The legacy of compost and slurry amendments to soil on physical resilience to compaction

Organic amendment of soils with composts or slurries affects compaction resistance and resilience, but the longevity of these impacts has not been explored, and few data are available from controlled field studies.  Here we carried out a rapid soil compaction resilience test, where coarsely sieved soil to simulate a freshly prepared seedbed are exposed to controlled compaction and cycles of wetting and drying in the laboratory. Agricultural soils with long history of compost and slurry amendments were sampled from the Lower Pilmore field of the James Hutton Institute, Dundee, UK.  Replicated plots received three levels of compost (35, 100 and 200 Mg ha-1), three levels of slurry (10, 20 and 40 Mg ha-1) and control (no amendment) from 2005 to 2009. Subsequently, normal rates of 35t ha-1 and 10 t ha-1 were applied until 2014. Loose soils sampled in 2023 were sieved to 4mm and then compressed cyclically under uniaxial stresses of (i) 50kPa to simulate a roller, (ii) 200kPa to simulate a tractor, and (iii) again at 200kPa. Between each of these stress cycles the soils were saturated (wetting) for 24 hours and drained (drying) to field capacity (5kPa) for 12 hours on a sandbox. Changes in void ratio during the loading and unloading phases were obtained directly from sample displacement, while the void ratio after wetting and drying was calculated from soil mass-volume relationship. Void ratio increased with increase in organic carbon in both compost and slurry soils. Soil wetting-drying following the first and second 200kPa compression cycles caused significant recovery of void ratio for both compost and slurry. Final void ratio (measured after the wet-dry cycle that followed the second 200kPa compression) was 0.40 m3 m-3 in the control, versus 0.45 m3 m-3 in 35 Mg ha-1 compost and 0.49 m3 m-3 in both 100 and 200 Mg ha-1 compost. For slurry soils, final void ratio was 0.40, 0.41 and 0.45 m3 m-3 for 10, 20 and 40 Mg ha-1, respectively. Organic carbon accounted for a significant percentage (R2 = 0.48; p = 0.00) of variability in the final void ratio for compost soils whilst there was no significant relationship between void ratio and organic C in slurry soils. Compression and recompression indices increased more with increase in compost than with increase in slurry, but overall, they displayed no significant (p≤0.05) relationships with organic carbon. Soils treated with compost are therefore, better able to absorb compressive stresses than their slurry counterparts and could significantly recover their form and capacity to perform their ecological functions following stress withdrawal. Moreover, legacy applications of compost than slurry can affect compaction resilience for several years.