Subangular Blocky Structure Research Articles

GENESIS OF INCEPTISOLS ON A VOLCANIC LANDSCAPE IN TAIWAN

Very little data are available regarding the genesis of Inceptisols derived from pyroclastic materials in Taiwan because most of the earlier research focused on Andisols. This study was conducted to explain the formation of Inceptisols derived from the pyroclastic material in northern Taiwan. Three pedons representing the summit, backslope, and footslope positions of a volcanic landscape were examined and sampled for physical, chemical, and mineralogical analyses. Results revealed that the soils were moderately developed and characterized by an A-BA-Bw-BC horizon sequence, loam to clay loam texture, firm and subangular blocky structure, low bulk density, high water, and P retention. The soils were very acid, with low exchangeable bases but very high exchangeable Al as well as considerable amounts of Al associated with humus. Gibbsite and quartz were found to be dominant in the sand fraction, whereas gibbsite, kaolinite, and 2:1 minerals were the major minerals in the clay fraction. Formation of the soils was relatively fast because of the easy weather-ability of the pyroclastic materials, perhumid climate, and good drainage of the volcanic landscape. Since these Inceptisols possessed several Andic soil features, we hypothesize that they probably developed from earlier formed Andisols. Therefore, it is likely that the sequence of soil formation was Entisol→Andisol→Inceptisol. The most important soil-forming processes in the formation of the Inceptisols were likely structure formation, loss of bases and acidification, braunification, bioturbation, organic matter accumulation, weathering, and clay mineral formation.

Structure and structure‐related characteristics of dryland and long‐term irrigated xerochrept soils

Typical soils from the Ebro Valley (NE Spain) under different land use regimes (flood irrigation for more than 100 years and dryland) were studied from various analytical data, including classical physicochemical and mineralogical parameters, and submi‐croscopic and porosimetric properties, in order to assess potential changes in soil quality (particularly soil structure). The submicroscopic study revealed marked differences in structural characteristics between the dry (uniform, complex granular structure) and the irrigated (weakly developed subangular blocky structure) soils, which may be explained in several ways. When dry soil is flooded, soil aggregates are stressed by swelling of the clay present and entrapped air causing immediate break‐up and slaking of aggregates. Another source of soil structure changes is the ploughing and traffic of agricultural machinery under moist conditions in the irrigated soil; these practices create mechanical stress in the top soil (Ap horizons) and lead to the disappearance of packing voids and the formation of a very dense fabric. The B horizons in the irrigated soil show clear signs of degradation typically associated with saline‐sodic soils irrigated with low‐salinity water (mainly structure collapse and reorganization of fine materials). The use of submicroscopic and porosimetric techniques proved very powerful for assessing soil structure degradation: It allowed detection of some processes that cannot be properly evaluated with conventional analyses. Based on the results, improved water management practices are required to avoid land degradation in semiarid irrigated areas similar to the ones studied.

Aggregation indices to characterize structural breakdown of dry soil samples by air slaking

AbstractThe objectives of this study were to develop a simple test to assess air slaking effects on structural breakdown of dry soils and with the help of the test, to verify whether observations made by Gath (1995) on artificial soil samples are also valid for undisturbed soil samples. The proposed slaking test and the derived aggregation indices allow for an indirect assessment of the effect of air slaking. It is possible to use the test as a means of grouping soil structure with respect to slaking susceptibility. Thus, soil samples with clay contents < 10% and granular or weakly coherent structure are practically completely dispersed upon immersion. Soils with > 30% clay are well aggregated in angular blocks and prisms, and slaking will stop at a microaggregate level. Loess soils, which usually have a range of 10 to 30% clay content and coherent to subangular blocky structure show an intermediate behaviour. Finally, the results obtained here using undisturbed soil samples are in line with those obtained by Gäth (1995), indicating that at a given water content rate of water uptake as influenced by hydraulic conductivity, sample size and bulk density is a major factor controlling effects of air slaking. However, when comparing samples of different origin, increase in clay content is the principal factor that reduces slaking effects.

