Differences in Sipaconcentrations in topsoils betweenthe regions can be explained by geo-/pedologic conditions.We assume that the large Sipaconcentrations in Philippinesoils are mainly due to a large Sipainput due to weather-ing processes in recent geologic history. Potential effects ofdifferences in climate between regions are masked by thedominant effect of geo-/pedologic conditions. In some of theVietnamese paddy fields, Sipaconcentrations are below crit-ical values proposed in literature (Dobermann & Fairhurst2000). Si may therefore limit crop production on these fields.Many of the Vietnamese farmers remove part of the rice strawfrom the fields. This might cause decreasing Sipaconcentra-tions in soils. Hence, an improved straw management mightimprove the Si supply to plants. We plan to test this hypothesiswithin LEGATO.