Porosity and soil water properties of Caribbean volcanic ash soils

Volcanic ash soils are generally recognized as soils with excellent and sta ble physical properties. Here we characterized the porosity and water prope rties of volcanic ash Andosols and Nitisols from Guadeloupe in contrasting banana systems: (1) perennial crop without mechanization, (2) mechanized an d regularly replanted crop. Desiccation from 1 kPa to 1550 kPa moisture ten sion leads to significant shrinkage in the Andosol, representing a 50% redu ction of the void space. The clayey Nitisol exhibited limited shrinkage. So il clods from the mechanized plots had a significantly smaller macroporosit y than that from perennial plots. The soil hydraulic conductivity was also drastically reduced in the compacted layers of the mechanized plots. Howeve r, Nitisols appeared to be less affected than Andosols. Laboratory compress ion tests showed that both soils were susceptible to compaction at soil moi sture close to field capacity. The shrinkage properties of the Andosol were due to microaggregation of non-crystalline components upon drying. The rel ative stability of the macroporosity in the Nitisol was probably related to the presence of stable microaggregates made of halloysite and iron oxide. Two major processes promote soil structure degradation in the Andosol under mechanized banana cropping, surface desiccation and soil compaction. They are both induced by repeated tillage after clearing.