Revision of the Classification of the Soils of Trinidad ...

Revision of the Classification of the Soils of

Trinidad and Tobago

Based on Keys to Soil Taxonomy, 12th ed. 2014

By Craig Ditzler, Ph.D. June, 2017

Table of Contents

Purpose ------------------------------------------------------------------------------------------------------------------ 1 Changes to Soil Taxonomy ------------------------------------------------------------------------------------------- 1 Procedure ---------------------------------------------------------------------------------------------------------------- 2 Revised Classification ------------------------------------------------------------------------------------------------- 3

Trinidad (Table 1) -------------------------------------------------------------------------------------------- 3 Tobago (Table 2) --------------------------------------------------------------------------------------------- 7 Alternative Classification --------------------------------------------------------------------------------------------- 9 Trinidad (Table 3) ------------------------------------------------------------------------------------------- 9 Tobago (Table 4) ------------------------------------------------------------------------------------------- 10 Future Considerations ------------------------------------------------------------------------------------------------ 10 References ------------------------------------------------------------------------------------------------------------- 11

Purpose

The purpose of this project is to update the classifications for the soil series of Trinidad and Tobago in accordance with the latest version of Keys to Soil Taxonomy (Soil Survey Staff, 2014b). The taxonomic classifications were originally assigned by Dr. Guy D. Smith, working with the University of the West Indies, as part of a larger project to classify the soils throughout the Caribbean region (Smith, 1983). Since then Soil Taxonomy has been revised numerous times and this has resulted in nearly all of these soils having obsolete classifications. This is now corrected. Tables 1 and 2 below present the old and new taxonomic names for the soil series.

This report can also be used to help in any planning for future updating of the soil survey of Trinidad and Tobago. The soil survey field work for the islands was completed in the 1960's. There have been many advances in soil survey technology and its application in the 50 plus years since this work was done. Some of the more important changes include the standards used for describing soil profiles, laboratory tests for documenting chemical and physical properties, cartographic procedures for making accurate maps, new kinds of soil interpretations to assist in land use management, and advances in soil classification. The section titled "Future Considerations" (below) details several issues involving the description of soils in the field and analysis of samples in the laboratory that would significantly improve the classification of the soils in any future work.

Changes to Soil Taxonomy

There have been many improvements to Soil Taxonomy. Four changes that most significantly impacted the classification of the soils of Trinidad and Tobago are described here.

Recognition of highly weathered, low activity clays. Many soils of semi-tropical and tropical areas are unique due to the nature of their highly weathered clay fraction as characterized by low cation exchange capacity, inherently low natural fertility, and the unique challenges they present to fertility management. These properties are now addressed through the recognition of the kandic horizon at the great group level (e.g. the Acono series ? a Kanhapludult), or at the subgroup level (e.g. the Austin Road series ? a Kanhaplic Haplustult). In addition, a new family level consideration, cation exchange activity class was introduced for use in many soils with mixed or siliceous mineralogy to provide insight into the inherent capacity of the soil to retain plant nutrients. For example, the Buenos Aires series has a subactive class while the La Retraite series has a semiactive class. Otherwise their classifications are identical. The different cation exchange activity classes reflect the lower nutrient supplying character of the Buenos Aires series compared to the La Retraite series (something noted in the original map unit descriptions). Together these changes help to better group soils based on the kind and nature of the clay minerals making up the soil.

Dropping the use of the Trop category throughout Soil Taxonomy. At the time that Dr. Smith first classified these soils, many (but not all) soils in tropical regions were placed in Trop suborders e.g. Piparo (Tropept), or great groups e.g. Acono (Tropudult), Barataria (Tropohemist), Montserrat (Tropudoll), or Couva (Tropaqualf). There are many examples shown under the "old classification" for Trinidad and Tobago soils. The Trop categories were intended to group soils that have both warm temperatures and little seasonal fluctuation in temperature (isomesic or warmer). Eventually the Trop classes were dropped for two major reasons. First, in practice it resulted in situations where some soils in tropical landscapes were classified in trop categories while their adjacent neighboring soils in the same landscape were not because other factors were deemed more important. Also, the use of trop categories at a high taxonomic level was redundant with the use of isohyperthermic temperature regimes at the family level. Both convey similar information so there was no real advantage to providing it at two categorical levels in the taxonomy.

