Measuring water retention in undisturbed samples of stony soils

Abstract

Stony soils have been increasingly used for agriculture production; however, little is known about their hydraulic properties due to problems, such as sample deformation and hydraulic continuity between samples and suction devices when the sampling and measurements are accomplished with traditional techniques. In this study, the traditional ring sampling technique was replaced by the sampling of undisturbed soil blocks coated with paraffin wax to preserve their structure. A saturated paste of fine-grained mineral particles was used to ensure contact and hydraulic continuity between samples and suction devices (sand table and ceramic plates). This allowed us to determine 30 water retention curves for three stony soils with coarse particle contents (> 2 mm) ranging from zero to 69 %. The van Genuchten model was fitted to the measured retention data and the root mean square errors were between 0.0034 and 0.0331 m 3 m -3 , with no outliers or odd behavior in the retention curves. These results showed that consistent water retention curves for stony soils can be determined with the technique proposed.Fine-grained minerals sandwiched between the surface of suctions sources and sampled blocks improve hydraulic continuity between them. These techniques can be applied to determine water retention properties in structured soil samples with coarse particles where it is unfeasible to collect structured soil samples with metal sampling rings.

Particle arrangement and internal porosity of coarse fragments affect water retention in stony soils

“…One of the most important results from this study is the distinct and complex contribution of CFs on soil porosity and water retention. The effect of CFs properties like their size and internal porosity on the shape of soil water retention curve of the same soils evaluated in this study can be seen in a previous paper (Pereira et al, 2023). Here we focus on water retention at some specific suctions of great interest to evaluate soil porosity and plant available water.…”

“…The description of methodology in this study is based on the description presented by Pereira et al (2023). Their proposed techniques for measuring water retention in undisturbed blocks of soils with CFs may be unknown to many researchers and readers.…”

“…The soils sampled in this study are those described in Pereira et al (2023). They are located in southern Brazil and contain CFs with an apparent diameter between 2 and 250 mm (Figure 1).…”

“…This is one of the factors that makes the technique not so feasible for soils with CFs. Recently, Pereira et al (2023) proposed a technique that consists of coating soil blocks with paraffin wax and using a fine‐grained mineral paste sandwiched between the surface of suction sources and sampled blocks to provide a good hydraulic continuity between them. This technique proved suitable to determine the volumetric water content in soil blocks with CFs from saturation up to 10 m of water suction.…”

“…Using the strategy developed by Pereira et al (2023), the present study was performed with two objectives: (i) to characterize the bulk water retention capacity of soils with CFs from three different parent materials, and (ii) to get insights into the effect of porous‐ and non‐porous CFs on water retention to explain multilinear regressions of soil bulk water retention capacity as a function of particle sizes class and soil bulk density.…”