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44MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEwater repellence. These materials break down andeither mix with or coat soil particles making themhydrophobic, or water repellent. Most often waterrepellence is associated with the soil surface becausethis is where most of the organic matter is located.Despite being associated with organic matterthere is generally no direct relationship betweentotal soil organic matter and water repellence. Thisis because organic matter content is one of severalinteracting variables that impact on the severity ofsoil water repellence (Harper et al. 2000). Ratherit is a combination of the amount, type of organicmatter present and soil texture that influences thesusceptibility of soils to water repellence.Blue lupins (Lupinus consentinii) are renownedfor their capacity to cause severe water repellence.This is most likely not only associated with plantwaxes, but also the large biomass associated withthis plant type in many environments. For example,sandplain soils will develop moderate to severewater repellence following five years of continuousblue lupin production. Heavier soil types generallyrequire higher amounts of organic matter to inducewater repellence and as such it is less prevalent onthese soil types. Sheep camps also tend to be morewater repellent because they concentrate repellentorganic matter and waxy substances that have notbeen broken down during sheep digestion. Similarly,in native vegetation a strong link exists betweenparticular species such as Eucalyptus astringens(brown mallet), E. patens (blackbutt) and Banksiaspeciosa (showy banksia) and the occurrence ofwater repellence (Blackwell 1996).Only a relatively small amount of organic matter(between 1-4 per cent of the soil mass) is requiredto inhibit water absorption and slow infiltration insands. Water repellence has a similar impact onboth deep and shallow sands in terms of limitingwater entry (Moore 2001).In general, coarse textured sands require lessparticulate organic material and develop repellencemore rapidly than finer textured clay soils, but thesemay still exhibit water repellence (see Figure 5.3).It is also evident that increasing amounts of soilorganic matter is associated with the developmentof more severe repellence for any given clay content(see Figure 5.3).These results contrast to some extent with previousstudies that found water repellence only occurred insoils with less than 10 per cent clay and was mostsevere in coarse textured sandy soils, with less thanfive per cent clay (Harper and Gilkes 1994). Figure 5.3 A complex relationship exists betweensoil organic carbon, clay content and the severityof water repellence as measured at 400 sitesacross Western Australia (Hoyle and Murphyunpublished) for MED severity (nil – black, very low– red, low – green, moderate – dark blue, severe –light blue, very severe – pink; King 1981).
45MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEPlate 5.1 A sub-surface compaction layer shows a dense impenetrable soil layer.Source: Tim Overheu, DAFWAPlate 5.2 Bulk density core.Plate 5.3 a) Water droplet sitting on thesurface of a non-wetting soil (Van Gool etal. 1999) and b) typical sub-surface drynessobserved after rain in water repellent sand.
Page 1 and 2: MANAGING SOILORGANIC MATTERA PRACTI
Page 3 and 4: FOREWORD1The one constant in agricu
Page 6 and 7: TABLES, FIGURES AND PLATES4MANAGING
Page 8 and 9: 6INTRODUCTIONMANAGING SOIL ORGANIC
Page 10 and 11: 018SOIL ORGANIC MATTERMANAGING SOIL
Page 12 and 13: Table 1.1 Size, composition, turnov
Page 14 and 15: Table 1.2 Functional role of soil o
Page 16 and 17: Figure 1.7 The influence of soil ty
Page 18 and 19: 16HOW MUCH ORGANIC CARBONREMAINS IN
Page 20 and 21: 0218BIOLOGICAL TURNOVEROF ORGANIC M
Page 22 and 23: 20MANAGING SOIL ORGANIC MATTER: A P
Page 26 and 27: 2403MANAGING SOIL ORGANIC MATTER: A
Page 29: less than 15 per cent of carbon inp
Page 36 and 37: Figure 4.1 Nitrogen cycling in soil
Page 39: mycorrhizal associations have been
Page 42 and 43: Table 5.1 Influence of soil charact
Page 44 and 45: Figure 5.2 Adjustment of organic ca
Page 48 and 49: 0646ORGANIC MATTER LOSSESFROM SOILM

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