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Alkaline clayey soil is considered to be a degraded soil, which causes serious issues in engineering and construction projects. The formation and development of desiccation cracks in alkaline clayey soils is a complex process affecting the strength, stability, and permeability of these soils. Therefore, to investigate how soda content can inhibit the development of desiccation cracks in alkaline clayey soils in Ulanhot, Inner Mongolia, China, free desiccation tests were performed to examine the cracks in alkaline clayey soil samples with different soda (Na2CO3) contents in the laboratory. During the cracking process, the changes in water content and the occurrence and evolution of cracks were recorded with a camera. Digital image processing was performed to analyze the structure characteristics of the surface cracks. The results indicated that the characteristics are influenced by the different soda contents. The soda inhibited the development of desiccation cracks in alkaline clayey soils. Three stages were found in the process of desiccation cracking in accordance with the rate of water loss: the residual, normal, and structural shrinkage stages. At <0.4 mol L-1 Na2CO3, the "three straight lines" model could be used to describe the volume shrinkage. However, at >0.4 mol L-1 Na2CO3, the structural shrinkage stage disappeared. Moreover, based on the general soil volume change equation and fractal theory, a model was formulated to quantitatively explain the relationship between the fractal dimension and the volume changes in the soil. The experimental results and equations are important for reducing subgrade deformation and slumps.

There is a need for improved knowledge of the limits to the available water range for root growth in the subsoil. The objective of this study was to recalculate the upper and lower limits of the least limiting water range (LLWR) concept by using respectively the air-filled porosity (εa) at which 0.005 of the relative gas diffusivity (Ds/Do) is reached and readily available water (RAW). The refined upper limit estimates the variation in εa related to pore connectivity and the refined lower limit expresses the boundary at which plants suffer physiological water stress. This study was based on soil sampled in compaction trials on two sandy loam soils. Soil samples were taken from plots with no compaction (Control), and compaction with 78 kN (M8) and 58 kN (M6) wheel loads with multiple wheel passes. The soil cores were analyzed for εa, Ds/Do, bulk density (rb) and penetration resistance (PR). Heavy farm machinery impact of M8 and M6 led to subsoil compaction up to depth of 0.5 to 0.7 m for the soils under study. The subsoil structure was affected by compaction across depths with the decrease in εa (~33-46%) and Ds/Do (~37-61%) and increase in rb (~4-8%) and PR (~40-50%, at -100 hPa at 30-cm depth). The refined LLWR showed a wider water range compared to the original approach. We anticipate that the refined LLWR well reflects the limiting soil physical conditions for root growth for the studied soils, but validation by combined soil physical and plant growth measurements is needed.

Little is understood about chemical weathering processes in Alaskan arctic soils, where moisture is generally not limited but acidity varies and the average soil temperature is close to or below freezing. Weathering reactions in soil convert primary minerals into secondary clay minerals. Silty loam textured soils from three sites in moist acidic tundra (MAT) and three sites in moist nonacidic tundra (MNT) in the northern Arctic Foothills, Alaska, were characterized with emphasis on the origin of the clay minerals. The MNT soils had a discontinuous and thinner organic layer, which leads to a deeper summer thaw and greater cryoturbation than the MAT soils. The MNT had higher cation exchange capacity and base saturation than MAT. These buffer against acidification and account for the pH differences of MAT and MNT. Other chemical characteristics including C and N content as well as Fe and Al were similar (by horizon) across the MAT/MNT boundary. X-ray diffraction of coarse (0.0002-0.002 ram) and fine clay (<0.0002 mm) fractions indicate that illite, vermiculite, and kaolinite are the predominant clay minerals. Presumably, kaolinite is detrital and vermiculite is weathered from illite. The proportion of vermiculite to illite is higher in MAT and the illite to vermiculite proportion is higher in MNT. This shows that soil acidity does affect weathering processes despite the low soil temperature. [ABSTRACT FROM AUTHOR]

The objective of this work was to construct and validate a model that shows how the clay shrinkage curve, under the influence of a silt-sand admixture as well as an inter- and intraaggregate structure, is transformed to the soil shrinkage curve. To meet this objective, we investigated (i) the reference shrinkage curve, that is, one without cracks, because cracks lead to a multivalued shrinkage curve; (ii) the rigid superficial (interface) layer of aggregates, with changed pore-size range and distribution, compared with the intraaggregate matrix; and (iii) soils with sufficiently high clay content when large pores inside the intraaggregate clay (so-called lacunar pores) are nonexistent. The methodology is based on detail accounting for contributions of the interface aggregate layer and intraaggregate matrix to the soil volume and water content during shrinkage. The reference shrinkage curve is determined by six physical soil parameters: oven-dried specific volume; maximum swelling water content; mean solid density; soil clay content; oven-dried structural porosity; and the ratio of aggregate solid mass to solid mass of the intraaggregate matrix. Only the last parameter was fitted for lack of data and compared with an unfitted estimate. The model was validated using data for eight soils. The important new conclusion is that the mere existence of the rigid superficial aggregate layer leads to a soil reference shrinkage curve that is convex upward in the structural shrinkage area, unlike the shrinkage curve of the clay contributing to the soil. The model can have numerous applications. [ABSTRACT FROM AUTHOR]

The fate of pig (Sus scrofa) slurry labeled with 15N was investigated when applied on a clay soil (fine, mixed, frigid, Typic Humaquept) and a sandy loam (loamy, mixed, frigid, Typic Dystrochrept) cropped to maize (Zea mays L.) in 2000 and to barley (Hordeum vulgare L.) in 2001. The slurry was applied in spring 2000, and plant and soil samples were collected at 6 h (Day 1) and at Days 14, 42, 96, and 413 after application. The samples were analyzed for 15N content in plant, in whole soil, and in soil NH4+-N, NO3--N, organic N, and clayfixed N pools. Percentage 15N recovery was >93% in both soils at Day 1 and decreased slowly thereafter. Rapid clay fixation of slurry 15 occurred at Day 1, and was greater in the clay soil (34% of applied 15N) than in the sandy loam (11%). At Day 96, less of the applied slurry 15N was recovered in maize grown on the clay soil (29%), as compared with the sandy loam (50%). At the same period, the residual soil 15N was mostly present as organic N and NO3-N in the sandy loam, and as organic and clay-fixed N in the clay soil. At Day 413, 15N recovery in barley was about 3% of the initially applied N in both soils. The 15N recovery was generally higher in the clay soil than in the sandy loam, but total 15N recovery from the soil-plant system was similar for both soils. We conclude that soil type had little influence on the total 15 recovery from the soil-plant system, but significantly influenced the fate of slurry N in the various soil pools and plant N uptake on the year of application. [ABSTRACT FROM AUTHOR]