RELATIVE MEASURES OF SOIL BULK DENSITY TO CHARACTERIZE COMPACTION IN TILLAGE STUDIES ON FINE SANDY LOAMS

Abstract

Field studies concerned with soil physical properties require methods to quantify and characterize soil compaction, especially for tillage experiments. The bulk densities of a Charlottetown and Gowanbrae fine sandy loam were related to a maximum or standard compaction state for each soil to obtain a measure of relative compaction. The two soils were both classified as Orthic Humo-Ferric Podzols. Relative compaction was closely related to the volume of macropores (r² = 0.85). Over a 3-yr period, mouldboard ploughing loosened the soil to give an average relative compaction of 77%. Subsequent soil consolidation and settling increased relative compaction to 84% over the growing season. Direct-drilling maintained relative compaction at a limited range of 88–91%. Relative grain yield of cereals was related (r² = 0.69) using a polynomial curve to relative compaction. A range of 77–84% relative compaction was associated with a relative grain yield ≥ 95%. A relative compaction of 84–89% was considered the equilibrium soil density level for the two soils under study. This range was related to a macropore volume of 13.5–10% which is adequate for permeability but possibly inadequate for optimum soil aeration under a humid soil moisture regime. Overall, relative compaction provided a useful index or standard to assess changes in soil bulk density and proved to be a biologically meaningful soil physical parameter. Key words: Soil compaction indices, relative compaction, Podzolic soil, fine sandy loam, cereal yield