Characterizing equilibrium physical condition near the surface of a fine sandy loam under conservation tillage in a humid climate

The adoption of conservation tillage may cause change in soil physical properties over time, especially at the soil surface. A study was initiated in the cool, humid region of Atlantic Canada to characterize soil physical quality, under quasi-equilibrium conditions, at the surface 0 to 10 cm depth of a fine sandy loam, Orthic Podzol (Haplorthod) subjected to long-term direct-drilling (DD; 5-cm soil disturbance) with a hoe-drill and moldboard ploughing (MP; at 20-cm soil depth) for cereals and protein crops. A range of methodology, including unsaturated infiltration, preferential flow, uniaxial compression, and non-limiting water range (NLWR), was used to assess changes in soil structural form and strength. Monitoring soil bulk density (Db) and organic carbon (OC) over a 12-year period indicated that the equilibrium Db established after 8 years under DD (Db, 1.2 Mg m at 22 g C kg) and MP (Db, 1.1 Mg m at 20 g C kg) was below the level associated with poor aeration (1.3 Mg m) for this soil type. Unsaturated hydraulic conductivity was lower in the soil matrix under DD compared with MP; however, preferential water flow through soil pores (<1.5 mm equivalent diameter), as determined using bromide breakthrough curves, was not different between tillage systems. Compression curves indicated that tillage influenced soil compressibility only at relatively high water potential (−1 kPa), with the DD soil showing greater strength than the MP soil. Plant available water (ACW) parameters were similar between tillage treatments. Greater soil water contents associated with a limiting penetration resistance of 2 MPa resulted in a lower NLWR and soil structural index (NLWR:AWC) for DD (9.3 mm; 0.35), compared with MP (19.9 mm; 0.71), for the 6 to 14 cm soil depth. Overall, long-term direct-drilling had little effect on soil permeability and pore continuity but, compared with MP, increased soil strength parameters. The latter indicates that the soil would have greater bearing strength after long-term DD, but it may also present more adverse conditions for seedling root growth at low water potential compared with MP.