Studies on depth of placement of urea on nitrogen recovery in wheat grown on a red-brown earth in Australia

Field and greenhouse experiments were carried out at the University of Sydneyto examine the influence of depth of placement of urea on crop nitrogen (N) uptake and N recovery in wheat grown on a red-brown earth in Australia. In the greenhouse, an ¹⁵N source of urea was used in examining the pattern of N availability, while field experiments using an unlabelled urea assessed the usefulness of deep placement of urea as a tool for improving N use by wheat.Placement at a depth of 15 cm slightly delayed the accessibility of N to the plant only in the early stages of growth, i.e., about 12 days after sowing. Large differences in N content and N concentration observed as a result of placement was only transient and disappeared later in the season. Total N recovery was 93.8% in the deep placement and 79.9% in the shallow placemen, but these differences were due to differences in soil N recovery, as crop N recovery was approximately 48% in both treatments.In the field, there was very little advantage in the deep placement compared with the shallow placement. Also, no residual benefit was observed as a result of increased depth of placement. Thus deep placement may not be an important strategy for increasing N uptake over a conventional shallow depth of 3–5 cm.