test and the plant's age, not from the neighbour's habit.
For fruit trees, a surface spread of autumn urea is not a Farkesht prescription. Tree nitrogen is better given in spring and early growth, split and with irrigation. Farkesht's macro Chalkood Cake places nitrogen on an organic bed at root depth so it is released gradually and there is less leaching and volatilisation; that is for the season's base nutrition, not for an overnight greening shock. If micronutrients are short at the same time — which is common in calcareous soil — pair it with the micro Chalkood Cake in the winter programme, but do not treat yellow old leaves with iron alone. In urban space where the soil is out of reach, an injection capsule is a separate route and does not replace an orchard's nitrogen programme.
- Describe old leaves and young leaves separately and look at the root-depth moisture with a spade.
- If the soil is dry, irrigate and wait a few days; if the pattern remains, carry on with nitrogen.
- Choose the nitrogen source by soil texture and pH; in alkaline soil take ammonium sulphate seriously.
- Split the quantity: several small applications are better than one heavy shock.
- For a tree, prefer placement in the canopy shade or a winter fertiliser pit to spreading at the foot of the trunk.
- Two to three weeks later, look at the colour of the new leaves; a fully yellow old leaf usually does not turn green again.
Compost, legumes and what urea does not replace
Rotted organic matter raises the capacity to hold nitrogen and water and, in the light soils of the south, is more durable than any sack of top dressing. Fresh manure does not do this; ammonia and salt burn the roots. Legume cover crops fix atmospheric nitrogen, but in Iranian fruit orchards they are not a replacement for the season's programme; they make more sense in arable rotation and in young orchards with managed ground cover. Regular irrigation completes uptake: nitrogen does not move in dry soil and goes out of reach under flooding. If after a nitrogen fertiliser the young leaves are still yellow with green veins, return to the iron route and the article on chlorosis in calcareous soil. Farkesht writes the treatment programme for that particular plot, because the urea prescription for a lawn in Isfahan and for pistachio in Rafsanjan are not the same.
The treatment in summary: recognise nitrogen deficiency by the uniform yellowing of old leaves, separate it from iron and drought, match the source to Iran's alkaline soil, and split the quantity. Do not expect a dead leaf at the bottom of the crown to green up; the measure of success is the colour and growth of new leaves in two to four weeks. If the tree is bearing, do not tie nitrogen to unchecked vegetative growth; a leafy crown with poor fruit has merely swapped a deficiency for an excess. For a precise programme for your own orchard or greenhouse, give the soil and leaf tests to the Farkesht specialists so the fertiliser cake and the nitrogen top dressing are written onto one plan rather than onto last season's guess.
