CLAY DISPERSABILITY IN MOIST EARTHWORM CASTS OF DIFFERENT SOILS

Earthworms were fed soil from two polders, differing in soil age and l and use (grass and arable). Sterilised and non-sterilised moist earthw orm casts were, directly or after ageing (for 2, 4, 8 and 20 weeks), a nalysed for clay dispersability and polysaccharide content, either as such, or after treatment with water or K-periodate/K-borate. Results f or casts were compared with those for field aggregates of comparable s ize that were treated similarly. Fresh earthworm casts were very unsta ble. Soil age and land use influenced the stability of fresh casts: th ose from old polder in grassland were most stable, those from young po lders under arable land the least. Casts and field aggregates from ara ble soil were much more susceptible to periodate treatment than those from grassland. Casts and field aggregates from the grassland in the o ld polder were least affected by periodate. When measured after water or periodate treatment, casts became more stable within the first 4 we eks of ageing. As this occurred both in sterile and non-sterile casts, microbial activity was not necessary for the stabilisation. Polysacch aride 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 parti cles, leading to closer contact between particles and/or binding subst ances probably explains stabilisation of continuously moist earthworm casts.