Diagnosing phosphorus deficiency in oats
Nearly all soils in Western Australia are phosphorus deficient in their natural state but the continual use of phosphorus fertiliser means acute deficiency in broadacre crops is rare, with the exception of Darling Range gravels. Induced phosphorus deficiency in seedlings can occur in very dry conditions, but is often transitory and plants recover when the top soil rewets.
Phosphorus is a major nutrient for improved oaten hay and grain production.
What to look for
- Phosphorus( P) deficiency reduces seedling establishment and root development. The deficiency symptoms usually only occur if the deficiency is severe and are more noticeable in young plants as they have a greater relative demand for P than more mature plants.
- Symptoms of P deficiency in oats are usually non-specific and difficult to diagnose in the field. The most noticeable feature in oats is reduced growth and vigour. Acute P deficiency generally occurs when other factors such as pests, root diseases, or dry soil restricts the plants ability to access soil P.
- Early signs are smaller erect plants with darker leaves and fewer tillers.
- More acute deficiency causes orange to purple colouration on old leaves.
- Orange to red colours then necrosis moves down older leaves from the tip.
What else could it be
Dry soil can induce phosphorus deficiency in young plants
|Darker leaves and fewer tillers||Symptoms disappear when the soil rewets|
|Diagnosing yellow dwarf virus||Progressive reddening and death of older leaves||Yellow dwarf virus is associated with streaks and occurs in patches|
Where does it occur?
- Phosphorus deficiency is a problem on high phosphorus-retaining soils, particularly in the Darling Range where soil acidity and water repellence markedly reduce phosphorus uptake.
- Dry topsoil can lead to temporary phosphorus deficiency on all soils particularly during early crop growth and on water repellent soils.
- Water repellent and acidic soils require more phosphorus.
- Plants have a high requirement for phosphorus during early growth. As phosphorus is relatively immobile in the soil, top dressed or sprayed fertiliser cannot supply enough to correct a deficiency.
- Phosphorus does leach on very low PBI (a measure of phosphorus retention) sands, particularly on coastal plains. Top dressing is effective on these soils.
- The optimum P requirement for hay and grain appear to be different. Oat varieties may differ in their P requirements.
How can it be monitored?
- Soil test to determine phosphorus fertiliser requirements. Soil tests may underestimate available phosphorus on very low PBI sands and overestimate it on acidic and water repellent soils (particularly in the Darling Range).
- Use whole-top plant test to diagnose suspected phosphorus deficiency, and compare paired good/poor plant samples where possible.