Seedling emergence as influenced by aggregate size and bulk density

Abstract

Producers in western Canada are becoming increasingly interested in conservation tillage. A thorough understanding of how seedlings interact with the soil surrounding them is required to develop criteria for designing effective furrow openers and packing devices suitable for use in conservation tillage systems. To facilitate interpretation of the results of a field evaluation study of furrow openers for zero tillage seeders,·we conducted a greenhouse experiment designed to assess the impact of bulk density and aggregate size distribution of the seedbed on the emergence of Hard Red Spring Wheat (Triticum aestivum L). Seeds of wheat (c.v. Lancer) were germinated in seedbeds with five aggregate size distributions with geometric mean diameter ranging from 0.44 to 12.67 mm, and four bulk densities ranging from 1.0 to L6 Mg m-3 arranged in a factorial design. The soil used in this study was taken from the Ap horizon of a Swinton silt loam (Orthic Brown Chernozemic). Number of seedlings emerged and speed of emergence were affected by bulk density and aggregate size of the seedbed, and by the interaction of both variables. In general increasing bulk density or aggregate size reduced and delayed emergence, but in seedbeds with high bulk density or with large aggregates, the effect of the other variable was negligible. Increased bulk density delayed emergence mainly by decreasing the volume of voids in the soil. This elevated the interfacial stress to. the elongating coleoptile. The detrimental effect of increased aggregate size was mainly due to increase in the length of the path the coleoptile had to traverse to reach the soil surface, as it elongated through the inter-aggregate voids. Compaction of the seedbed to achieve the higher bulk densities in the coarser-aggregate seedbeds resulted in substantial breakdown of larger aggregates. Consequently, as the interfacial stress was increased by compaction, the path length was decreased, and both effects cancelled each other.