and discuss factors determining spatial differences in Sipaconcentrations. Results show that Sipa concentrations in top soils of paddies (assessed by acetate extraction) greatly differ between Philippine (141–322 mg Si kg−1) and Vietnamese (20–51 mg Si kg−1) regions. This can be explained by differences ingeo-/pedologic conditions between the countries. Large Sipa concentrations in the Philippines are due to recent rock formationby active volcanism, hence, by a large Sipa input due to mineral weathering in recent geologic history. Land-use can also affect Sipain top soils: in Philippine regions, Sipa concentrations were significantly larger for paddies than for other land-use types.In some of the Vietnamese paddies, Sipa concentrations are below critical values, thus might limit plant growth. Many Vietnamese farmers export Si from fields by removing straw residues with the harvest. Our subsequent research thus aims totest whether changes in harvest residue management can improve the Si supply to plants in Vietnamese regions