Potatoes have high nitrogen (N) and phosphorus (P) demand. High N inputs increase the risk of leaching and denitrification losses exacerbated by heavy rain events and poor irrigation scheduling. Aside from correct N fertiliser placement and timing, growers can retain soil N by holding it in the ammonium form. This can be done by treating ammonium and urea-based fertilisers with a nitrification inhibitor (NI). A potential benefit of ‘ammonium nutrition’ is improved P use efficiency on highly buffered red soils where potatoes receive high P rates.

NIs such as Entec® and eNpower® have been commercially available in Australia for over a decade and have gained widespread acceptance in crops like sugar in the wet tropics.

To better understand how NIs can be used by potato growers, Incitec Pivot Fertilisers commissioned independent field studies on highly productive red soils near Atherton (Queensland) and Devonport (Tasmania).

NIs work by suppressing bacteria that convert ammonium-N into nitrate-N. While plants use both forms of N, nitrate can be lost by leaching and denitrification. Ammonium-N attaches to clay and organic matter and is resistant to such losses. Ammonium is a more energy efficient N source for plants as nitrate-N must be converted back to ammonium-N in the plant for amino acid production.

This paper builds on a study conducted at Atherton in 2011 that showed statistically significant yield increases of >4 t/ha to Entec treated fertilisers in Sebago potatoes grown on a red soil. The latest study was conducted by The Tasmanian Institute of Agriculture at its Forthside Vegetable Research Facility in north-west Tasmania. The trial was set out with a randomised complete block designed with three replicates, and Russet Burbank potatoes were planted on 1 November 2019. The soil is classified as a red Ferrosol with pH (CaCl2) 5.8, organic carbon 4.7%, Bray P 16.7 mg/kg and PBI averaging 1,100. The trial was split into an N rate trial +/- Entec / Enpower and a P rate trial +/- Enpower.

Nitrogen trial

N treatments included basal applications of 0, 81 and 135 kgN/ha combined with side dress applications of 0, 73 (3 applications) and 195 (4 applications) kgN/ ha. The P trial had a basal N application of 310 kg/ha based on district practice and P rates of 0, 80, 159 & 265 kg/ha.

Inhibitor treatments with the higher basal N rate (135 kg/ha) showed yield increases of 3.3-6 t/ha, whereas lower yields were reported with the lower basal N rate (81 kg/ha).

This suggests that the benefits from the inhibitor application are only realised above a minimum nitrogen input, whether they are applied at pre-plant or as side dressings. When basal application and side-dressing treatments were ignored for each inhibitor, the slopes of each regression estimated similar gross yield increases of 0.09 t/ha per unit of nitrogen where no inhibitor was applied; 0.08 t/ ha where Entec was applied; and 0.1 t/ ha where Enpower was applied (F1,19, p = <0.0001).

The phosphorus trial

The P trial had a basal N application of 310 kg/ha based on district practice and P rates of 0, 80, 159 & 265 kg/ha.

Responses in the P trial were somewhat confusing, with plant available P being higher at the two lower P rates (0 and 80 kg/ha), equal at 159 kgP/ha and lower at 265 where Enpower was applied.

Curiously, the gross yield data suggests no penalty from reducing P rate from 265 kgP/ha for conventional P fertiliser to 159 kgP/ha where Enpower was used. It is suggested that further work is needed with a greater range of P rates to better understand this relationship.

The bottom line

Where high basal rates of N and P are used to grow potatoes on red soils in high rainfall and/or irrigated environments, there is evidence to support the use of NIs such as Entec and Enpower. The use of such technology may facilitate a reduction in side dress number as well as N rate, labour and cost, with minimal production downside. The impact on P response is less clear; however, it is apparent that NIs improve available soil P on red soils at lower P rates – implying improved P efficiency.

Find out more

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