NITROGEN NEEDS OF SUGARCANE CROPS: LESSONS FROM TESTING THE N REPLACEMENT CONCEPT

DECIDING how much N fertiliser to apply to a sugarcane crop traditionally requires estimates of the likely crop yield and the amount of N needed to achieve that yield. In this paper we examine these questions by analysing data from onfarm experiments testing the N Replacement system, which aims to have N applications aligned with actual cane production. In 11 experiments in one-year crops, yields in the N Replacement treatment were similar to those achieved with the farmers’ conventional N management which had average N applications 18 to 157 kg/ha greater than in the N Replacement treatments. In four sites with two-year crops in NSW, N rates with N Replacement were similar to or greater than the farmer’s current N applications. N concentrations and N uptake in cane in most experiments were considerably lower than those previously reported. This unexpectedly low N requirement explains why yields were maintained with the lower N applications in the N Replacement system. The lower N concentrations also meant that the crop N surplus was greater than expected. The results also show that it is not critical to accurately predict the yield of the coming crop in developing an N management strategy, provided N applications and production are matched in the longer term. We suggest that focussing on the minimum surplus needed to maintain productivity and soil N reserves may usefully guide future thinking on sustainable N fertiliser management.