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Nitrogen balancing act

THE wet change in July highlighted the need for farmers to focus on their nitrogen applications as cereal and canola crops across the Central West show signs of deficiency.
Nanjing Night Net

IMAG Consulting agronomist Glenn Shepherd, Dubbo, said crop condition depended on the quantity of nitrogen applied upfront.

He said any crops that received the majority of their nitrogen requirements upfront or were top-dressed early on were okay.

However, wet soils exposed any deficiencies as the anaerobic environment (which occurs in waterlogged soils) caused denitrification, leading to reduced levels of plant-available nitrogen, and was apparent particularly in the heavier grey and black clay soils due to their poorer drainage.

A crop’s nitrogen uptake was further compounded by the reduced respiration of the plant’s roots in the waterlogged soils, he said.

“The last significant fall (of rain) was about a fortnight ago, but in many areas the profile was already full so we don’t need much moisture to make things wet,” he said.

Mr Shepherd said the lower lying areas were usually the first hit, with cereals displaying yellowing of the lower leaves as the plant “cannibalises” them to translocate nutrients to the upper leaves which are more important for the plant’s own reproduction (ie grain fill).

In canola the lower leaves were also affected, but they turned purple, and in severe cases a purplish red.

However, Mr Shepherd said care should be taken to not confuse these symptoms with sulphur deficiency which presented similarly.

Many growers in Mr Shepherd’s region had begun addressing nitrogen requirements, but not everybody had been able to access their paddocks.

“Most of the canola has had nitrogen already,” he said.

Mr Shepherd said it was important to supply nitrogen (if deficient) before the plant was running up to flower, otherwise the opportunity to increase yield potential was lost.

Given the high moisture availability this season, Mr Shepherd said it was an excellent opportunity to increase yield with low risk of a moisture shortage come crop finishing.

“This year we’ve got a full profile of moisture heading into spring so we’ve got a good opportunity to maximise yield if we give it enough nitrogen to reach that potential,” he said.

At this point in the season urea (applied before rain) was still an option, as was UAN either applied before rain, or alternatively as a foliar spray.

However, he said targeting root uptake as opposed to foliar uptake would allow greater quantities of nitrogen to be applied safely.

The results also depended on the stage of the crop.

He said with wheat, if tillering was occurring there was still opportunity to increase yield potential, but if the crop had finished tillering then yield potential was set and it was about getting the most from within that potential.

“Late applications can still be effective providing the potential yield is there.”

The movement in the markets also meant the cost-benefit was improved from six weeks ago, as the price for wheat (and canola) had lifted.