Dispersion and flocculation of Vertisols, Alfisols and Oxisols in Southern Brazil

- We tested dispersion techniques for subtropical soils granulometry analysis.
- Soils tested presented varying soil texture, clay mineralogy and organic matter.
- Clay dispersion was affected mainly by horizontal reciprocating shaking times.
- The standard method can be replaced by horizontal shaking for 240 min.
- The new technique will speed-up laboratory routines to obtain reliable results.

Incomplete soil dispersion in particle-size analysis reduces clay content and increases silt and sand contents, whereas abrasion affects size distribution by large particles' breakdown into smaller size-fractions. We evaluated different dispersion techniques on particle-size distribution in ten soil horizons of seven soils from southern Brazil, with varying soil texture, clay mineralogy and organic matter. We tested two contact times of chemical dispersant (sodium hydroxide 1 M): immediate shaking and shaking after 12 h of contact; three types of spheres: glass, technyl and nylon; five horizontal reciprocating shaking times (30, 60, 120, 240 and 480 min); and standard method (electric mixer) with chemical dispersant (sodium hydroxide 1 M), 12 h of contact time, plus 15 min of physical stirring at 12,000 rpm. Soils with high contents of clay and iron oxides or exchangeable aluminum were less dispersible, respectively, because of organo-mineral complexes and aluminum affinity to organic matter and clay minerals. Soils with kaolinite and quartz and with low organic matter content in the B horizon were easily dispersible. Nylon spheres are adequate for physical dispersion, either immediately or 12 h after addition of chemical dispersant. Horizontal shaking for 60 or 120 min is sufficient for adequate dispersion of kaolinitic soils, while for soils of difficult dispersion, such as oxidic soils, 240 min of horizontal shaking is necessary. For water dispersible clay, horizontal shaking during 240 min is suggested. These results allow for standardizing the contact time of chemical dispersant, horizontal shaking timing, and sphere types, for soils with varying texture, clay mineralogy and organic matter, in laboratories working in tropical environments, with improved dispersion and time saving.