SEEDLING EMERGENCE AS INFLUENCED BY AGGREGATE SIZE, BULK-DENSITY, AND PENETRATION RESISTANCE OF THE SEEDBED

Producers in western Canada are concerned with producing good seedbeds in conversation tillage systems. 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 field evaluations 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 0.44-12.67 mm, and four bulk densities ranging from 1.0-1.6 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). A logistic model including bulk density, geometric mean diameter and time since initiation of the experiment explained 90% of the variation observed during the course of emergence, and a logistic model that used penetration resistance and time explained 86% of this variability. 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 delayed emergence and reduced total emergence. However, the effect of bulk density was small in seedbeds with large aggregates, and the effect of aggregate size was negligible in compacted seedbeds. 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 interaggregate voids. Compaction of the seedbed, necessary 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.