Sugarcane yield and nitrogen uptake in relation to profiles of mineral-nitrogen in the soil

Efficient use of fertiliser nitrogen in sugarcane production requires a matching of N supply from all sources with N demand by the crop. Major sources include fertilisers, mineral N in the soil and within-season mineralisation. This paper reports one aspect of a case study that sought to document the dynamics of soil mineral N, N uptake by the crop and crop growth and yield in relation to variation in N supply from fertilisers. Two ratoons of CP51-21 were studied over the 1992/93 and 1993/94 seasons growing under irrigation on a red-yellow podzolic soil near Bundaberg. Approximately 500 kg ha-1 of mineral-nitrogen was found in the top 300 cm, and 190 kg ha-I in the top 150 cm prior to commencement of the trial. This reserve of mineral N was apparently sufficient to supply the crop's needs and additional fertiliser N had no significant effect on sugar yield in the 1992/93 season. However, two successive ratoons of cane grown without fertiliser N additions reduced mineral-N in the top 150 cm of soil to 88 (end of 1992/93 ratoon) and then to 22 kg ha-1 (end of 1993/94 ratoon), and reduced sugar yields. The implications of these results for improved profitability, and reduced off-farm impacts of fertiliser N, are discussed in terms of the questions they posed - how widespread are reserves of mineral-N under sugarcane; can sites that will not respond to fertiliser N due to soil reserves be recognised reliably; could fertiliser applications be varied to reflect mineral-N reserves in sugarcane soils?