Optimal timing of Nitrogen fertiliser applications can maximise crop uptake and reduce losses to the atmosphere. Soil Nitrogen (N) dynamics in Victoria show some big differences between regions. High rates of N losses recorded from soils in the High Rainfall Zone (HRZ) emphasise how important timing of N applications can be in some scenarios. That study site had recently come out of long term pasture had very high background organic carbon levels, a different scenario to a typical continuous cropping paddock.
Applying all N fertiliser at sowing leads to much higher losses of N to the atmosphere compared to split applications. Losses of N from fertiliser applied at sowing can be up to 85% in the HRZ when background mineral and organic N levels are high. Applying N fertiliser later in the season can reduce those losses by around half.
In the Wimmera, a lower rainfall area, N losses from fertiliser applied at sowing are around half of those seen in the HRZ, in the range or 20-40%. These losses were also halved by splitting N applications with topdressed urea.
Nitrification inhibitors can significantly reduce the emission of greenhouse gases and overall losses of N from the soil in the HRZ. In lower rainfall areas nitrification inhibitors do not make a significant difference.
The best guide to how much N fertiliser to apply is a soil test to measure mineralised N in the soil profile at sowing. In years where mineralisation of N has happened naturally over the fallow there may be less need to apply nitrogen. In years where wet conditions have led to denitrifying conditions in the soil pre planting the N requirement may be higher than expected.
Researchers on this project were Roger Armstrong, Rob Harris & Ash Wallace. Their paper ‘Recovery of 15N urea fertiliser applied to wheat under different management strategies, in the High Rainfall Zone of south western Victoria‘ was presented at the 2015 Australian Agronomy Conference.
Jeff Kraak, Fertilizer Australia, Charlie Walker, Incitec Pivot.
Photos used in video courtesy of Ash Wallace and Rob Harris.