Gardening glossary
Nitrogen
Nitrogen is the first of the three macronutrients in the NPK ratio and the engine behind leafy, green, photosynthesising tissue. Plants pull nitrogen out of the soil as nitrate (NO₃⁻) and ammonium (NH₄⁺) ions, then use it to build chlorophyll, amino acids, proteins, and DNA. Without enough nitrogen, lower leaves yellow uniformly (older leaves first, because the plant cannibalises them to feed new growth), stems thin out, and the whole plant looks pale and stalled.
Where nitrogen comes from in a home garden:
- **Compost and well-rotted manure** release nitrogen slowly as soil microbes break them down — a season-long feed at low rates. - **Liquid feeds** (fish emulsion, seaweed, synthetic high-N blends) deliver an immediate hit; use them when foliage is the goal. - **Legumes** (peas, beans, clover) fix atmospheric nitrogen into the soil via root nodules — about 50–150 kg N/ha per season for well-nodulated crops, although the gardener-scale benefit is modest. - **Mineral fertilisers** like ammonium sulphate or urea provide the most concentrated, fastest-acting form.
When to push nitrogen and when to ease off matters. Leafy greens, lawns, sweetcorn, and brassicas appreciate steady nitrogen through the first half of their growth. Once tomatoes, peppers, or cucumbers start flowering, switch to a potassium-leaning feed — extra nitrogen now means lush foliage and few fruit.
Two common nitrogen mistakes home gardeners make. The first is over-feeding houseplants in winter — short days mean slow growth and unused nitrogen accumulates as salt in the pot. The second is dumping fresh, "hot" manure on a bed before planting; the rapidly-decomposing nitrogen burns young roots. Let manure age six months, or work it in the autumn before a spring crop.
For a quick visual test, see our [yellow plant leaves guide](/blog/yellow-plant-leaves) — uniform pale yellow on older leaves is the classic nitrogen-deficiency tell.