Balance and distribution of sulphur in volcanic ash-derived soils in Chile

Sulphur distribution with an insight in S-organic fractions was studied in volcanic-ash derived soils of Chile (Typic Dystrandepts, Palehumult) by isolating each fraction and measuring S directly by instrumental elemental analysis. S-pattern followed closely C and N distribution in these soils. Light organic-S forms accounted for more than 40% of total S in soil, indicating that this S-form is the actual vehicle through which mineralization to S-SO4 occurs. The role of this type of S-molecules, especially of those with lower molecular weight, needs to be clarified regarding its direct availability for plant roots. In younger Andisol soils, the main fraction of organic S was linked to high molecular weight humins (20–34%). In Ultisol soil, apparently a final degradation step occurs where organic soluble-S has almost disappeared and S is nearly exclusively represented by S in humic acid (HA), fulvic acid (FA) and humin (H).