ACQUISITION OF NITROGEN BY RATOON CROPS OF SUGARCANE AS INFLUENCED BY WATERLOGGING AND SPLIT APPLICATIONS

WHILE NO yield benefits have been demonstrated to split applications of nitrogen (N) fertiliser, uncertainty existed within the sugar industry as to the opportunity to improve fertiliser N uptake efficiency by splitting applications. A rates-of-N fertiliser trial, with split applications was conducted over third- and fourth-ratoon crops in the Rocky Point area where the site was waterlogged for 24 hours by irrigation in the third ratoon and by local flooding and heavy rain in the fourth ratoon. Fertiliser treatments were 0, 100 and 200 kgN/ha, with single and split applications of the applied fertiliser rates. Major waterlogging events occurred between application of basal fertiliser and the split application. The zero-N treatment on the humic gley soil accumulated approximately 80 kgN/ha from soil mineral N, and N acquisition in the fertilised treatments was ranked in accordance with applied N. There was no benefit to final N uptake or yield from the split applications of N fertiliser. Fertiliser N use efficiency peaked at 35– 40% for the third-ratoon crop and 15–20% in the wetter fourth-ratoon crop. The difference was attributed to early loss of fertiliser N in the latter crop. The generally small and non-significant response to the 200 kgN/ha rate over the 100 kgN/ha rate supports the current recommendation of 110 kgN/ha for this soil type.