Dropping the use of Pell and Chrom great groups in Vertisols. These great groups were originally intended to separate the mostly dark colored, wetter Vertisols on level to concave slopes from the somewhat lighter colored, better drained Vertisols on more sloping areas. In practice this was not particularly effective. New great groups based on the presence of important diagnostic horizons and features were established to replace the Pell and Chrom great groups. In addition, poorly drained Vertisols were better recognized with the introduction of the Aquerts suborder. Several Vertisols are recognized in Trinidad and Tobago. Examples include La Fortune, Marac, Milford, and Minister.

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Recognizing both fine and very-fine particle-size classes in Ultisols. At the time that Dr. Smith classified these soils, Ultisols with 35 to 100 percent clay in the subsoil were all placed in the clayey particle size class. These have since been separated into two classes, fine (35-59 percent) and very-fine (60% or more). This allows for better separations at the family level for Ultisols with high clay contents. A few examples include Anglais, Long Stretch, and Mon Pariel, all in Trinidad. No Ultisols were recognized in Tobago, primarily due to the prevalence of parent materials with inherently higher base status.

Procedure

In the introduction to his report, Dr. Smith pointed out several factors contributing to uncertainty about the classification of the soils (Smith, 1983). These included the minimal level of standards for describing the soils in the field, lack of some key laboratory data, and the challenge of applying a taxonomic system after the soil survey was completed rather than concurrently with the field work so that class limits and criteria could be tested and verified. These all continue to contribute to uncertainty of the proper classifications now. Despite these challenges, some key steps and considerations used to reclassify the soils included the following.

Acceptance of the previous work. Dr. Smith made many assumptions about likely soil moisture regimes (udic vs. ustic), and the presence or absence of key features such as argillic horizons, slickensides, plinthite, etc. that were not explicitly described in the original soil survey report. In addition, with no laboratory data available documenting the mineral makeup of the soils, he made informed estimations about likely mineralogy classes. These assumptions were generally accepted and carried forward for the new classifications.

It was necessary to evaluate the soils for the possible presence of a kandic horizon, a feature previously not recognized by Soil Taxonomy. Key laboratory data for clay content and cation exchange capacity was entered into a spreadsheet and calculations were made for weighted average clay in the upper 7 inches so that it could be compared to the layer below and for apparent cation exchange capacity (CEC/clay% * 100) in the subsoil layers. It should be noted that effective cation exchange capacity (ECEC) is also a criteria required by Soil Taxonomy, but these data were not available. It was assumed that if the calculated apparent CEC was low enough, the ECEC would also have been met (something generally accepted as true).

A few soils with high organic carbon in the upper part were placed in the Humults suborders (Maracas, Matelot, and Spring Hill). To confirm this classification requires values for organic carbon and bulk density to a depth of 1m so that the mass of organic carbon per square meter can be calculated. However, no bulk density data was reported and organic carbon was reported for just the upper several inches. In order to estimate this, bulk densities of 1.3, 1.4, and 1.5 were assumed for the surface layer, the next layer to 20 inches, and for the zone from 20 to 40 inches respectively. Organic carbon values as reported were used and (decreasing) values were assigned for the remaining depths having no reported carbon levels. If the result qualified for the suborder (i.e. 12.0 kg / m2), then a class of Humult was assigned. This result seemed reasonable for the soils identified.

Mineralogy classes as estimated by Dr. Smith were generally accepted. In a few instances changes were made if clues in the descriptions suggested otherwise. Apparent CEC values provided some clues for clay mineralogy. In most cases these data simply were not available to make a determination with high confidence.

Several soils required assignments for cation exchange activity classes, which were not used in Soil Taxonomy when Dr. Smith classified the soils. Calculations for apparent CEC were calculated with the spreadsheet (as described previously) and cation exchange activity classes were assigned. Most soils were placed in either a subactive or semiactive class, several are active, and a few are superactive (Tables 1 and 2).

It should be noted that while the laboratory data for both Trinidad and Tobago soil surveys were incomplete by today's standards, these data for Tobago were particularly limited. Most significantly, there was no particle-size data, which made it impossible to evaluate family particle-size class. The only information available was the texture class from the field description. The assignments of particle-size made by Dr. Smith were accepted without change. Also, apparent cation exchange activity could not be calculated with the spreadsheet. In addition, in many instances the sampling was limited to sublayers of the horizons described in the field rather than each entire horizon.

