Description and working steps
The method is a physical fractionation that separates the aggregate size classes by wet sieving. Therefore, the forces involved in soil disruption are slaking and dispersion. The first one causes the break-up of large aggregates into smaller ones, the second one the release of primary particles from aggregates.
An aliquot of 5 g is used to separate aggregates by wet sieving on two sieves (250 and 53 µm) piled up and immersed in water (2 L). After 10 min of slaking time and 20 min of “up and down” movement of the sieves three fractions are obtained: Macroaggregates, microaggregates and silt and clay. The Macro and microaggregates are directly recovered from the sieves. The silt and clay fraction is recovered after flocculation with CaCl2 (147 mg). All the fractions are dried at 60°C.
The method is used to study the secondary structure of soils by separating three aggregate size classes (macroaggregates > 250 µm; microaggregates 250-53 µm; silt and clay <53 µm) and to evaluate where the organic matter is mainly accumulated and stabilized through physical protection mechanisms.
Since the forces applied to disrupt the soil are weak, the risk of redistribution and mixing of organic matter among the different aggregate size classes is very low.
On the other hand, for the same reason, the largest aggregate size class can hold high amounts of the smaller fractions.
Elliott, E., 1986. Aggregate structure and carbon, nitrogen, and phosphorus in native and cultivated soils. Soil Science Society of America Journal 50, 627-633.
Gioacchini P., F. Cattaneo, L. Barbanti, D. Montecchio, C. Ciavatta, C. Marzadori. 2016. Carbon sequestration and distribution in soil aggregate fractions under Miscanthus and giant reed in the Mediterranean area. Soil and Tillage Research 163, 235-242.
Gioacchini P., D. Montecchio, E. Gnudi, V. Terzi, A. M. Stanca, C. Ciavatta, C. Marzadori. 2016. Fate of N in soil amended with 15N-labelled residues of winter cereals combined with an organic N fertilizer. Soil Research 54, 182-190.