Page 77 - E-BOOK English

Gleysols

Definition

Gleysols are saturated soils as influenced by groundwater for sufficient time that reduced

conditions occur, resulting in a thick layer (≥ 25 cm) of gleyic properties within 40 cm from the
surface soil, including those soils that are formed under fluctuations of seawater. This pattern is

made up of reddish, brownish, or yellowish colours at aggregate surfaces and/or in the topsoil
layers, in combination with greyish/bluish colours inside the aggregates and/or in the subsoil

layers. Gleysols with a thionic horizon or hypersulfidic material (acid sulfate soils) are common.
Redox processes may also be caused by up-moving gases, such as CO and CH .
2 4

Distribution

Gleysols cover more than 4,500 million rais worldwide. They occur at all topography.
In the tropics, Gleysols occupy 1/3 of their all extent. Most areas of Gleysols with a thionic

horizon or hypersulfidic material are found in the coastal lowlands of Southeast Asia, especially

Indonesia, Viet Nam, and Thailand.
In Thailand, Gleysols are found about 27 million rais. The largest extent of them is in
the central plains, coastal lowlands of the southeast and southern

regions and peninsula. Minor areas of Gleysols are seldomly found

in the flood plains in the north and northeast regions of Thailand.

Formation

Gleysols are developed under the influence of saturation by
groundwater for sufficient time that reduced conditions take place.

Iron and manganese minerals are chemically reduced from the

ferric to the ferrous state, causing the soil a bluish-grey colour when
permanently saturated. An orange-brown mottling characteristic also
may be found when reducing and oxidizing conditions alternate.

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