Page 152 - E-BOOK English

Vertisols

Definition

Vertisols are heavy clay soils with higher amounts of swelling clays from the surface
soil. In most years, when these soils dry out, they form deep-wide cracks from the surface
downward, resulting in the constant interval turnover of soil material (churning). Vertisols have
a vertic horizon shallower than 100 cm from the surface soil with clay contents of >30%
from the surface to the vertic horizon. Cracks produced by alternate swelling and shrinking of
expanding clays are common at the surface soil or underneath the tilled horizon (or having crack
width ≥ 3 cm on the surface crust).

Distribution

Vertisols cover 2,093 million rais worldwide, mostly in the semi-arid tropics with an
-1
average annual rainfall of 500-1,000 mm yr . The largest Vertisol areas are in Australia, India, and
South Sudan.
In Thailand, these soils cover about 2.1 million rais, mostly distributing in the central
region and central plains border where the lower part of their riverbed is relatively
new to the floodplain, located mainly in Saraburi, Lop Buri, Phra Nakhon Si Ayutthaya,
and Phetchabun provinces. Besides, Vertisols are in some parts of the northeast region such as
Buriram, Nakhon Ratchasima, Ubon Ratchathani, and Nong Bua Lam Phu provinces.

Formation

Vertisols are developed from the weathering of base-rich parent materials. Typically,
weathering products from upland areas are washed on to the plains during the wet season. In
the dry season, crystallization of smectitic clay minerals occurs within the soil. The alternate
wetting and drying cycles result in these clays to expand and contract. This phenomenon further
causes deep cracks from the surface soil, which permits surface material to fall downward and
produces an undulating microrelief (gilgai). In the subsoil, expansion, and contraction cause
slickensides indicated by the presence of oblique shear planes in the profile.

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