 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com 696 DOI: 10.1002/jpln.200800066 J. Plant Nutr. Soil Sci. 2009, 172, 696–703 Pore-size distribution and particle arrangement in fragipan and nonfragipan horizons Gloria Falsone 1 * and Eleonora Bonifacio 1 1 DIVAPRA – Chimica Agraria, Università degli Studi di Torino, via L. da Vinci 44, Grugliasco 10095, Italy Abstract Fragipan is a widely distributed subsoil horizon that induces severe limitations to plant growth and land use, mainly because of its high bulk density. In this work, we evaluated the pore-size distribution through the analysis of the cumulative curve of intruded mercury volume in some soils with fragipan horizons. This approach provides information also about the arrangement of particles, thus we compared the results obtained for fragipan and nonfragipan horizons to relate porosity and particle arrangement with the specific physical properties of fragipans. The total volume of intruded mercury did not allow to discriminate between fragipan and nonfragipan hori- zons. However, from the variation of the pore volume as a function of the radius, two modal classes of pores were found, coarse and fine, respectively. The fine-pore class arose from the arrangement of clay particles, and its volume was correlated to clay contents (r = 0.787) and to clay packing density (r = –0.621). The clay fractions in fragipans were less densely packed than in the other B horizons, even if they had similar clay contents. The coarser-pore modal class is known to arise from the interactions between clay, silt, and sand particles, and its volume was different among horizons. Fragipan had a low volume of this modal pore class. In addition, a packing density for the coarser phase of 0.74, corresponding to a rhombohedral packing, was found only in fragipans. Thus, the low permeability and high bulk density of fragipans are linked to specific arrangements of the particles: an open packing of the clay phase is associated to an extremely dense packing of silt and sand. This combination is not present in any other soil horizon. Key words: mercury porosimetry / soil aggregates / packing density / Alfisol Accepted August 6, 2008 1 Introduction The pore space affects important soil qualities and soil global functionality. Conventional farming practises lead to soil- porosity degradation, soil compaction, and increase of the bulk density, with consequent hindrance of water movements and restrictions to root growth (e.g., Cannell and Hawes, 1994; Pagliai et al., 2004). However, when not cultivated, some soils show horizons with a natural low permeability and high bulk density, therefore percolating water and roots can penetrate only along interped faces and seams. These condi- tions may cause the temporary waterlogging of the overlaying layers and constitute a severe limitation to plant develop- ment. The fragipan is one of these soil horizons; it has a worldwide distribution (Witty and Knox, 1989) and may severely limit plant growth and land use mainly because of its high bulk density, ranging from 1.5 to 2.0 g cm –3 (Rhoton and Tyler , 1990; Lindbo and Veneman, 1993). The high bulk den- sity is associated to low porosity; Olson (1985) found that fra- gipans have lower pore volumes and higher amounts of resi- dual pores (<0.5 lm equivalent diameter) than adjacent hori- zons. Ajmone-Marsan et al. (1994) found that residual pores were dominant in the fragipan horizons of some soils in Italy, suggesting that the pore-size distribution may be useful in characterizing the fragipan. Even when a specific class of aggregates is considered, fragipan porosity keeps its unique characteristics. Recently we found that in the 1–2 mm aggre- gate class, the porosity of fragipans was different from that of nonfragipan horizons with similar amounts of clay, thus indi- cating that the typical physical properties of fragipans are not only linked to the presence of large and dense clods in the soil (Falsone and Bonifacio, 2006). A detailed insight into the complexity of the soil pore system can be obtained by using mercury (Hg)-intrusion porosimetry to quantify the volume of pores inside the soil aggregates (Fiès, 1992), under the assumption on full pore connectivity. When summarizing the porosity results into classes of pores according to their functionality, i.e., as transmission, storage, and residual pores (Greenland, 1977), the informations resulting from the Hg-intrusion curve are not fully exploited. An interesting procedure of data exploration was proposed by Bruand and Prost (1987); they related the pore volume to a modal equivalent pore radius, thus describing the pore-size distribution from the cumulative Hg curve. Fiès and Bruand (1998), using this procedure, obtained information on the arrangement of elementary particles in clay-silt-sand mix- tures. They found that the total pore volume could be divided into two classes arising from the interactions of sand with silt and clay, with an additional class that derived from the pack- ing of the clay alone. Lamotte et al. (1997) applied the same approach to a soil, a hard sandy loam, and found two main * Correspondence: Dr. G. Falsone; e-mail: gloria.falsone@unito.it