Improving seed yield and N use efficiency of sunflower by deep placement of N fertilizers and straw mulch on saline clay soils of the Ganges Delta

Context

Achieving high nitrogen (N) fertilizer use efficiency remains a challenge for crop production, especially when the soil is dry during critical growth periods and under zero-tillage. Appropriate placement depth of N fertilizers may increase the N use efficiency and yield of crops by enhancing availability, uptake and utilization of N, but it is unclear whether these gains can be made in saline clay soils.

Objective

To test the hypothesis, sunflower was grown in consecutive dry seasons on a saline clay soil of the Ganges Delta to evaluate the effects of placement depth of N and mulch on growth, yield, net return and N use efficiency of the crop.

Methods

In both the years, the treatments consisted of eight placement combinations [P₀= prilled urea at surface (0 cm), P₁= prilled urea at 5 cm depth, P₂= prilled urea at 10 cm depth, P₃= prilled urea at 15 cm depth, P₄= urea super granules (USG) at 5 cm depth, P₅= USG at 10 cm depth, P₆= USG at 15 cm depth and P₇= USG at 20 cm depth] with and without rice straw mulch.

Results

Main effects of N placement method and mulch significantly (p < 0.01) influenced the yield, N use efficiency and net return but their interaction for those parameters was non-significant. Prilled urea at 10–15 cm depth was associated with highest yield components, seed yield (3.8–4.0 t ha⁻¹), oil yield and net return while the agronomic N efficiency (26 kg kg⁻¹) was higher in USG at 15 cm depth than other treatments in both the cultivation years. The rice straw mulch improved the seed yield, oil yield, net return and N use efficiency by 7, 4, 11 and 8 %, respectively, relative to un-mulched fields.

Conclusion

We conclude that placement of prilled urea at 10 cm depth and mulch on the soil surface increase apparent N recovery and N use efficiency for zero-tilled sunflower in the saline clay soils of the Ganges Delta.

Implications

Deep placement (10–15 cm) of N from prilled urea enhanced the yield and N use efficiency due to enhanced soil water content around the placed urea, and increased uptake and utilization of N. Mulch also conserved the soil water, leading to higher solute potential and lower topsoil salinity that were positively related to growth, yield and N use efficiency of sunflower, but the effects were independent of N placement depth or form.