Effect of rock fragments on soil porosity: a laboratory experiment with two physically degraded soils

Abstract

There are still few studies on the role that rock fragments (RFs) have in the change in soil structure based on direct observation of the soil‐pore system. Physically degraded soil is of particular interest because small RFs might be considered a factor in its remediation. In our laboratory experiment we mixed five different proportions of 4–8‐mm sized RFs with a Luvisol and a Regosol that have poor ability to self‐structure and were characterized by a massive structure in the field. Nine wetting and drying cycles were applied to repacked samples (15 cm in diameter and height) of soil–RF mixtures to facilitate the formation of soil structure. Image analysis was used to quantify development of the pore system at varying RF contents. The physically degraded soils studied in this research initially showed a decrease and then an increase in porosity with increasing amounts of RFs in the soil–RF mixtures. The results indicated that the Regosol responded more than the Luvisol to RF content. Threshold values of RF content at which the mechanism of pore formation prevails over that of pore reduction depended upon the soil type and can be attributed reasonably to small differences in the coefficient of linear extensibility (COLE). We also identified a mechanism for the propagation of porosity downwards from the soil surface with increasing RF content, together with a vertical homogenization effect on porosity. Our results contribute to understanding the mechanisms by which small rock fragments embedded in physically degraded topsoil induce changes in the pore system and confirm the potential of rock fragments to protect soil structure in soil susceptible to compaction. Highlights Physical interaction between rock fragments and fine earth in degraded soil. Soil porosity examined by combined experimental laboratory approach with image analysis. Soil porosity first decreases then increases with increasing content of rock fragments. Coexistence of two opposing mechanisms: one of porosity reduction and one of formation of new pores.

Effect of Caliche Nodules on Soil Water Storage in the Northern Loess Plateau of China

“…The increasing SWC was found to be positively related to caliche nodule content in the soil. This finding could be attributed to caliche nodules increasing soil macro‐pores and further changing the pathway for soil water movement . The water infiltrated in the deep layer via preferential pathways created by caliche nodules.…”

“…Many studies have reported that large particles can increase macro-pores, which mainly occur at the contact point between large particles and soil particles. [22,23] Macro-pores are the main channel for soil water movement. [24][25][26][27] There is a positive correlation between the content of large particles and the number of macro-pores in soil.…”

“…Large particles can change soil structure and porosity because they occupy some of the space in the soil that should be occupied by soil particles. Many studies have reported that large particles can increase macro‐pores, which mainly occur at the contact point between large particles and soil particles . Macro‐pores are the main channel for soil water movement .…”

Effect of Stoniness on the Hydraulic Properties of a Soil from an Evaporation Experiment Using the Wind and Inverse Estimation Methods

“…One hypothesis for explaining this result could be the presence of rock fragments changing the pore-size distribution by creating voids and pores of big size that Equation 3was not accounting for [16,18,40]. Different authors' findings support this hypothesis.…”

“…However, their existence can create lacunar pores at the rock fragment-fine earth interface that could become paths for preferential water flow and thus increase the saturated hydraulic conductivity [11][12][13][14][15]. Additionally, their presence reduces the total porosity and the spatial pore network architecture depending on their volume, size, shape and the type of soil studied [16]. Some rock fragments are even able to hold water depending on their origin [4,17].…”