Phosphorus deficiency
Dark, dull, stunted plants — often purple-tinged on the older leaves — with weak roots and delayed flowering. Early damage is the costliest.
- Nutrient type
- Primary (macro)
- Mobility in plant
- Mobile
- Symptoms start on
- Old (lower) leaves
- Most at risk
- Cold, acid or calcareous soils
Overview
Phosphorus drives root growth, early establishment, tillering, flowering and grain or seed set. A shortage does not usually yellow the crop; instead it makes it small, dark and dull, and in many crops throws a purple or reddish-purple colour onto the older leaves and stems as sugars build up unused.
Because phosphorus is almost immobile in the soil, roots have to grow to it — so anything that slows early root growth (cold, waterlogging, compaction) can trigger a temporary deficiency even where the soil test is adequate. Applied phosphorus also gets "fixed" and locked up in very acidic and in calcareous (high-pH) soils, so placement matters as much as amount.
How to spot it
- Plants are stunted and thin, with an unusually dark, dull blue-green colour rather than a healthy bright green.
- A purple or reddish-purple tinge appears on the older leaves, leaf edges and stems — clearest in maize, and common in wheat, cotton and mustard.
- Tillering, branching and flowering are delayed; the crop matures late and unevenly.
- Roots are sparse and short when you pull a plant up — the least visible but most important symptom.
Why it happens
- Cold, wet soil early in the season slows root growth so the crop cannot reach soil phosphorus — a common cause of purpling in early-sown maize and wheat that fades as it warms.
- Very acidic soils fix phosphorus as iron/aluminium phosphates; calcareous, high-pH soils fix it as calcium phosphates. Either way much of the applied P is locked up.
- Broadcasting phosphatic fertiliser on the surface instead of placing it near the seed, where immobile phosphorus can actually be reached by young roots.
- Compacted or waterlogged soil that restricts the root system.
Confirm before you spend
- Purpling from cold usually clears on its own as the soil warms and roots extend — don’t mistake a cold snap for a true soil shortage.
- A soil test is the only reliable way to know your field’s phosphorus status; unlike nitrogen it changes slowly, so one test guides several seasons.
How to correct it
- Phosphorus is a basal nutrient — the correction is mainly for next season: place the full recommended dose (DAP or single super phosphate) at or below seed depth at sowing, not broadcast on top.
- For a standing crop showing early purpling, a foliar spray of a water-soluble phosphate (e.g. 0.5–1% DAP or mono-ammonium phosphate) can tide it over until roots reach the soil supply.
- On acid soils, lime to correct pH before the season so applied phosphorus stays available; on high-pH soils, band-place P near the seed to limit fixation.
- Single super phosphate (SSP) also supplies sulphur and calcium, which is a bonus on the light soils where P deficiency is common.
Prevent it coming back
- Always place phosphatic fertiliser near the seed at sowing rather than top-dressing it later — immobile P has to start where the roots are.
- Use phosphate-solubilising bacteria (PSB) and add FYM/compost, which release fixed phosphorus and improve its availability.
- Correct soil pH over time (lime on acid soils) so less of every future dose gets locked up.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
My young maize is purple. Is it always phosphorus deficiency?
Not always. Early-season purpling is very often caused by cold soil slowing the roots, not a real shortage — it clears as the soil warms. If the purpling persists into warm weather, the plants stay stunted, and a soil test is low, then it is a true phosphorus deficiency.
I applied enough DAP but still see deficiency. Why?
Placement and soil pH are the usual reasons. Phosphorus barely moves in soil, so broadcast DAP on the surface never reaches young roots; it needs to be placed near the seed. And in very acid or very alkaline soils much of it gets chemically fixed. Band-place it, and correct pH, so more stays available.
Can I fix it mid-season?
Only partly. A 0.5–1% DAP foliar spray helps a standing crop through an early shortage, but phosphorus is really a basal nutrient — the proper fix is correct placement at sowing next season. Don’t expect a top-dressing to work the way it does for nitrogen.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.