Assessing the compaction susceptibility of South African forestry soils.
The widespread use of heavy machinery during harvesting and extraction operations in South African timber plantations has led to concern that soil compaction is causing long term site productivity declines and environmental damage. This study was conducted with the intention of establishing a framework for the routine prediction of compaction susceptibility of South African forestry soils. Principal facets of compaction behaviour were established for a wide range of soils and these were related to changes in soil physical conditions resulting from compaction. Soils were chosen from a broad range of geological and climatic regions and varied greatly in texture (8 to 66% clay) and organic matter content (0.26 and 5.77% organic carbon). A quantitative description of compaction behaviour was obtained using a simple uniaxial compression technique. Bulk density was related to applied pressure, water content and initial bulk density as independent variables. Statistical analysis of the coefficients in the model enabled the relative importance of applied pressure and water content during the compaction process to be evaluated and related to commonly measured soil physical properties. Compressibility was strongly correlated with clay plus silt content and to a lesser extent with clay content and organic carbon determined by loss-on-ignition (LOI). Though significant correlations were obtained between maximum bulk density (MBD) and clay plus silt content, MBD was more strongly correlated with organic carbon (LOI). A classification system for compaction risk assessment is presented, based on the relationship between compactibility (MBD) and organic carbon (LOI), and between clay plus silt and compressibility. The effect of soil compaction on soil physical quality was assessed by examining changes in penetrometer soil strength (PSS) and water retentivity curves of compacted soils. Clay content strongly influenced the relationship between PSS, bulk density and water content. The PSS at wilting point (-1500 kPa) increased with increasing clay content whereas PSS at a matric potential of -10 kPa and was most strongly related to organic carbon (LOI) and increased with increasing organic carbon content. Compaction generally resulted in an increase in field capacity and wilting point on a volumetric basis and a flattening of the water retentivity curves. However, no simple effects of compaction on available water capacity were observed. Changes in PSS, aeration porosity and water retention following compaction allowed the definition of a single parameter, the non-limiting water range (NLWR), to describe more precisely the changes taking place in the air-soil-water matrix following compaction. "Compaction envelopes" were constructed to illustrate these complex inter-relationships and to relate changes in NLWR to compactive effort and relative bulk density. The use of NLWR is recommended as a sensitive parameter for assessing compaction risk of forestry soils.