Why Are My Plant Leaves Turning Yellow? Nitrogen Deficiency Explained

If the lower, older leaves on your plants are fading from green to pale yellow while the growing tips stay relatively green, nitrogen deficiency is the most likely cause. It is one of the most common nutrient problems in the garden, and it is usually easy to fix once you have identified it correctly.

Why Nitrogen Is So Important

Nitrogen is the engine of leafy growth. It is a key building block of chlorophyll — the pigment that makes leaves green and captures sunlight for photosynthesis — and it forms part of every protein and enzyme in the plant. Without enough nitrogen, a plant cannot build new tissue quickly, cannot photosynthesize efficiently and gradually starts to look sick.

Recognising the Symptoms

Nitrogen deficiency has a very characteristic pattern. Because nitrogen is mobile within the plant, when supplies run short the plant withdraws it from older tissues to send to new growing points. This means yellowing starts at the bottom of the plant and works upward. Look for:

• Pale yellow-green or bright yellow older leaves
• Stunted, slow growth overall
• Thin, weak stems
• Small leaves that are lighter in colour than normal
• Premature leaf drop on older growth

The yellowing tends to be uniform across the whole leaf rather than patchy or interveinal (that pattern suggests magnesium or iron issues instead).

Common Causes in Garden Soil

Nitrogen deficiency does not always mean you have failed to feed. Several other factors can trigger it even in well-tended gardens:

• Waterlogged or cold soil: roots cannot take up nutrients from saturated or very cold ground
• Sandy soil: nitrogen leaches rapidly through fast-draining sandy soils
• Fresh wood chip mulch: microbes decomposing woody material temporarily lock up soil nitrogen
• Heavy cropping: hungry fruiting vegetables strip nitrogen fast
• Container growing: pots exhaust nutrients quickly and need regular feeding

How to Fix Nitrogen Deficiency

The fastest correction is a high-nitrogen liquid fertiliser applied to the soil around the roots. A feed with an NPK ratio weighted toward the first number — such as 20-10-10 — will show results within a week. For a more organic approach, diluted liquid nettle feed or comfrey tea are rich in nitrogen and work well as a soil drench. A top-dressing of well-rotted chicken manure pellets provides a steady release over several weeks.

If you suspect cold or waterlogged soil is the cause rather than actual deficiency, focus on improving drainage and wait for the soil to warm before feeding heavily — adding more nitrogen to soil that roots cannot access will not help.

Preventing Nitrogen Deficiency

Regular feeding through the growing season is the simplest prevention. For heavy-feeding crops like brassicas, courgettes and sweetcorn, apply a nitrogen-rich feed every two to three weeks once the plants are in active growth. Incorporating compost into the bed before planting adds a slow-release base. For containers, switch to a balanced liquid feed at every other watering.

How Much Is Too Much?

It is worth noting that over-correcting is also a problem. Excess nitrogen drives lush, soft, sappy growth that is attractive to aphids and vulnerable to fungal disease. Aim to fix the deficiency and then settle into a balanced feeding routine rather than repeatedly applying high-nitrogen products throughout the season.