Use it or lose it – Nitrogen (N) fertiliser considerations for cotton crops in the ORIA
Jack Daniel
In the Ord, a high yielding cotton crop (12+ bales/ha) will require at least 180 N (kg/ha) to be recovered from the soil. This amount usually exceeds the N that can be supplied by the soil alone. Therefore, external N fertiliser inputs are required.
External N inputs will mitigate the limitation of N, which in turn will maximise yield and profitability. However, this is only applicable if N is used correctly, in a way that gets it into the crop. There are several challenges for N recovery in the ORIA.
The current wet season planting system (Jan-Mar) poses challenges for this on a number of fronts.
- First, paddocks are not conducive to mechanical soil disturbance for much of the wet season. Whether fertiliser is broadcast or drilled, soil disturbance will be required to incorporate N into the soil profile.
- Second, the clay soils are prone to waterlogging, which will exacerbate denitrification and lead to losses of N.
Faced with these challenges, NACRA undertook 3 years of replicated field trials, both on commercial farms and the DPIRD research station.
Several distinct fertiliser application methods were investigated. Most treatments were applied at a rate of 220 N (kg/ha), unless otherwise stated. Urea was either; drilled in advance to the wet season in November/December (‘Up-front’) or applied prior to the first flower in March/April (‘in-crop’). Also, a combination of both of these was implemented as a 50% application split (‘50:50’). Other enhanced efficiency fertilisers (EEF’s)were also investigated. This included polymer coated urea (PCU), nitrification inhibitor coated urea (NI) and urease inhibitor coated urea (UI). Rates ranging from 36 N (kg/ha) right up to 400 N (kg/ha) were also studied, although most treatments used a rate of 220 N (kg/ha), so to compare application method alone.
It was found that having a large portion of N applied ‘in crop’ would maximise yields. These treatments often exceeded 12 bales/ha and recovered >180 N kg/ha. Also, the ‘50:50’ split nearly equalled the best treatments, providing a good option. The ‘up-front’ fertiliser options severely underperformed the ‘in-crop’ options. In 2023 there was a yield penalty of 4 bales/ha. Therefore urea ‘up-front’ is not recommended, as the wet season will severely lessen its efficacy.
Good performance of ‘in-crop’ urea applied for both drilled and broadcast options, although more datapoints were collected for the drilled options during this study. Previous research on these soils found nitrate release more rapid with broadcast options (Wetselaar et al., 1972). This indicates timing and proper incorporation is more critical when broadcasting urea. High N rates of 400 N (kg/ha) were also investigated but only served to enrich the stover, rather than maximise yield. Low rates of 128 N (kg/ha) were also found to underperform the 220 N (kg/ha) rate by 1.1-1.8 bales/ha, indicating little opportunity apply a rate much below 220 N (kg/ha).
No enhanced fertiliser products (PCU, NI and UI) were found to provide a productivity benefit above urea. This suggests that timing of N application will outweigh any benefit from enhanced fertiliser products, when productivity is the only metric. However, several other studies have quantified a nitrous oxide emission reduction from nitrification inhibitors (Schwenke & McPherson, 2018). Therefore, it may be worth further consideration in the ORIA, especially as the price difference between enhanced fertiliser products and urea continue to reduce or if a market becomes available (or an emissions reduction policy mandated).
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