Page 59 - E-BOOK English

Ferralsols

Definition

Ferralsols are very deep, strong weathering, reddish, or yellowish with unclear boundaries

of their horizons. These soils contain low activity clay minerals, mostly kaolinite, and high amounts
of sesquioxides. A ferralic horizon within 150 cm from the surface soil, but no argic horizon is
typical for Ferralsols. If an argic horizon is found in the soil profile, however, there will be either
water-dispersible clay below 10%, geric property, or organic carbon equal to or higher than 1.4%,

which would qualify for Ferralsols.

Distribution

Ferralsols are restricted to tropical regions together of 4,688 million rais, especially in South
and Central America and Central Africa with scattered areas elsewhere. These soils are associated
with Acrisols, Nitrisols, Plinthosols, and Cambisols.

In Thailand, Ferralsols distribute in the southern part of the northeast plateau and
southeast coast, approximately 785,482 rais. These soils mostly occur in the areas where hill slope
and Karst topography take place, especially in the southern part of the northeast region, followed

by that of the east region.

Formation

Ferralsols are developed from strong weathering and leaching over a long period of time,
which causes the soils rich in iron and aluminum oxides. One of the most important processes is

oxidation and the accumulation of organic matter in the topsoils. Throughout the intense weathering
process, the loss of silicon (desilication), but the accumulation of free irons occurs. The considerable
amounts of exchangeable bases are also leached from the soil profile during the desilication process.
Long term weathering of aluminosilicates in parent material controls the equilibrium of the desilication
process. Subsequently, the accumulation of iron in the soil profile takes place in well-drained soils

in the forms of insoluble and stable iron oxides. Clay translocation may also be present during the
development of Ferralsols, indicated by clay coated on soil pores and ped faces; however,
this process is not typically dominant. It should be noted that the presence of clay coats may not

be the diagnostic characteristics of Ferralsols. The absence of clay coats may be due to a rapid
leaching process.

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