THIS PROMOTIONAL FEATURE WAS PROVIDED BY BALLANCE AGRI-NUTRIENTS
Micronutrient deficiencies can impact wheat yield and grain nutritional quality, but does applying micronutrients always improve these factors?
It is not quite as straightforward as that, according to results from a comprehensive trial in Canterbury, New Zealand’s main wheat growing area1.
The trial demonstrated that if soil pH is maintained in the optimum range of pH 5.8–6.2, wheat is unlikely to show a yield response to the micronutrients boron (B), copper (Cu), manganese (Mn) or zinc (Zn). The trials were carried out over two growing seasons on commercial farms, across 20 sites in Year 1 and three in the Year 2.
In Year 1, crops received either no micronutrients (control), Cu (soil or foliar applied), Mn (foliar applied), and Zn or B (soil applied). There was no significant yield response to any of these treatments in the first year, despite about half the trial sites being below the suggested critical soil thresholds for Cu and Zn. Furthermore, soil test results for Cu and Zn did not correlate with the plants’ micronutrient status.
After Mn foliar application, two sites showed a significant yield reduction. Tissue Mn at ear emergence in treated plants ranged from 21–164 mg/kg, and was in some cases above
Hill Laboratories’ normal range of 25–100 mg/kg. On untreated plants, tissue Mn was always within the normal range.
After Cu foliar application, overall yield reduced by more than 0.5 t/ha at four sites, and visual symptoms of damage were seen in some plots, indicating crop damage. However, in treated plants, tissue Cu levels at ear emergence ranged from 5.6–22.6 mg/kg, which is within the normal range of 5–25 mg/kg.
In Year 2, responses to soil applied Zn and foliar applied Cu and Mn were assessed (as grain yield and micronutrient levels in tissues at mid-late tillering and ear emergence) under three lime application rates (0, 5, 10 t /ha). Two sites showed averaged yield responses across treatments of 0.4 t/ha (at Rakaia) and 0.9 t/ha (at Barhill) to Mn, but not to Zn or Cu.
At the Barhill site, applying lime without Mn caused a significant yield reduction (averaged 8 per cent), and this effect w as overcome when Mn was included with lime treatments.
It appears Mn deficiency was yield limiting and the effect was exacerbated by liming.
Learnings and recommendations
According to the trials, the only micronutrient likely to limit yield in Canterbury is Mn, particularly in known localised areas of deficiency such as Barrhill, and also if overliming raises soil pH above optimum. If soil pH gets too high the risk, particularly in cereals, is that plants will not get enough micronutrients. Application of lime, particularly at above optimal pH, can worsen micronutrient deficiencies such as Mn, so overliming should be avoided, particularly on soils prone to low Mn levels.
To identify a micronutrient deficiency, crop tissue tests are recommended, as in the trial soil tests for micronutrients did not correlate with the plants’ micronutrient status. Crop
tissue levels correlate well with relative yield.
Micronutrient application is not an effective way of improving the grain’s nutritional quality; in the trials, applying micronutrients increased concentrations in vegetative tissue but not in the grain (with the exception of Zn in Year 2 sites).
Although direct yield responses to micronutrients are unlikely, there may still be value in applying these micronutrients to prevent their depletion in intensively cropped soils using a product such as YaraMila Actyva S. This ideal base/starter fertiliser for spring (or autumn) sown cereals contains Mn, Zn and B as well as nitrogen, phosphorus, potassium and sulphur in a convenient granule to help promote an even establishment. It is suitable for broadcasting or drilling.
The best approach is to plan cropping well in advance and apply lime a year before the most pH-sensitive crop in the rotation. Lime needs at least 12 months to take full effect, and 1 t/ha will shift pH by 0.1. If pH is less than ideal, take this into account in yield estimates.
1 Curtin D, Martin RJ, Scott CL 2008, Wheat (Triticum aestivum) response to micronutrients (Mn, Cu, Zn, B) in Canterbury, New Zealand, New Zealand Journal of Crop and Horticultural Science 36: 169–81