A plant cannot move, and it cannot complain. What it can do is change colour, change shape, and slow down. Almost every gardener has stood in front of a pot wondering why the leaves look wrong, and the honest answer is that the plant said something three weeks ago and nobody was reading.
The good news is that plant nutrient deficiency symptoms are surprisingly consistent. A nitrogen-starved tomato in Pune looks like a nitrogen-starved tomato in Lucknow. Once you know where to look first, most diagnoses take under a minute with no lab test, no kit, no guesswork.
This guide gives you the reading order the experienced growers use: location first, pattern second, colour third.

Start with one question: where is the damage?

This single question eliminates half the possibilities before you look at colour at all.
Some nutrients move freely inside the plant. When they run short, the plant pulls them out of its older, lower leaves and sends them to the new growth at the top, because new growth is the future. Those nutrients are nitrogen, phosphorus, potassium and magnesium. symptoms appear on the oldest leaves first, at the bottom of the plant.
Other nutrients cannot move once they are locked into a leaf. Iron, calcium, boron, zinc, manganese. When those run short, the old leaves stay fine and the newest leaves at the top show the problem.

So before anything else, look at the plant as a whole and ask which end is suffering.

● Bottom leaves affected, top looks fine - mobile nutrient. Think nitrogen, phosphorus, potassium, magnesium.
● Top and newest leaves affected, bottom looks fine - immobile nutrient. Think iron, calcium, or a pH problem blocking uptake.
● Everything affected at once, evenly - usually not a deficiency at all. Look at water, roots, light or heat first.

That third case is the one people misread most often. If the entire plant declines together, adding fertiliser rarely helps and often makes it worse.

Symptom by symptom - the five you will actually meet

Nitrogen. The most common shortage in container gardening by a wide margin. The whole leaf goes pale, evenly, not patchy, not veiny, and it starts with the oldest leaves at the bottom. Growth becomes slow and thin, and the plant looks generally washed out. If you have ever searched why are my plant leaves turning yellow, this uniform bottom-up paleness is the answer about half the time. Nitrogen deficiency in plants shows up fastest in leafy crops — spinach, methi, coriander — and after heavy rain, which washes nitrogen straight down past the roots.
Phosphorus. Subtler and easier to miss. Leaves turn a dull, dark, slightly blue-green rather than pale, and older leaves and stems develop a purple or reddish tinge, often strongest on the underside. Growth stalls, flowering is late and poor. Common in cold soil and in heavy soils where roots are struggling, because phosphorus moves very little in soil, the root has to reach it.
Potassium. Look at the leaf edges. The margins of older leaves go yellow, then dry brown and crisp, as if scorched, while the middle of the leaf stays green. Edges may curl. Stems feel weaker than they should and fruit sets small or unevenly. Tomatoes, chillies and brinjal show this clearly when they start fruiting, because fruit is where potassium goes.
Magnesium. The classic Christmas-tree pattern, yellow spreading between the veins while the veins themselves stay distinctly green, on older and middle leaves. In bad cases the yellow patches go brown and the leaf drops. Very common in pots that have been watered heavily for months and in light, sandy mixes.
Iron. The same between-the-veins yellowing as magnesium, with one decisive difference: it appears on the newest leaves at the top. The veins stay a sharp green against a pale, almost cream-coloured leaf. In severe cases new leaves come out nearly white. Curry leaf, lemon, hibiscus and jasmine are the usual complainers in Indian gardens.

Quick reference - read the table backwards from what you see

The last row deserves a note. Calcium problems are almost never a shortage of calcium in the soil. They are a shortage of steady moisture moving calcium into the plant. Blossom-end rot on tomatoes is a famous example. Water consistency fixes it far more often than any product does.

Before you feed: three things that fake a deficiency

This is the part most beginner guides skip, and it is the reason so much fertiliser gets wasted.
One - pH lock. The nutrient can be sitting right there in the soil and still be unavailable, because at the wrong pH it binds into a form roots cannot take up. Iron is the worst offender: above roughly 7.5, iron locks up almost completely. Adding more iron to alkaline soil is money into a hole. Fix the pH and the yellowing often clears without any new input at all.

Two - too much water, or too little air. Damaged roots cannot absorb nutrients no matter how well fed the soil is. A plant sitting in soggy soil shows pale leaves and stalled growth that look exactly like nitrogen hunger. Check the soil with your finger and check whether the drainage hole actually drains before you reach for feed. Three - not enough light. A plant in deep shade goes pale, leggy and thin. No amount of fertiliser substitutes for light, and feeding a light-starved plant usually produces weak, soft growth that pests find first.

