The effect of soil compaction on the water retention characteristics of soils in forest plantations

Abstract

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 ofiowering 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 deperident 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 s.oils 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.Die rela~iewe effek van kompaksie op die waterretensie eienskappe van 'n reeks gronde in bosbouplantasies van die somerreenvalgebied in Suid Afrika is bestudeer. In aile gevalle het kompaksie die'afplatting' van die S-vormige waterretensiekromme tot gevolg gehad, hetsy die kromme op 'n massa-of volumebasis uitgedruk is. Dit impliseer 'n afname in die watervrylatings-indeks (d.i. die gradient van die lineere gedeelte van 'n grafiese voorstelling: matrikspotensiaal versus waterinhoud op 'n log-lineere skaal). 'n Duidelike verwantskap tussen beskikbare waterkapasiteit en brutodigtheid of grondtipe kon nie ge'identifiseer word nie omdat veranderinge in die waterretensiekurwes na kompaksie afhanklik is van die komplekse verwantskap tussen kompakterende prosesse, grondeienskappe en die vorm van grondporiee. Kompaksie het die beskikbare waterkapasitiet (BWK) op drie maniere beTnvloed: Toenemende kompaksie het (i) BWK verminder in meeste gronde; .(ii) BWK vergroot in spmmige sande~ige en kleierige gronde en (iii) BWK verhoog tot op 'n punt waarna dit verlaag is in sommige . gronde. Met waterinhoud uitgedruk op 'n volumebasis toon veldwaterkapasiteit oor die algemeen 'n toename met toenemende k6mpaksie, maar wanneer waterinhoud op 'n massabasis uitgedruk word, is 'n duidelike tendens nie waarneeri,baar nie. Veldwaterkapasiteit is gewoonli~ 'n arbitrer gekose syfer omdat dit kan v~rieer tussen grondtipes en verskille...

A modeling framework to quantify the effects of compaction on soil water retention and infiltration

“…Hill and Sumner (1967) showed that the effect of compaction on the water retention curve (WRC) is mainly due to changes (reductions) in the proportion of large pores, the distribution of smaller pores, and the overall reduction in total pore space and pore connectivity. Smith et al (2001) showed that compaction tends to flatten the typical S-shaped WRC (when plotted on a semi-log scale), which agrees with observations made in earlier studies (Connolly et al, 1997) for soils under long-term (50-yr) conventional tillage.…”

Advances in soil physics and soil water management research in South Africa, 1979–2003

“…Research workers at the Institute for Commercial Forestry Research (ICFR) and elsewhere have informed the debate (Boden, 1992;Roberts, 1993;Roberts, 1994;Musto, 1994;Hensley et al, 1997;Smith et al, 2001).…”

Modeling Subsurface Drainage in Compacted Cultivated Histosols

“…a, inverse of the air-entry value; n, index parameter related to the pore-size distribution. on water retention is such that there is a flattening of the water retention curve with almost no changes in the middle part of the curve (Smith et al, 2001;Lipiec and Hatano, 2003). As reported by Lipiec and Hatano (2003), for the high matric potential (h) range (from 0 to −10 kPa) compaction decreases volumetric water contents, whereas for low h values (from −250 to 1550 kPa) it increases it.…”