Modification at Subgroup Level of Paleustults: A Case of Some Thai Soils.
Somchai Anusontpornperm1, Stephen Nortcliff2, and Irb Kheoruenronme1. (1) Department of Soil Science, Faculty of Agriculture, Kasetsart University, Chatuchak, Bangkok, 10900, Thailand, (2) Dep. of Soil Science, The University of Reading, Dep. of Soil Science, Univ of Reading PO Box 233, Reading, RG66DW, United Kingdom
This study on the proposed modification at subgroup level of Paleustults was conducted on nine Typic Paleustults found in the Northeastern region of Thailand. The aim was to provide some possibly modified taxonomic units within the great group so as to make them more suitable for agronomic interpretation. This is necessitated by the structure of the classification in the latest version of Soil Taxonomy (1999), where all Paleustults are put only into one subgroup, Typic Paleustults. This has given rise to considerable problems when seeking to derive agronomic interpretations based on soil classification. All soils chosen were deep soils, derived from sandstone and siltstone, with similar genetic horizon sequence, Ap-Bt. Whilst similarly classified the soils exhibited considerable differences in their properties, morphologically, physically and chemically. Based on morphological and analytical data, these soils can be experimentally classified into a number of possible subgroups. One soil located on the lower part of middle terrace periodically has water saturation in some layers within a depth of 75 cm from the mineral soil surface, particularly in the rainy season of wet year and by the presence of mottles and matrix of low chroma; therefore Aquic subgroup is suggested. Two soils can be placed into Psammentic subgroup because the upper 75 cm of their argillic horizon has sandy particle-size throughout with sand contents in these layers ranging from 833 to 900 g kg-1. A soil found in an area of higher and better distributed precipitation with an average annual rainfall of 1600 mm will have a greater amount of moisture and longer periods of moisture storage. This soil, in normal year, tends to be dry in some or all parts for fewer than 120 cumulative days per year, as a result, the Udic subgroup is better for the soil. Three soils are mainly composed of low CEC clay in their argillic horizons thus the Kanhaplic subgroup is suitable for these soils. By definition, soils in this subgroup must have the CEC (by 1N NH4OAc pH7) of less than 24 cmol(+) kg-1 clay in 50% or more of the entire argillic horizon. Another two soils are Typic Paleustults due to not having any other features that can be used for identification of different subgroups. After modification at this level, new taxonomic classes have provided more meaningfully interpretations and they quite clearly demonstrate soil characteristics almost entirely in response to the position on the landscape where the soils occur, soil parent material and climatic conditions. In the context of Typic Paleustults, crops can be grown without few limitations related to water logging in the rooting zone as many have found in the proposed Aquic subgroup. They have a slightly better nutrient and water holding capacities than those placed in Psammentic subgroup and vertical movement of water is also slower as a result of finer texture of the soils. In addition, inherited plant nutrients are potentially higher as well. Limited available soil moisture during drought periods can commonly be experienced in the Psammentic subgroup especially in dry years in contrast to the soil classified into the Udic subgroup where the moisture storage period in this soil is longer. Clay in these Typic Paleustults has higher CEC than in the soils of Kanhaplic subgroup, thus, they are positively able to retain plant nutrients to some degree. This is clearly shown by their active cation-exchange capacity class. To make it more practical, interpretation with respect to soil classification unit can also be made at lower categories, for example, soil family or phase of soil series. However, more soils in the region should be studied further to verify the modification and to eventually be added into the new version of Soil Taxonomy should it be revised.