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Effective strategies for micronutrients in the minimum till age

Australia’s leading authority on micronutrients has helped agronomists with ways to achieve better results from their fertiliser recommendations at Incitec Pivot Fertilisers’ recent Agronomy Community forums.

Dr Ross Brennan devoted many years of his career to micronutrient research, both starting out and in his final years before retiring in January this year from the WA Department of Primary Industries and Regional Development.

This has given him a unique perspective and an incredible bank of work to draw on to identify micronutrient deficiencies and recommend effective ways to address them.

Dr Brennan confirmed that the best way to identify micronutrient deficiencies was tissue testing, using the youngest emerged blade.

“This is the most reliable method because it tells you what the plant is getting from the soil,” he said.

“Soil testing has some value for zinc and to a lesser extent copper, in that it can tell you if you’re well and truly above critical soil concentrations, but when you get towards the responsive range, it’s less precise.

“If you rely on seeing deficiency symptoms, it’s often too late to recover from the yield setback, so a better approach is to review fertiliser history and tissue test regularly,” he said.

The Nutrient Advantage^® laboratory offers both DTPA soil testing and plant tissue testing for a range of micronutrients, including zinc and copper.

Dr Brennan said crop yield losses of up to 20% could occur before copper deficiency symptoms were evident in cereal crops.

“Applying a bit of copper or zinc with your starter fertilisers is one way, and a quite cheap way, to avoid these hidden losses,” he said.

“If you do find a severe deficiency, which is pretty rare these days, arrange a foliar application without delay.

“You can expect good responses to foliar sprays if they’re made early, with chelate forms of zinc and copper tending to be more effective.”

While widespread severe deficiencies have largely been addressed in Australia’s cropping country, Dr Brennan said there were still deficiencies occurring in individual cases and opportunities for growers to improve yields by supplying optimum levels of micronutrients.

Some of his recent research has focused on the residual value of copper and zinc applications in cropping country and their reapplication in minimum tillage situations.

This has provided some guidance on the effective placement and form of nutrients to enhance their availability to plants.

“For example, we get better responses by incorporating copper with cultivation and using smaller granule sizes of less than 1 mm diameter,” he said.

“The greater the number of cultivations mixing the copper through the soil, the better it is for plant availability,” he said.

“Smaller granules increase the number of particles in the soil, increasing the chance of root interception.”

However, with minimum tillage used extensively in modern cropping systems and growers preferring easy to handle granulated fertilisers, not powder, these options are unlikely to be widely adopted.

Dr Brennan said agronomists could still consider ways to increase the number of copper and zinc particles applied and improve the distribution of those particles throughout the entire soil.

“One option is to band between the rows on a different inter-row line each year for several years, to improve spatial availability through the soil,” he said.

“Another option might be to apply micronutrient fertilisers on diagonal rows.”

He said some growers might consider a strategic cultivation of the top 10 cm of soil every five to seven years following an application of micronutrients.

“We know that copper and zinc will last a long time and continue to be effective in crop for many, many years,” he said.

In fact, Dr Brennan presented some of his research which showed that 5.5 kg/ha of copper sulphate drilled with the seed 12 years ago was more effective in supporting current day grain yield than a fresh or current 5.5 kg/ha of copper sulphate drilled with the seed.

“One of the reasons for this is that over the 12 years the original copper has been mixed through the soil, whereas the current application was drilled with the seed, so there’s obviously a placement effect,” he explained.

Another application method showing some promise is banding copper and zinc below the seed.

He showed the results of a recent experiment comparing copper and zinc drilled with the seed, banded below the seed, or cultivated.

The concentration of zinc in young wheat leaves at boot stage increased from 15 mg/kg for nil zinc to 24 mg/kg for zinc drilled with the seed. Banding the zinc below increased zinc concentration further to 34 mg/kg. The best results were from cultivation.

“As soon as you include some sort of cultivation or mixing you improve the ability of the plant to get to the copper or zinc and take it up,” he said. “Both of those nutrients require some mixing through the soil.”

Dr Brennan advised against copper seed coatings, because they could negatively affect seed germination. Spraying liquid copper on the soil surface was also shown to be ineffective unless followed by cultivation.

“As agronomists, we just need to keep in mind the principles of what works with micronutrients,” he said.

“They’re not mobile so they need to be in the soil where the roots can intercept them and if you can increase the number of particles through the soil, that can help.”