The Effect of supplemental irrigation and nitrogen fertilisation on wheat (Triticum aestivum L.)

Summary

Four irrigation treatments: no irrigation; early irrigation (150 mm); late irrigation (150 mm); and early+late irrigation (275 mm), with 363 mm of rain; and four basic applications of nitrogen (0, 60, 120, 180 kg ha⁻¹), with and without an additional nitrogen top dressing of 60 kg ha⁻¹, were applied to autumn-sown wheat.

For any given total nitrogen rate, there was no difference between the single and the split application.

Grain yields ranged from 3040 kg ha⁻¹ for the unirrigated, zero-nitrogen treatment to 6340 kg ha⁻¹ for the two irrigations, 180 kg ha ⁻¹ N treatment. There was a strong interaction of irrigation and nitrogen on grain yields which was due mainly to the late irrigation: in the absence of the late irrigation the optimal nitrogen rate was 120 kg hat, followed by a marked decline in yield with additional nitrogen, whereas the application of the late irrigation shifted the optimum nitrogen rate to 180 kg ha⁻¹. In the absence of the late irrigation, increasing the nitrogen rate from 0 to 240 kg ha ⁻¹ reduced kernel weight from 42 to 32 mg, whereas late irrigation largely prevented this decrease (42 to 39 mg). The reduction in kernel weight was evident even at the first nitrogen increments, in the range where grain yield was still increasing. Lack of nitrogen reduced soil moisture extraction during the grain filling stage, particularly from soil layers deeper than 60 cm.

Stomatal aperture in the irrigated treatments was markedly larger in nitrogen-supplied than in nitrogen-deficient wheat, although the leaf hydration was similar; in the unirrigated treatment, the nitrogen-supplied plants had a lower hydration and smaller stomatal aperture than nitrogen-deficient plants.