Diagnosing phosphorus deficiency in narrow-leafed lupins
Nearly all soils in WA were phosphorus (P) deficient when cleared for agriculture, but continual use of P fertiliser means acute deficiency in broadacre crops is rare, with the exception of very acidic and high PBI (phosphorus buffering index) soils, P deficiency is often transitory and compounded by dry soil with symptoms disappearing when topsoil re-wets following rainfall.
What to look for
- Smaller later flowering plants with worse symptoms on higher P-fixing, water repellent and very acidic soils, and in very dry seasons.
- Most common symptoms are smaller plants, narrower stems and petioles and fewer laterals.
- Petioles and leaflets are more upward-angled giving affected plants a more upright appearance.
- Growth is severely reduced before leaf symptoms show.
- As deficiency progresses older leaf leaflets turn muddy greyish-green and tend to droop.
- Severely deficient leaflets wither from the tip and die, often twisting, then drop-off, followed by the petioles.
What else could it be
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Where did it come from?
- P deficiency is more likely on high PBI soil, particularly Darling Range gravels.
- Dry topsoil can lead to temporary P deficiency on all soils particularly during early crop growth and on water repellent soils.
- Plants have a high P requirement for phosphorus during early growth and a deficiency can't be corrected within the growing season. Apply P fertiliser to crops according to soil test.
- The best way to apply phosphorus fertiliser to lupin crops depends on rainfall and the capacity of the soil to retain P, measured as soil test PBI.
- On low PBI sands it is best to separate seed and fertiliser to avoid toxicity.
- On most soils drilling P with or near the seed is effective.
- Deep banding places the poorly mobile nutrient like P in soil that remains moist for longer periods, making it more accessible to the plant roots.
- On very high PBI soils such as Darling Range gravels, applying P with the seed avoids possible deficiency during early growth before seedling roots reach the deeply placed P.
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.
- Critical plant phosphorus levels vary with plant age and size. Critical levels at the vegetative stage up to 80 days, and 80 to 140 days after seeding are 0.27-0.37, and 0.13-0.23 % respectively.
- The required seed concentration for early seedling vigour is 0.25%.