A comparative analysis of the pore system in volcanic soils by means of water-retention measurements and image analysis

For the range of volcanic soils investigated, water retention analysis showed that near-saturation and field capacity water contents are more representative of the whole water retention behavior than the most commonly used “wilting point” water content. The Integral Retention Index,

IRI

, showed a strong relationship with the different environments; the index also showed a clear relationship with the andic properties (as estimated by Al

o

+1/2Fe

o

;

r

=0.84), the soils with the lowest Al

o

+1/2Fe

o

and also a high content in pumice departing from this linear relationship. This is especially relevant because

IRI

, has a very important physical meaning with respect to ecosystem fertility.

The fraction of the pore space occupied by macroporosity, inferred from the parameter

φ

1

, was relevant for half the soils investigated; however, it was not possible to detect any correlation with other environmental variables.

The

IRI

index, related to static hydrological properties, was approximately normally distributed. The distribution of the

φ

1

parameters, related to dynamic hydrological properties, was far from normality as it was better approximated by others distributions (i.e. Cauchy) than the log-normal.

Image analysis showed the great complexity of the pore system of these volcanic soils which generally present multimodal pore size distributions.

Comparison between the results from these two techniques suggests that hydrological pore analysis is more suitable to represent pore systems in the size range lower than 500 µm while a much more detailed description is provided by image analysis for pores larger than 500 µm. Specifically, the water retention approach is feasible in describing only simple pore size distribution while fails in describing more complex porous systems with micro ad macropores.

The overall results reported in the present chapter confirm the necessity of different approaches in order to thoroughly understand the soil pore system, especially when complex volcanic material is examined.