Field peas are susceptible to zinc deficiency, but is rarely seen in WA.
What to look for
Paddock
Plants with white necrotic older leaves and considerable variation in severity of symptoms between plants.
Deficiency will vary with soil type, being worst on highly alkaline soils.
Deficiency may be worse in cold wet weather.
Plant
Older leaves of young plants may initially wilt.
Cream-coloured necrosis on older leaf margins moves to the midrib, leaving a small green residual at the leaf base. The whole leaf turns white and dies.
Tendrils go limp, curl and finally die.
Later, new leaves are small, pale and cupped.
Red-brown lesions develop on new leaf and upper main stems.
Where does it occur?
Soil type
Temperature
Dry conditions
Most sandy surfaced soils required copper and zinc when initially cleared for agriculture.
Zinc deficiency is also common on highly alkaline soils, and may be induced by cold wet weather.
Zinc is relatively immobile in soil and becomes unavailable to crops in dry soil.
Where soil levels are marginal, zinc deficiency may be induced by applications of lime to acidic soils.
Management strategies
Spraying foliar
Foliar spray (effective only in current season) or soil fertiliser drilled with the following crop.
Zinc foliar sprays need to be applied as soon as deficiency is detected to avoid irreversible damage.
As zinc is immobile in the soil, topdressing is ineffective, as it would only be available to the plant when the topsoil is wet.
Zinc drilled deep increases the chances of roots being able to obtain enough in dry seasons.
Zinc has a 15 to 20 year residual life in soil.
Zinc present in compound fertilisers often meets the current requirements of the crop.
How can it be monitored?
Soil test
As zinc is immobile in the plant, young shoot or young leaf sampling is most accurate, but Australian laboratory standards have not been established for field peas. A guide from literature is 20mg/kg. Sample paired good and poor plants at the same growth stage.
A DTPA zinc soil test provides at best a rough guide to soil zinc status. The critical concentration for cereals varies with soil type. For example acid sands about 0.2 mg/kg while alkaline clay require about 0.45 mg Zn kg soil.
