The effect of soil compaction on the water retention characteristics of soils in forest plantations
Аннотация
A study was carried out to evaluate the relative effects of soil compaction on the water retention characteristics of a range of soils in forest plantations in the summer rainfall regions of South Africa. In all cases compaction resulted in the ‘flattening’ of the S-shaped water retentivity curve expressed on either a mass or volumetric basis. This had the implicit effect of lowering the water release index (a log-linear plot of matric potential against water content). A clear relationship between available water capacity (AWC) and bulk density and soil type was not discernible since changes in water retentivity curves following compaction are dependent upon the complex relationship between compressive processes, soil properties and pore geometry. AWC responded to compaction in three ways: (i) AWC was reduced with increasing compaction (most soils); (ii) increasing compaction resulted in increasing AWC (some sandy and clayey soils) and (iii) increasing compaction resulted in increasing AWC up to a point after which it declined. In general, field capacity (FC) increased with increasing compaction when water content was expressed on a volumetric basis but no clear trends were apparent on a mass basis. Effects of compaction on AWC depend upon the designated matric potential for FC which is often arbitrarily defined since it may vary for different soils and crops. Changes in pore geometry are better reflected by the expression of the water content on a mass basis without consideration of volume effects. From a practical point of view, however, changes in available water are better expressed on a volumetric basis.