Recovery of nitrogen by wheat from various depths in a cracking clay soil

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Abstract
The ability of wheat to recover nitrogen from topsoil and subsoil was studied in two experiments on the Darling Downs, Queensland. Nitrate was placed at the required depth by injecting solution at regular intervals over a small plot (<1 m2) on soil almost depleted of mineral nitrogen to a depth of 120 cm. In experiment 1 a lower grain yield with shallow (7.5 cm) compared with deep (30, 60, or 90 cm) placement was partly attributed to a temporary nitrogen deficiency induced by a 26-day drought around heading. The interaction between moisture regime and nitrogen placement depth was further studied in experiment 2. The absence of any useful falls of rain during the first 78 days of crop growth induced a temporary deficiency of nitrogen with topsoil placement (7.5 cm). Deficiency was rapidly relieved by rain (74 mm) between boot and flowering and the difference in nitrogen uptake between topsoil and subsoil placements had disappeared at flowering. Two irrigations each of 36 mm were sufficient to avert early nitrogen deficiency, almost doubling (57 kg ha-1 to 108 kg ha-1) the uptake of nitrogen at boot. In contrast, drought during early crop growth caused a negligible reduction in the uptake of subsoil (45 cm or 90 cm) placed nitrogen. Soil mineral-nitrogen or fertilizer nitrogen that is leached into the subsoil should provide a more satisfactory reserve of available nitrogen than fertilizer applied to topsoil at planting, for winter cereal crops in Queensland.