DOES PROLONGED GREEN CANE TRASH RETENTION INFLUENCE NITROGEN REQUIREMENTS OF THE SUGARCANE CROP IN THE WET TROPICS?

SINCE THE INTRODUCTION of green cane trash blanketing (GCTB), there has been ongoing interest in its potential to contribute to the nitrogen (N) requirements of the sugarcane crop. An experiment was established at Tully to determine if N fertiliser inputs could be reduced following the implementation of a long-term GCTB system. The experiment also assessed if the amount of cultivation within the GCTB system during a fallow impacted on N requirements. The site had been set up in 1990 to investigate the long-term effects of GCTB. Three farming systems (FS) treatments were included: (1) long-term burnt cane, conventional cultivation of plant and ratoons; (2) long-term GCTB, conventional cultivation of plant crops, zero cultivation of ratoons; (3) long-term GCTB, zonal tillage of plant crops, zero cultivation of ratoons. Four N treatments were incorporated into the three FS treatments: 0, 50, 100, and 150 kg N/ha in the plant crop and 0, 80, 160 and 240 kg N/ha in ratoon crops. Soil carbon and N levels increased following 15 years of GCTB. However, the response to fertiliser N was similar for cane grown in long-term burnt or GCTB systems. This indicates N rates should not be reduced following long-term adoption of GCTB at this stage. Different degrees of cultivation of the GCTB fallow did not influence N requirements. Crop yields and level of response varied between years probably due to variation in rainfall. The variable response was not considered to be associated with N fertiliser loss as leaf N values were generally above the accepted critical value of 1.8% N. Although several possible explanations are possible, excess rainfall and the resulting waterlogged soils may have limited normal growth. Evaluating systems to improve drainage and aeration around the plant’s roots is a priority to offset this kind of effect.