Insights into Permian Pedogenesis and Climate: Magnetic Stratigraphy of Four Stacked Paleosols, Lower Permian (Wolfcampian) Roca Shale, Central Kansas: ABSTRACT

Abstract This study focuses on the rock magnetic character of four stacked paleosols (P1 at base, P4 at top) in the Lower Permian (Wolfcampian, Council Grove Group) Roca Shale exposed near Manhattan, Kansas. Magnetic characterization of Quaternary soils has proven a valuable tool for evaluating climatic conditions and pedogenic processes, yet no systematic study of pre-Quaternary soils has been undertaken. The Roca paleosols are protosols, vertisols, calcic vertisols, and calcisols. Cross-cutting relationships and superposition principles suggest that macro- and micro-climatic conditions evolved as soil formation progressed. In particular: 1) in P2, clasts of the lower (vertic) part are entrained in the upper (calcic, leached) part suggesting a shift from subhumid to semiarid; and 2) in P3 and P4, carbonate nodules have argillic coats that are at times overlain by (hematitic) ferrans, suggesting climate transitions from semiarid to subhumid back to semiarid. Rock magnetic data provide further insight into pedogenesis and climate. A preserved magnetic signature is suggested by: l) distinct magnetic susceptability, natural remanent magnetization (NRM), anhysteric remanent magnetization (ARM) and isothermal remanent magnetization (IRM) in B, Bt, Bk, E, and R horizons; and 2) carbonate nodules and adjacent subangular blocky structure which preserve the same susceptability trends. Overall, susceptability ranges from <2.0 to 11.6 x 10{-8}m{3}/kg, but within P2, P3, and P4, there is a general upwards decrease. PI contains upwards increasing susceptability. These changes correspond to a shift in S-ratios (IRM[0.3T]/SIRM) ratios from ~0.9 to ~0.4, reflecting a possible change from magnetite to hematite upwards through the paleosol profile. NRM values range from almost 1 x 10{-8}`to 5 x 10{-5} Am{2}/kg. NRM/susceptability and SIRM/susceptability ratios (proxies for ferrimagnetic vs. paramagnetic contributions to susceptability) decrease from PI to P4 and the position of the highest ratio within each paleosol profile is lowered from the B (P1) to top Bt (P2) to base Bt (P3, P4) horizons. Rock magnetic data and field observations suggest that weathering intensity increased through time from the development of P1 to P3 (P3 represents the most leached and oxidized paleosol profile), but decreased during development of P4. The oxidizing conditions prevalent in upper soil horizons led to increased lessivage in these intervals and subsequent concentration of clays and iron oxides in lower, less oxidizing horizons. Gleyed intervals are characterized by (SD?) magnetite. The results of this study show that coupled field observation and rock magnetic characteristics of pre-Quaternary paleosols potentially will yield significant climatic and pedogenic information.

A three‐dimensional field study of solute transport through unsaturated, layered, porous media: 2. Characterization of vertical dispersion

Solute plumes were created in an unsaturated field soil with either flux application or by leaching an initial resident distribution (see Ellsworth et al., this issue). The spatial variance of the plumes initially increased with time between the soil surface and a depth of 2.5 m, within which the soil was a nearly structureless loamy sand. Below this depth, the plumes were observed to compress in the vertical direction as they moved into, and through, a region of subangular blocky structure and loam texture (between 2.5 and 4.0 m depth). As the solute moved below the layer of fine texture, the plume variance again increased with time. Using a transformed advection‐dispersion equation description, two constant, field‐averaged transport coefficients, V* and Dzz*, were determined in a scaled coordinate system from the moment equations. These two constant parameters were then used to predict the observed local, or plot scale, transport. Results indicate that the two constant parameters describe transport reasonably well at each plot site and over all sampling depths.

Development of soil structure in some turbic cryosols in the Canadian low Arctic

This study was undertaken to examine the relationship between periglacial processes and the development of soil structure in some Turbic Cryosols. Three active nonsorted pattern ground features (mud hummocks) were examined for field macrostructure and micromorphological characteristics. The surface of unvegetated mud hummocks exhibited granic fabric expressed as strong granular structure. Cryoturbic movement caused surface materials to be cycled downward toward the permafrost table and upward into the hummock core. Resultant compression caused coalescence of discrete structural units resulting in matrigranodic fabric and subangular blocky structure. Porphyroskelic fabric associated with massive and occasionally prismatic structure was observed in the core of the hummocks although some remnant granularity was evident in thin section. Cycled fragments of surface vegetation 10–1000 μm in length were observed dispersed through all soil horizons. All horizons within the active layer contained more than 2% organic carbon. Key words: Turbic Cryosol, micromorphology, soil structure, cryoturbation

Morphological and hydraulic properties of a silt loam soil in New Zealand as affected by cropping history

