Fertilising guide

How to fertilise White Onion (Allium cepa 'Sturon')— schedule & NPK

Also called Sturon onion, white onion, globe onion.

More about white onion

About White Onion

Allium cepa 'Sturon' · also called Sturon onion, white onion · edible

The bulb onion is a biennial allium grown as an annual for its swollen storage bulb. 'Sturon' is a popular, reliable globe variety usually grown from heat-treated sets, giving uniform, well-keeping, mild bulbs that resist bolting. Bulbs swell through summer as daylength lengthens, then ripen and are lifted and dried in late summer for long storage.

Growth habit: Forms a basal swollen bulb of fleshy concentric scales topped by upright hollow blue-green leaves; a biennial grown as an annual, flowering to a globe umbel only if it bolts or is left a second year.

What fertiliser white onion actually wants — and why

White Onion stores its crop underground, so the rule is the reverse of leafy plants — go easy on nitrogen, which sends energy into tops at the expense of roots.

For the language behind the three numbers on the bottle — what nitrogen, phosphorus and potassium each do — see the NPK ratio explained entry. The short version for white onion: match the feed to the job the plant is doing right now, not to a generic “plant food” on the shelf.

How often to feed white onion, and which months

Feeding only earns its keep while the plant is in active growth and can use the nutrients — pour feed into a dormant or low-light plant and it simply builds up as root-burning salt. For white onion:

Moderate feeder. In a compost-improved bed, give a balanced or slightly nitrogen-leaning feed early in the season to build leaves, then stop feeding once bulbing begins so the bulbs ripen and store well rather than staying soft. In practice: prepare the bed with well-rotted compost (not fresh manure), then little or no extra feeding through the season (spring through early autumn); a light potassium feed mid-growth at most.

The dormant-season rule matters more than the exact interval: skip feeding entirely when white onion is resting. For the wider context on indoor feeding rhythms across the seasons, the houseplant fertiliser schedule walks through the year month by month.

What strength to mix for white onion

Less is more for white onion. If you feed at all, keep it light and low-nitrogen — the soil preparation does the work, and over-feeding actively spoils the crop.

Feeding always goes onto already-damp soil, never dry roots — water white onion first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the white onion watering schedule.

Signs you are over-feeding white onion

Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for white onion:

Large lush leafy tops and small, forked or hairy roots.
Split or cracked roots from a nitrogen-and-water surge.
All foliage and no usable crop at harvest.

Signs you are under-feeding white onion

Genuinely uncommon in reasonable soil — these are not hungry plants.
Pale, weak tops and small roots only in very poor, exhausted ground.
Slow growth across the whole bed in long-uncultivated soil.

If the symptoms point at watering, light or roots rather than nutrition, the full white onion care brief covers soil, humidity and the common problems for this species.

Flushing and leaching the salts

Flushing is not the issue for white onion — the equivalent care is avoiding fresh manure and high-N feeds entirely, and rotating beds so the soil is not over-rich from a previous hungry crop.

Organic vs synthetic feeds for white onion

Organic options

Well-rotted compost worked in the season before, or for a previous crop, is ideal — never fresh manure. UK: garden compost, low-N blends; US: Espoma Garden-tone sparingly or finished compost. Lean and well-worked beats rich.

Synthetic / liquid feeds

If anything, a low-nitrogen, potassium-leaning feed only — UK: a high-potash feed mid-season at most, never a general high-N; US: a 5-10-10 sparingly. Most root crops crop best with no synthetic feed at all.

Brand names are examples, not endorsements, and UK and US ranges differ — check the label’s own NPK and dilution rate, since formulations change.

Fertilising white onion — frequently asked questions

What fertiliser does white onion need?

Low-nitrogen, with modest phosphorus and potassium for root development — ideally compost-improved soil rather than a high-N feed. Excess nitrogen forks the roots and grows lush tops instead of a crop. White Onion stores its crop underground, so the rule is the reverse of leafy plants — go easy on nitrogen, which sends energy into tops at the expense of roots.

How often should I feed white onion?

Moderate feeder. In a compost-improved bed, give a balanced or slightly nitrogen-leaning feed early in the season to build leaves, then stop feeding once bulbing begins so the bulbs ripen and store well rather than staying soft. Moderate feeder. In a compost-improved bed, give a balanced or slightly nitrogen-leaning feed early in the season to build leaves, then stop feeding once bulbing begins so the bulbs ripen and store well rather than staying soft. In practice: prepare the bed with well-rotted compost (not fresh manure), then little or no extra feeding through the season (spring through early autumn); a light potassium feed mid-growth at most.

What strength of feed for white onion?