Rule of thumb: if the symptom pattern does not match the location rule from earlier, suspect one of these three before you suspect a nutrient.

How to fix a nutrient deficiency without guessing

Once you have ruled out water, light and pH, the question of how to fix nutrient deficiency becomes fairly simple. Work in this order.

1. Identify old-growth versus new-growth. You already know how. This narrows it to two or three candidates.

2. Match the pattern, not just the colour. Uniform pale is different from between-the-veins yellow, which is different from scorched edges. The pattern is the diagnosis.

3. Check pH if the symptom is on new growth. Especially with iron. Tap water in much of India runs alkaline.

4. Correct with a balanced organic feed, not a single-nutrient shot. Deficiencies rarely arrive alone, and heavy single-nutrient dosing pushes other nutrients out of balance. Sodium-heavy, one-note chemical feeds are how a small problem becomes a soil problem.

5. Give it two weeks and watch the new growth only. This is the discipline that matters. Damaged leaves do not turn green again. Recovery shows up as healthy new leaves at the top, not as repaired old ones.

6. Write down what you did. Date, plant, what you fed. Next season this becomes your own reference table, worth more than any generic guide.

Why fixing the soil beats chasing symptoms

Every symptom in this guide is a downstream effect. The upstream cause, in the vast majority of pots and beds, is the same: soil with too little organic carbon, too little biological life, and a structure that has collapsed into something roots struggle to work in.

Soil in that condition fails in three ways at once. It holds nutrients poorly, so feeding leaks away. It holds water badly, so roots suffocate or dry out. And it has almost no microbial population, which matters because microbes are what convert nutrients into the forms roots can actually absorb. You can keep topping up a dead soil forever and never fix why it keeps emptying.

This is where fermented organic manure does work that a quick-release chemical feed cannot. FOM is made by putting digested biogas slurry through controlled fermentation, which means it carries organic carbon, micronutrients and living beneficial microbes together rather than a narrow burst of N-P-K. In practice that gives you three things: nutrients released slowly instead of all at once, better soil structure over successive seasons, and a microbial population that keeps nutrients cycling.

A workable routine for pots and terrace beds: mix FOM granules into the soil once a month, and use LFOM liquid diluted as directed once in between. Balanced, steady, and aimed at the soil rather than at a single yellow leaf.

Do / Don't

Do:

● Check old growth versus new growth before anything else
● Look at the pattern of the yellowing, not only the colour
● Rule out water, light and pH before buying fertiliser
● Test pH if the newest leaves are affected
● Feed balanced and organic, in small regular doses
● Judge recovery only by new growth

Don't:

● Don't diagnose from one leaf - look at the whole plant
● Don't dose a single nutrient heavily on a guess
● Don't feed a plant sitting in waterlogged soil
● Don't expect damaged leaves to turn green again
● Don't feed a plant that is short of light instead of moving it
● Don't change three things at once, or you learn nothing

Two-minute diagnosis:

(1) Old leaves or new leaves?
(2) Uniform pale, between the veins, or scorched edges?
(3) Soil wet or dry to the finger?
(4) Enough light?
(5) Water or soil pH known?
(6) Anything changed in the last three weeks?

Frequently asked questions

Why are my plant's lower leaves yellow but the top looks healthy?
That bottom-up pattern points to a mobile nutrient, most often nitrogen - the plant is stripping its old leaves to supply the new ones. It is also normal for a plant to shed a few of its oldest leaves as it grows, so one or two at the very base is not an emergency.

How do I tell iron deficiency from magnesium deficiency?
Both show yellow between green veins, so use the location. Magnesium hits older and middle leaves; iron hits the newest leaves at the top. If it is iron, check pH before you add anything - alkaline soil locks iron away regardless of how much is present.

How long until a corrected plant looks better?
Expect ten to fifteen days for visible new growth, longer in cool weather. Watch the growing tip. The already-damaged leaves will not recover, and waiting for them to is how people conclude, wrongly, that the fix did not work.

Should I use a chemical fertiliser for a quick correction?
For a severe, clearly identified single deficiency, a targeted correction has its place. But most home-garden cases are mild, mixed, and caused by tired soil, and repeated heavy chemical feeding degrades structure and microbial life, which makes the next deficiency more likely. Balanced organic feeding solves the cause rather than the symptom.