Pore size distribution and stability of ortstein and overlying horizons in podzolic soils under forest

Abstract

Podzolic soils with ortsteins are used for forestry worldwide. Pore size distribution (PSD) and strength of cemented ortstein horizons affect soil transport properties and plant growth but they are scarcely understood. The aim of this study was to quantify the PSD and tensile strength of ortstein and overlying horizons in two podzolic soils under forest. The PSD in the range of pore size from 0.0016 to 178 μm in radius was determined in undisturbed aggregates (6–8 mm in diameter) using a mercury porosimeter. The results were presented in the form of both a cumulative pore volume curve and a logarithmic differential pore curve as a function of the pore radius. The tensile strength (kPa) of the soil horizons was assessed by crushing an aggregate and measuring the breaking force. The results showed that the total mercury intrusion volume, median pore radius (volume and area), and average pore radius were lower and that the total pore area was higher in the ortstein than in the overlying horizons in all soils. The lower volume of larger pores (58–32μm) in the ortstein horizons of the forest soils was accompanied by a greater volume of smaller pores. The differential PSD curves were unimodal in all horizons and soils. The representative peaks in the ortsteins, compared to those in respective overlying horizons, were of lower magnitude (0.31 to 0.41cm3g−1 vs. 0.49 to 0.60cm3g−1). Irrespective of the soil, the peaks in the ortstein, compared to overlying horizons, occurred at lower equivalent pore radii. The tensile strength of the ortstein horizons (112.5–123.9kPa) was many times greater than in the overlying horizons (7.1–14.0kPa). Both the greater contribution of smaller pores and the tensile strength in the ortsteins than in the overlying horizons were ascribed to an increased concentration of carbon, iron, aluminum, and silica. The study provides new insights into understanding the structural and strength discontinuity in soils with ortsteins.