Clay dispersability in moist earthworm casts of different soils

Earthworms were fed soil from two polders, differing in soil age and land use (grass and arable). Sterilised and non-sterilised moist earthworm casts were, directly or after ageing (for 2, 4, 8 and 20 weeks), analysed for clay dispersability and polysaccharide content, either as such, or after treatment with water or K-periodate/K-borate. Results for casts were compared with those for field aggregates of comparable size that were treated similarly.Fresh earthworm casts were very unstable. Soil age and land use influenced the stability of fresh casts: those from old polder in grassland were most stable, those from young polders under arable land the least. Casts and field aggregates from arable soil were much more susceptible to periodate treatment than those from grassland. Casts and field aggregates from the grassland in the old polder were least affected by periodate.When measured after water or periodate treatment, casts became more stable within the first 4 weeks of ageing. As this occurred both in sterile and non-sterile casts, microbial activity was not necessary for the stabilisation. Polysaccharide content within treatments did not vary over time, so changes in stability of moist earthworm casts were not related to changes in the amount of polysaccharide. We conclude that reorientation of clay particles, leading to closer contact between particles and/or binding substances probably explains stabilisation of continuously moist earthworm casts.