Abstract. A rotation trial of four years’ pasture followed by two years’ arable was used to study the effect of cropping on the morphological and hydraulic properties of soil. An adjacent paddock in grass for the past 35 years was included as a permanent pasture reference. Initial infiltration and field saturated hydraulic conductivity (Kfs) were least for cultivated soil and increased with increasing time under pasture. This could be explained by the contrasting porosities of resin‐impregnated blocks of undisturbed soil which had been infiltrated with methylene blue dye. Small Kfs values for cultivated soil resulted mainly from a thin surface crust, although pore discontinuity at the depth of the cultivation pan (130 mm) could also have contributed. Greater Kfs values under short‐term pasture resulted primarily from water flowing through biogenic pores connected to the surface. The greatest Kfs values were in soil that had been under pasture for 35 years (P35). This was attributed to flow through biogenic pores and fissures associated with the strongly‐developed subangular blocky structure. The amount of water that infiltrated the two‐ and four‐year pasture soils (P2 and P4) under ponding was 2.5 and 5 times greater, respectively, than the soil that had been cultivated for two years (C2).As irrigation duration cannot be varied under the border‐dyking system used on the Canterbury Plains, the interval between irrigations must be varied if the same total amount of water is to be applied to each of these soils through the season. The interval should be less for the cultivated soil than for those under pasture, and should increase with increasing time under pasture (i.e. P35 &gt; P4 &gt; P2 &gt; C2).

Water Movement Through Pedal Soils: I. Saturated Flow

AbstractApparent dispersion coefficients were determined with a chloride tracer in 55‐cm long undisturbed columns from four pedal soil horizons of three pedons. Five medium subangular blocky structures were compared with five coarse prismatic structures, all with a silty clay loam texture. Dispersion coefficients (D), which could not always be calculated due to “nonideal” behavior of the flow system, were significantly different for the two types of structures. Application of an unlimited quantity of traced water to drained columns resulted in very high dispersion particularly for the subangular blocky structures. Some practical implications of the occurrence of hydrodynamic dispersion are discussed.

Water Movement Through Pedal Soils II. Unsaturated Flow

AbstractVery high hydrodynamic dispersion, occurring during application of an unlimited dose of chloride‐traced water to 10 drained 55‐cm long columns with undisturbed pedal soils, was strongly reduced by a daily application of 1 cm of water. Dispersion in five columns with subangular blocky structures remained significantly higher than that measured in five columns with prismatic structures. Differences are explained by a hypothetical analysis of flow patterns in both types of structure. Application of a surface crust strongly reduced dispersion in the subangular blocky structures, even though the flow rate remained approximately 1 cm/day. This was attributed to lack of flow into the larger vertical pores extending through the columns. Pronounced differences in terms of hydrodynamic dispersion were found among two types of structures from three soil series, all with a silty clay loam texture. This demonstrates the potential usefulness of routine soil structure descriptions as a correlative tool for predicting certain aspects of physical behavior.

Dark Colored, Acid, Forest Soils of Western Oregon

AbstractVery acid, strongly leached soils of the Cascade and Coast Ranges of Oregon have been called Brown Latosols, Yellowish Brown Lateritic soils, and Sols Bruns Acides. These soils are in an area of relatively high precipitation and relatively uniform temperatures under coniferous forest and dense understory vegetation. The A horizons are dark colored and have strong granular or subangular blocky structure. The B horizons are commonly dark yellowish brown and have weak or moderate subangular blocky structure. The clay content increases from A to B in some cases, but clay skins are not evident in thin sections. These soils are high in organic carbon, low in bulk density, and low in base saturation. They are Haplumbrepts according to the 7th Approximation.

Sols Bruns Acides of the Northeastern United States

AbstractSols Bruns Acides is the name now used in Belgium, France, and possibly other European countries for a class of soils at the great soil group level. Similar soils have developed under forest vegetation in the Northeastern United States, and it is proposed that the same name be applied here. In the past these soils have been included with Brown Podzolic, weak Podzol, weak Gray‐Brown Podzolic or Red‐Yellow Podzolic soil groups. Morphologically, Sols Bruns Acides have a thin A1 horizon; a paler A2 or A2‐like horizon (possibly a B1 horizon) which is poorly differentiated from the B2 horizon; a B2 horizon with uniform color from top to bottom, weak subangular blocky structure, and silicate clay accumulation not evident or appearing only in traces. Sola of these soils are strongly to very strongly acid and have low base status. Distinction between Sols Bruns Acides and the Gray‐Brown Podzolic and Red‐Yellow Podzolic soils rests mainly on amounts of clay accumulation in the B horizon. Sols Bruns Acides differ from weak Podzols and Brown Podzolic soils mainly in color and structure of the B horizon. The latter have in the upper B horizon strong colors which fade with depth and weak, fine granular structure.