Diagnosing potassium deficiency in narrow-leafed lupins
Narrow-leafed lupins have a high potassium (K) requirement, but are able to access soil reserves at a greater depth than cereals.
What to look for
Paddock
Smaller paler plants that have lost older leaflets but retained the petioles.
Symptoms are worse on lighter sands and often appear as patches of poor growth amongst patches of better growth, in patches (livestock manure), or lines (harvest rows).
Plant
Plants are smaller and paler with thinner stems and fewer laterals.
Older leaves are affected first, some or all leaflets turn chlorotic then drop-off.
Middle and younger leaves on severely deficient plants can become twisted and claw shaped.
Petioles remain intact.
Plants are more susceptible to diseases and water stress.
Where does it occur?
Soil type
Sandy soils and deep grey sandy duplex soils tend to be more susceptible to K deficiency.
High rates of hay or grain removal can result in K deficiency.
Lupin roots can access nutrients at greater depth than cereals
Management strategies
Top-dressed or banded K fertilisers will correct the deficiency.
Excessive K fertiliser leaching can occur if applied earlier than four weeks after sowing on very sandy soils in high rainfall areas, because roots are insufficiently developed to capture all of the K.
Potassium chloride can be toxic when drilled with the seed.
How can it be monitored?
Tissue test
Use whole-top plant test to diagnose suspected potassium deficiency.
Critical potassium levels vary with plant age and size, but K concentration in whole shoots below 3.1 % and 0.9 % at 28 and 140 days after seeding respectively indicate deficiency.
Potassium soil test values (0-10%) higher than 50 mg/kg are unlikely to respond to added potassium. This is reliable in deep sands but may not be reliable for duplex soils that have potassium reserves in the clay.
