Rheological stabilization of wet soils by model root and fungal exudates depends on clay mineralogy

P. Barre, P. D. Hallett*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

56 Citations (Scopus)


Changes in soil rheological behaviour by a root mucilage analogue (polygalacturonic acid, PGA) and a fungal polysaccharide (scleroglucan) were studied for a range of soils with different clay mineralogies. An oscillation recovery test was conducted with a parallel plate rheometer with stress steps of 10 Pa (linear viscoelastic), 500 Pa (resistance) and then 10 Pa (resilience). Five soils that were either dominated by 2:1 or 1:1 clay minerals were examined: a pure Ca montmorillonite; a 2:1 Cambisol; a 2:1 Luvisol; a 1:1 clay loam Ferralsol; and a 1:1 sandy loam Ferralsol. The viscosity of all 2:1-dominated samples increased with decreasing water content, ranging from 151 Pa s to 237 800 Pa s. The model biological exudates increased viscosity by up to 10-fold in some 2:1 clay-dominated soils at the wettest water contents, with the relative effect diminishing as soils dried. The viscosity of 1:1-dominated soils also increased with decreasing water content, ranging from 17 320 Pa s to 1 333 000 Pa s. In these soils, scleroglucan increased viscosity 22-fold for the 1:1 clay loam soil, but PGA had little impact. For the 1:1 sandy loam soil, neither exudate influenced viscosity. When a greater stress of 500 Pa was applied, viscosity decreased in all samples because of shear thinning. Scleroglucan maintained greater viscosity under this greater stress for the montmorillonite, 2:1 Luvisol and 1:1 clay loam, whereas PGA did not have a significant impact. After the stress was removed, only the scleroglucan-amended 1:1 clay loam recovered to viscosities greater than the other treatments. The increased viscosity and resistance caused by biological exudates will help to ‘drive’ soil structure formation, particularly in the rhizosphere.

Original languageEnglish
Pages (from-to)525-538
Number of pages14
JournalEuropean Journal of Soil Science
Issue number4
Early online date24 Jun 2009
Publication statusPublished - Aug 2009


  • acid
  • plant
  • rhizosphere
  • organic-matter
  • flocculation
  • humic substances
  • polysaccharide
  • strain
  • sodium-montmorillonite
  • mucilage


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