The turnover of nitrogen from crop residues was investigated in two sequences of iceberg lettuce crops, grown on a retentive silt soil at HRI Kirton in 1991 and 1993. The two crops of iceberg lettuce were grown on the same site.
In 1991, crop 1, planted in May, was given five rates of nitrogen from 0 to 240 kg/ha N, for crop 2, planted in August, each plot was subdivided and received one of five rates of nitrogen from 0 to 120 kg/ha.
In 1993, crop 1 received 0, 50 or 100 kg/ha N and crop 2 received 0, 25, 50, 75 or 150 kg/ha N.
At harvest of the first crop, up to 69 kg/ha N was contained in the marketable crop and 80–116 kg/ha N in the crop residue of trimmings and unmarketable heads.
In 1991, approximately 75% of the crop residue N was accounted for 26 days later at planting of crop 2. In 1993, only 26–41% of the crop residue N was thus accounted for after 21 days.
It is suggested that lower temperatures in 1993 could account for the slower breakdown of the crop residue.
Mineral N measurements to 30 cm at planting provided an estimate of N response and would allow more economical use of nitrogen fertilizer whilst maintaining crop quality.
Failure to take account of mineral N at planting could lead to the application of excess fertilizer.