Assessing nitrogen application rates for subsurface trickle irrigated cane at Bundaberg

As trickle irrigation can be a more efficient means of applying crop nutrients, so nutrient application rates may be able to be reduced in trickle irrigation systems. To test this hypothesis, crop and sugar yields were measured in a trial comparing five nitrogen rates, and results are presented after three crop harvests. In all crops, the application of nitrogen significantly increased crop and sugar yield with the overall trend favouring the 90 (plant)l120 (ratoon) kglha nitrogen treatment. This was a lower application of nitrogen than would be conventionally used at this site. There was more than 200 kglha of mineral nitrogen in the soil to 2 m depth immediately prior to the site being planted. This nitrogen was reduced to 11 kglha at the start of the second ratoon crop. Least depletion occurred in the highest nitrogen treatment (180/240 kglha) with substantial nitrogen remaining deep (below 1.2 m) in the soil profile. This deep nitrogen was depleted in treatments receiving less nitrogen. A basic nitrogen balance suggested that at least 24 kglha of nitrogen was lost to the environment from the highest nitrogen treatment. Apart from reduced nitrogen applications, trickle irrigation allows for in-crop adjustments to nitrogen management to overcome problems such as loss of nitrogen in very wet periods. This is an economically and environmentally advantageous means of managing nitrogen for such problems compared with continual over-fertilisation.