Phosphorus deficiency in wheat and barley: signs and fixes

What phosphorus deficiency looks like

DPIRD (WA) says the symptoms are usually vague and hard to diagnose in the field. The most obvious sign is slow growth and poor vigour.

• Smaller, lighter green plants with dead leaf tips, often on sandier parts of the paddock or between header or swathe rows.
• Plants look water-stressed even when conditions are fine.
• Seedlings with induced deficiency can look pale olive-green and wilted.
• On older leaves, yellowing starts at the tip and moves down as a front, while the base stays dark green. The tip quickly dies orange to dark brown.
• Thin, poorly tillered plants. In barley, DPIRD says mild deficiency cuts tillering and grain yield with few signs apart from slower growth.
• Severely deficient barley may turn orange, red or purple. Yara and Soil Quality note purple or reddish tints, especially in cold weather.
• At some stages deficient crops look darker green, not paler (Soil Quality).

What else could it be

Soils and conditions at risk

• Soils that lock up (fix) phosphorus. DPIRD names the Darling Range gravels in WA, where sour (acidic) and water-repellent soils sharply cut uptake.
• Acidic and water-repellent soils, which need more phosphorus. Below pH 5.0 the soil's ability to fix phosphorus rises dramatically (Soil Quality).
• Dry topsoil early in the season, on any soil.
• Yara adds very alkaline (calcareous) soils, low organic matter, cold or wet conditions, low phosphorus reserves and iron-rich soils.
• Phosphorus-poor subsoils. In the northern cropping region, Soil Quality (NSW) reports yield penalties of up to 20%.

How to confirm it

• Tissue test: DPIRD recommends a whole-top plant test, comparing paired samples from good and poor areas.
• Soil test: use it to set phosphorus rates. The usual test is Colwell P, read alongside the soil's phosphorus buffering index (PBI), which shows how strongly it locks up P. See reading a soil test.
• Know the limits. DPIRD says soil tests can underestimate available P on very low PBI sands and overestimate it on acidic, water-repellent soils. Soil Quality says Colwell P has been unreliable on some soils, particularly calcareous ones, where the DGT test appears to work well.

Fixing it: mostly at sowing

Crops need a lot of phosphorus early, and it barely moves in soil. DPIRD says topdressed or sprayed phosphorus cannot supply enough to correct a deficiency. The exception is very low PBI sands, such as on coastal plains, where topdressing works.

• Apply at or before seeding. Soil Quality reports significant yield penalties when phosphorus goes on more than 10 days after germination.
• Starter options include MAP, DAP, Summit MAP 11-52 and Impact MAP. Compare MAP and DAP side by side and read MAP vs DAP for seed safety.
• Single superphosphate, such as SuPerfect, also supplies sulphur. Soil Quality says nitrogen-containing fertilisers such as DAP are more damaging to seed than superphosphate.
• Keep high phosphorus rates at least 2 cm below the seed, and take more care on dry seedbeds, wide rows, and with canola or lupins (Soil Quality).
• On sour soils, Soil Quality says years of phosphorus plus lime to keep pH above 5 reduce the soil's lock-up. See how much lime per hectare.
• If your agronomist suspects a phosphorus-poor subsoil, ask about deep banding. Soil Quality (NSW) reports yield benefits for up to five seasons in northern trials.

Budget to replace what you remove: Soil Quality puts it at about 3 kg of phosphorus per tonne of wheat or barley grain. Only 5–30% of applied phosphorus is used by the crop in the first year; the rest builds soil reserves. Browse phosphorus fertilisers. Always check with your agronomist before acting.