The effect of aggregate size in the seedbed on surface crusting and growth and yield of wheat ( Triticum aestivum L., cv. halberd) under dryland conditions

Abstract

The effect of aggregate size, a low level of stress (load of 2.7 kPa) and a surface crust on emergence and yield of wheat was determined, using field micro-plots of sieved aggregates with size ranges of <1, 1–2, 2–4 and >4 mm. The time to emergence was less with smaller aggregates, but in a wetter year the <1-mm aggregates caused a slight delay in emergence. Percentage emergence was greatest with the 2–4-mm aggregate-size range. Dry-matter production and grain yield both decreased with increasing aggregate-size range in the drier year, but the 1–2 mm aggregates gave maximum values in the wetter year. The presence of a crust increased the time to emergence and reduced the percentage emergence, but had no significant effect on yield. The low-level stress had no significant effect on any measured parameter. Crust strength at the completion of emergence was less on medium-sized aggregates (1–2 and 2–4 mm) than on the fine or large sizes, and these differences persisted until the end of the season. The erodibility and crust formation on aggregated seedbeds was assessed using simulated rainfall. Surface crusting was delayed, and total run-off volume was less from large aggregates (>4 mm) than from fine aggregates (<1 mm).