Defining appropriate nitrogen rates in low yielding locations with inherent waterlogging issues in the central cane growing region

Abstract

THERE IS an increasing requirement for more astute land resource management through efficiencies in agricultural inputs in a sugar cane production system. Precision agriculture (PA) provides a pathway and a framework for enhanced economic and environmental sustainability in the sugar growing sector with variable rate application (VRA) of nutrients regarded as an integral component of PA. Trials have been conducted in the Mackay region to determine the potential to reduce nitrogen (N) rates in defined zones with low yield potentials due to poor sub-surface drainage characteristics with inherent waterlogging issues. Results from these trials will assist in developing intra-paddock variable rate nitrogen guidelines for defined low yielding waterlogged areas. The two-year trial was established on a Victoria Plains series soil (black clay) with nitrogen application rates of 90, 160 and 230 kg N/ha respectively. The trial incorporated two water regimes, namely: a predominantly rain-fed system; and a waterlogged system maintained in an anaerobic condition through flood irrigation for the traditional three month wet season (January to March) Rainfall events over the traditional ‘wet season’ period resulted in both anaerobic and rain-fed zones being maintained in a relatively waterlogged state in consecutive trial years. Results from the February 2012 leaf analysis program showed increased leaf N concentrations with increasing N rates in the anaerobic zones. However, leaf N concentrations were below critical levels across N treatments and water regimes. In contrast, there was no statistical difference in leaf N concentrations in the rain-fed plots. In the 2012 and 2013 trial periods, results from April leaf analysis programs showed leaf N concentrations were well below critical levels across treatments and water regimes. In the 2012 trial period where sub-optimal moisture conditions post application of nitrogen treatments impeded the uptake of applied nitrogen, the 160 N treatment had a 27% higher yield than the 90 N treatment in the anaerobic zones. However, in the 2013 trial, two irrigations post application of N and prior to imposed waterlogged conditions resulted in no significant yield differences across N treatments in the anaerobic zones. The dry season irrigations in the imposed waterlogged plots resulted in a yield increases of 69% in the 90 N treatments, 34% in the 160 N and 23% in the 230 N treatments compared to the previous year’s yield data. In the rain-fed zones there were no statistical differences in yield between N treatments in consecutive trial years although rainfall over the traditional ‘wet season’ period had maintained the rain fed plots in a virtual anaerobic state for the three-month ‘wet season’ period (January to March). These results indicate that there is the potential to reduce N inputs in an intra- paddock VRA program in defined zones with low yield potential caused by inherent waterlogging issues. It is recognised that under extreme and sustained anaerobic conditions yield penalties may occur with reduction in N rates at or below 90 kg N/ha. However the 2013 trial results showed that yield can be maintained with low N inputs when strategic irrigations are implemented prior to the onset of waterlogged conditions.