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Revised Classification of the Soils

Table 1 presents the updated classification for the soils of Trinidad. Table 2 presents the updated classification for the soils of Tobago.

Table 1. Classification of soil series from Trinidad updated from the 1983 report to the 12th edition Keys to Soil Taxonomy, 2014.

Series

Old Classification

New Classification

Acono Anglais Antilles Aranguez Arena Aripo Austin Road Avocat

clayey, kaolinitic, isohyperthermic Orthoxic Tropudults clayey, kaolinitic, isohyperthermic Orthoxic Tropudults clayey, mixed, isohyperthermic Typic Haplustults fine, mixed, nonacid, isohyperthermic Tropic Fluvaquents mixed, isohyperthermic, coated Orthoxic Quartzipsamments clayey, kaolinitic, isohyperthermic Typic Densiaquults clayey, mixed, isohyperthermic Typic Haplustults clayey, kaolinitic, isohyperthermic Plinthic Haplustults

fine, kaolinitic, isohyperthermic Typic Kanhapludults

fine, kaolinitic, isohyperthermic Aquic Hapludults

fine, mixed, semiactive, isohyperthermic Typic Haplustults fine, kaolinitic, nonacid, isohyperthermic Typic Fluvaquents

isohyperthermic, coated Udoxic Quartzipsamments

fine-loamy, mixed, subactive, isohyperthermic, Typic Fragiaquults fine-loamy, mixed, isohyperthermic Kanhaplic Haplustults

fine, kaolinitic, isohyperthermic Plinthic Haplustults

Barataria

clastic, dysic, Fluvaquentic Tropohemists

Dysic, isohyperthermic Fluvaquentic Haplohemists

Basseterre

Bejucal Bel Aire Biche Blanchisseuse Bois Bourg Bois Neuf

Brasso

Brazil Brighton Buenos Aires Savannah

very-fine, mixed (calcareous), isohyperthermic Aquentic Chromuderts

very-fine, mixed, acid, isohyperthermic Entic Pelluderts very-fine, mixed, nonacid, isohyperthermic Entic Pelluderts very-fine, mixed, nonacid, isohyperthermic Aquentic Chromuderts fine, mixed isohyperthermic Typic Tropudalfs fine, mixed, isohyperthermic Aquic Haplustalfs very-fine, mixed acid, isohyperthermic Tropic Fluvaquents

very-fine, montmorillonitic, nonacid, isohyperthermic Aquentic Chromuderts

fine-loamy, siliceous, isohyperthermic Umbric Tropaquults fine, mixed, acid, isohyperthermic Sulfic Tropaquepts

very-fine, mixed, acid, isohyperthermic Entic Pellusterts

very-fine, mixed, semiactive, isohyperthermic Chromic Calciusterts

very-fine, mixed, semiactive, isohyperthermic Chromic Dystraquerts very-fine, mixed, semiactive, isohyperthermic Aquic Hapluderts very-fine, mixed, semiactive, isohyperthermic Aquertic Eutrudepts fine, mixed, subactive, isohyperthermic Typic Hapludalfs

fine, mixed, active, isohyperthermic Aquic Haplustalfs

very-fine, mixed, semiactive, isohyperthermic Chromic Dystraquerts

very-fine, mixed, subactive, isohyperthermic Aquertic Eutrudepts

fine-loamy, siliceous, active, isohyperthermic Typic Umbraquults very-fine, mixed, active, isohyperthermic Sulfic Endoaquepts

very-fine, mixed, subactive, isohyperthermic Chromic Dystrusterts

Cacandee Canterbury Cap-de-Ville Caracas

very-fine, montmorillonitic, nonacid, isohyperthermic Typic Pelluderts

fine, mixed, isohyperthermic Aquic Tropudalfs very-fine, mixed, isohyperthermic Aquultic Haplustalfs fine, montmorillonitic, isohyperthermic Fluvaquentic Tropudolls

very-fine, mixed, semiactive, isohyperthermic Chromic Endoaquerts

very-fine, kaolinitic, isohyperthermic Aquic Kandiudalfs very-fine, mixed, subactive, isohyperthermic Aquultic Haplustalfs very-fine, mixed, active, isohyperthermic Fluvaquentic Hapludolls

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