For a short-term fix, a foliar spray is the easiest, cheapest, and a reliable way to apply trace elements to crops. There are also options to help build soil reserves for future crops.
For a minor deficiency and/or in a known Zn-deficient paddock, Zn seed dressings can supply Zn to a young crop. Severe deficiencies may also need a foliar spray during the growing season. Use a foliar spray at the two-leaf stage, at a rate of 250–350 g Zn/ha. Early application is critical to minimise yield loss. Be aware that by the time you can tissue test to check trace element status the window for applying Zn has almost closed. But, slightly late applications are better than not treating the crop. Use test results to develop a Zn programme for the following year. Foliar sprays won’t have a carryover benefit for future crops.
To overcome the need to spray early, or build soil Zn reserves, use Zn enriched fertilisers at seeding or Zn spray onto the soil and incorporate at seeding. Apply at a rate of about 1 kg Zn/ha, up to 2 kg Zn/ha for a severe deficiency. Soil applications provide residual benefits in future years, so can be more cost-effective in the long run.
Foliar sprays at a rate of 75 – 100 g Cu/ha will overcome immediate Cu deficiency. Severe deficiency may need a second spray just before pollen formation.
In the long-term, manage Cu deficiency by incorporating Cu-enriched fertilisers into the soil. Most soils need about 2 kg/ha of Cu to fully correct a deficiency. This application may be effective for many years. Fluid application at seeding can also correct deficiency.
Mn deficiency often stems from high soil pH. To manage severe Mn deficiency on calcareous soil use:
- Mn-enriched fertilisers banded with the seed (3 – 5 kg Mn/ha), AND
- one to two follow up foliar sprays.
Neither foliar nor soil Mn applications have a residual benefit as they are bound by the high pH soils. They must be reapplied each year.
Heavy lime applications on acid soils can also reduce Mn availability.
Mn coated seed can be effective when used with foliar spray and/or Mn-enriched fertiliser. Fluid application at seeding can also correct deficiency.
Some sands contain very little Mn. On these soils a single foliar spray of 1 kg Mn/ha and/or Mn-enriched fertilizer is often sufficient to avoid yield loss.
Tissue test to check
Tissue test before applying trace elements. Soil tests and paddock history are not always good indicators of trace element deficiencies. Do not rely on visual symptoms alone – by the time they appear up to 20% of yield has already been lost. Plus, symptoms are not definitive. Copper deficiency and frost damage, for example, look similar. Zn deficiency produces unique symptoms in wheat and barley, but yield is lost before the symptoms are obvious.
This question was asked at the ‘Ask an Expert’ Panel in Adelaide, February 2019.