Characterization of the porous structure of Chilean volcanic soils by nitrogen adsorption and mercury porosimetry

Pore volume, specific surface area (SSA), and total intragranular porosity (TIP) of Chilean soils derived from volcanic materials were studied. Soil samples involving the 0-15 and 15-30 cm depth of virgin and cultivated Collipulli (Ultisol) and Diguillin (Andisol) soils at two particle size fractions (<1 mm and <2 mum) were considered. From mercury porosimetry and N-2 adsorption, mainly mesopores (pore diameter, dpore, about 10 nm) were determined for <1 mm Collipulli samples. Diguillin <1 mm soil shows macroporosity with dpore from 70 nm to 7000 nm. The clay fraction of Collipulli has macropores (dpore from 2000 nm to 40000 nm) and mesopores (dpore from 3 nm to 23 nm), while for Diguillin clay-size fraction most of the porosity comes from macropores (dpore from 50 nm to 800 nm). For all samples the SSA linearly correlates with the mesopore volume (r(2)=0.781; n=16) determined by N-2 adsorption, and with the mesopore + macropore volume (r(2)=0.771; n=12) when Collipulli <1mm samples are excluded; an inverse relationship between SSA and organic carbon content was found (r(2)=0.854; n=14). Thus, the SSA defined mainly by mesopores and macropores is probably related to the soil organic matter content. Mesopores and macropores mainly give the TIP, which increases as particle size decreases. No important changes in micropore and macropore volume, and in TIP were seen as result of cultivation. Mesopore volume is more important in samples dominated by kaolinite than in samples dominated by allophane (4 to 20 times). In general the soil pore distribution, its SSA and TIP are related to its mineralogy and organic matter content.