Argillic horizons

Coarse-sized particles dominate the particle size distribution of arid soils. Some soils are also quite gravelly. The subsurface horizons commonly exhibit accumulation zones of carbonates, gypsum and more soluble salts. Many arid zone soils are shallow and gravelly, some are alkaline. Their structure is weak. From most soils, clay accumulation horizons (argillic horizons) are absent, or are only weakly developed, and so are minerals that indicate an advanced degree of weathering. 

For an argillic horizon to form, the (coagulated) clay must disperse in the horizon of eluviation before it is transported to the depth of accumulation by percolating water. 

Weathering in older soils produces decreasing permeabilities due to in situ secondary mineral formation and the development of hard pans and argillic horizons. Such zones of secondary clay and iron-oxides are clearly evident in the increased aluminum and iron at a depth of a meter in the Panola regolith (Figure 3). Low permeabilities in such features are commonly related to the absence of continuous pores due to the formation of thick cutans of clay (O Brien and Buol, 1984). This process is enhanced by physical translocation and collapse of saprolite structures (Torrent and Nettleton, 1978). 

Argillic horizons often correlate with the maximum depth of effective evapotranspiration, commonly 1-2 m. Water loss initiates the precipitation of secondary clays and oxides from solutes. Such precipitation may also be related to the loss of dissolved organic species and the de-complexation of soluble aluminum. The resulting low permeabilities further retard the downward percolation of pore water, commonly creating transient perched water tables directly above the hardpans. Periodic drying in this zone focuses addition secondary mineral precipitation in the vicinity of the hardpan, which then leads to a lower permeability and more clay formation. 

The progressive development of such argillic features with increasing weathering intensity is commonly documented in soil chronosequence studies (Harden, 1990). An example of the development of argillic horizons is shown for the Merced chronosequenence in Figure 18 (White et al., 2003). Over a time span of... 

Fig. 4.23 Schematic diagram explaining the formation of argillic horizons in soils. The resulting horizons are comparable to those in the alfisol (luvisol) shown in Plate 4.2(a). The master horizons are typically centimetres to tens of centimetres thick.

Soils in southeastern Lee County are classified as Ultisols. They are moist soils that have argillic horizons and base saturations of less than 35 percent. Ultisols develop in areas that have long frost-free seasons, abundant rainfall, and adequate ground-water supplies. 

Ultisols. Mineral soils that have an argillic horizon with a base saturation of <35% when measured at pH 8.2. Ultisols are strongly leached, acid forest soils with relatively low native fertility. They are found primarily in humid temperate and tropical areas of the world, typically on older, stable landscapes. These soils are characterized by an accumulation of clays in the subsurface horizon and exhibit strong yellowish or reddish colors resulting from the presence of iron oxides. Ultisols are divided into five suborders Aquults, Humults, Udults, Ustults, and Xerults. 

Lessivage. Lessivage involves the vertical transport of particles and colloids of clay minerals. The downward horizons where the accumulation of the transported clays occurs are called argillic (denoted B,) horizons. Usually, due to the swelling of clay minerals with water, the plugging of argillic horizons ensures an impermeabilization of lower horizons. 

Argillic Formed by illuviation of clay generally a B horizon, where the accumulation of clay is denoted by a lower case t and illuviation argillans are usually observable unless there is evidence of stress cutans. Requirements to meet an argillic horizon are 1/10 as thick as all overlying horizons >1.2 times more clay than horizon above, or if eluvial layer <15% clay, then >3% more clay, or if eluvial layer >40% clay, then >8% more clay. 

Natric It is a subsurface horizons with accumulation of clay minerals and sodium. Requirements Same as argillic horizon Prismatic or columnar structure >15% of the CEC is saturated with Na+, or More exchangeable Na plus Mg than Ca. ... 

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