The cold snaps seen in southern Australia this year gave conditions where zinc deficiencies typically appear. Zinc deficiency tends to occur early in the growing season when soils are cold and wet. Shoot growth then outpaces root development. Root-pruning herbicides can induce zinc deficiency by the same mechanism. Zinc deficiency sometimes appears in young plants emerging in drying soil, disappearing when rainfall re-wets the topsoil.
Yield potential is lost through the period of deficiency affecting the plant. Moderate zinc deficiency can be corrected by applying soluble zinc fertiliser such as chelated zinc or zinc sulphate before stem elongation. Later applications up to flowering will have little effect on yield, but can increase zinc content in the grain. Too late now for this year in most cases!
Applying zinc at seeding with dry or fluid fertilisers balances cost and risk where additional zinc may be required. Using zinc-fortified seed can reduce the need for added zinc. Address zinc demand in the cereal phase of a crop rotation. Cereals are generally the more responsive to zinc fertiliser compared to pulses or oilseeds.
Photo: Severe zinc deficiency symptoms appear as oily grey green patches in the centre of leaves. The yellow and green necrotic patches gradually extend outwards towards the tip and base of the leaf. Young leaves are most affected. It is difficult to identify moderate deficiencies.
No single test can give a good indication of the likelihood of a direct response to the addition of Zn. Balancing crop removal is not useful as soil has a strong capacity to bind zinc. A combination of soil tests, paddock history, soil characteristics, crop demand and tissue tests is the best foundation for predicting a likely crop response to zinc:
- Critical soil test values (DTPA) are generally less than 0.5 mg/kg.
- Critical tissue concentrations in the youngest expanded blade of wheat is <14 mg/kg.
- Soil characteristics associated with higher risk of zinc deficiency are alkalinity, very high soil phosphorus, low organic matter, as well as high iron and manganese.
- Paddock history risks include previous observation of zinc deficiency, grain tissue tests of <15 mg/kg zinc, recent lime application and use of root pruning herbicides.
- What’s new with zinc? Maybe just some critical reminders, a 2015 GRDC update paper by Rob Norton.
- Balancing zinc nutrition, on page 3 within the 2014 GRDC Southern grains update.
- DAFWA’s guide to diagnosing zinc deficiency in wheat.
- Micronutrients – GRDC Crop Nutrition Factsheet for northern, southern and western regions.
Rob Norton, IPNI, Mark Conyers, NSW DPI.
Photo provided by IPNI.