Potential Contributions of Residual Soil Nitrogen to Subsequent Ratoon Sugarcane Crops in the Wet Subtropics

Nitrogen (N) fertiliser recommendations for ratoon sugarcane crops in the Australian subtropics do not specifically account for residual soil N. The present study was undertaken to determine whether residual soil N is sufficiently high to warrant incorporation into current N fertiliser guidelines in subtropical Australia. Nine soil cores were taken to 1 m depth (separated into 0–20, 20–40, 40–60, 60–80 and 80–100 cm layers) in 25 fields in the Australian subtropics after sugarcane harvest and assessed for soil pH, total carbon and nitrogen and mineral N (NO3− + NH4+) concentrations, and potentially mineralisable N (PMN) in the top 40 cm. Root weight in each soil layer was also measured in one core per field to determine rooting depth. When coupled with 14 d PMN in the top 40 cm, total available N ranged from 44–346 kg N ha−1, which could potentially contribute 30–100% of the typical 150 kg N ha−1 accumulated in shoots of ratoon cane crops in the region. Further field studies are required to determine the actual contributions that these N reserves can make to the N nutrition of ratoon cane crops, and the ramifications of those contributions to fertiliser recommendations.