Extended Abstract

Introduction

Soil structural stability is a fundamental component of soil physical quality because it regulates pore continuity, water infiltration, aeration, resistance to erosion, and the capacity of soil to maintain favorable conditions for plant growth. Soil aggregate stability is commonly evaluated using aggregate-size distribution and indices such as mean weight diameter (MWD) and geometric mean diameter (GMD). These indices are strongly influenced by soil texture, moisture conditions, organic and mineral binding agents, and the method used for aggregate-size determination. In addition to aggregate stability, the plasticity index is an important indicator of the consistency and mechanical behavior of soils and can be affected by changes in soil texture, moisture status, and the addition of mineral amendments. Pumice is a porous, lightweight volcanic material that has attracted attention as a potential soil amendment because of its high porosity, relatively low bulk density, and capacity to modify soil pore characteristics and water-related properties. However, the response of soil structural properties to pumice may depend on the initial soil texture and water regime. Furthermore, the effects of pumice on aggregate-size distribution under dry and wet sieving conditions have received comparatively limited attention. Therefore, this study was conducted to evaluate the effects of different pumice application rates and soil moisture regimes on the plasticity index and aggregate-size characteristics, including MWD and GMD, in soils with contrasting textures.

Materials and Methods

The experiment was conducted as a split-split plot experiment based on a randomized complete block design with 12 treatment combinations and four replications. Two contrasting soil textures, sandy loam and clay loam, were used as the main factor. The soils contained 14 and 38% clay, 28 and 42% silt, and 58 and 20% sand, respectively. Pumice was applied at three rates of 0, 4, and 8 g kg⁻¹ soil. Each experimental pot contained 7 kg of dry soil. Two soil moisture regimes were imposed based on depletion of available water (DAW), including 25% DAW, corresponding to irrigation when 75% of available water remained, and 50% DAW, corresponding to irrigation when 50% of available water remained. Soil moisture was monitored gravimetrically and irrigation was applied to restore the soil to the predetermined moisture condition. The plasticity index was determined using the Casagrande method. Aggregate-size distribution was evaluated using dry and wet sieving procedures. Mean weight diameter (MWD) and geometric mean diameter (GMD) were calculated from the distribution of soil aggregates in the different size classes. Analysis of variance was performed according to the experimental design, and significant treatment means were compared using the least significant difference (LSD) test at the 5% probability level. The interaction effects among soil texture, pumice rate, and moisture regime were evaluated to determine whether the response of soil consistency and aggregate stability depended on the initial soil condition and water regime.

Results and Discussion

The results demonstrated that soil texture and moisture regime significantly affected the plasticity index. The interaction between moisture regime and soil texture showed that the highest plasticity index (7.49%) occurred in the clay loam soil under 50% DAW, whereas the lowest value (6.60%) was recorded in the sandy loam soil under 25% DAW. Increasing water depletion from 25 to 50% reduced the plasticity index by 5.7% in the clay loam and by 2.1% in the sandy loam soil. The stronger response of the clay loam soil can be related to its greater clay content and the stronger influence of fine particles on soil consistency. Pumice application, in contrast, increased the plasticity index in both soils. The highest values were observed at 8 g kg⁻¹ pumice, reaching 7.86% in the clay loam and 6.70% in the sandy loam soil. These results indicate that the effect of pumice on soil consistency was not independent of soil texture and that the magnitude of its effect was greater in the finer-textured soil. Pumice application also improved aggregate-size characteristics, although the magnitude of the response depended on soil texture, moisture regime, and the sieving method. Under dry sieving, the interaction between irrigation regime and pumice application was significant. The highest MWD (3.23 mm) was obtained with 50% DAW and 8 g kg⁻¹ pumice, whereas the lowest value (2.83 mm) occurred with 25% DAW and no pumice. Relative to the control, application of 4 and 8 g kg⁻¹ pumice increased MWD by 5.6 and 6.7%, respectively, under 25% DAW and by 3.8 and 4.1%, respectively, under 50% DAW. The interaction between pumice and soil texture was also important. In the clay loam soil, MWD increased from 3.46 mm without pumice to 3.86 and 3.90 mm with 4 and 8 g kg⁻¹ pumice, respectively. In the sandy loam soil, the corresponding values were 2.34, 2.36, and 2.47 mm. A similar texture-dependent response was observed for GMD under dry sieving. In the clay loam soil, GMD increased from 1.54 mm in the control to 1.65 mm with both 4 and 8 g kg⁻¹ pumice. In the sandy loam soil, GMD increased from 1.09 mm to 1.16 and 1.27 mm with 4 and 8 g kg⁻¹ pumice, respectively. Thus, the relative response to pumice was particularly pronounced in the sandy loam soil for GMD, although the absolute values remained higher in the clay loam soil. Under wet sieving, the effects of soil moisture regime were also significant. Increasing DAW from 25 to 50% increased MWD by 13.5% in the clay loam and by 12.8% in the sandy loam soil. The highest wet-sieved MWD (2.68 mm) occurred in the clay loam soil under 50% DAW, while the lowest value (1.32 mm) was recorded in the sandy loam soil under 25% DAW. Wet-sieved GMD showed a similar pattern, with values of 1.21 and 0.81 mm under 50 and 25% DAW, respectively, in the clay loam soil, and 0.89 and 0.81 mm, respectively, in the sandy loam soil. Pumice also increased wet-sieved GMD, particularly in the clay loam soil, where the value increased from 1.14 mm without pumice to 1.17 and 1.24 mm with 4 and 8 g kg⁻¹ pumice, respectively. The contrasting responses obtained from...