Overconsolidation in agricultural soils .2. Pedotransfer functions for estimating preconsolidation stress

A series of soil survey interpretive procedures, or pedotransfer functions (PTFs), are presented that follow from the hypothesis corroborated in the companion paper that the saturated compressive behavior of structured and of corresponding remolded agricultural soils in southern Ontario were strongly related. The three PTFs were useful in characterizing the degree of overconsolidation as a physical indicator of soil quality on a regional basis. PTF1 identified soils as being highly overconsolidated when the void ratio difference between the normal compression Line (NCL) at unit stress and e(0) exceeded 0.36. The preconsolidation stress (sigma(c)') of a soil was estimated from the dry bulk density measured in situ and from other soil properties needed to estimate the NCL of remolded soils (PTF2) or the virgin compression line of structured soils (PTF3). The PTFs were tested on a data set comprised of soil horizons characterized in five county-level soil inventories that meet the minimum PTF data requirements and other defined criteria (n = 210). An PTFs showed that the degree of soil overconsolidation increased significantly (P < 0.0001) with depth and with increasing clay content. For PTF2; the mean estimated (T: for the Ap horizons (n = 47) was only 20 kPa due to the effect of repeated tillage and natural turbational processes. The untilled solum (B) horizons were found to be in a less consolidated condition than the C horizon subsoils (mean sigma(c)' = 97 and 138 kFa, respectively). This suggested that effective stresses from wheel traffic and tillage operations have generally been no greater than those originating from natural causes in this region, Many of the clay-rich subsoils were quite overconsolidated (mean sigma(c)' > 150 kPa), whereas the coarser textured soils appeared to be closer to a normally consolidated condition, PTF3 corroborated most of these findings.