Fate of urea applied to winter spinach in New Zealand

Field trials were conducted during three years (1996–1998) to follow the fate of N fertiliser applied to spinach (Spinacea oleracea L.) grown over winter (May–September) in New Zealand. Urea labeled with ¹⁵N was applied to the spinach at different rates (0, 200 or 400 kg N/ha), times (as two or three split applications) and placement methods (broadcast or banded). The results showed that the typical fertiliser practice of applying 350 kg N/ha near planting time followed by 50 kg N/ha 4 weeks later, resulted in leaching losses of 119–292 kg N/ha depending on the rainfall. Plants recovered only a small proportion of the fertiliser N (3–18% of the ¹⁵N). Better utilization of the fertiliser and lower leaching losses occurred when the fertiliser was applied strategically in split applications with only a small amount (10%) applied near sowing and the majority applied to match crop demand. For example, in 1998 applying 200 kg N/ha (20 kg N/ha initially+100 kg N/ha after 4 weeks and 80 kg N/ha after 8 weeks) resulted in twice the spinach yield (25 t/ha compared with 13 t/ha) and less than half the leaching loss (108 kg N/ha leached compared with 246 kg N/ha) compared with the typical fertiliser practice.