Cooperative Extension Service

DESCRIPTION SEQUENCE
by Richard Drees

The approach to describing thin sections depends on the purpose of the study. There are two main approaches:

  1. Start with the identification and description of the simplest units, and examine the way in which they combine through various stages of complex units to the fabric level. This goes from the microscopic towards the macroscopic level.
  2. Proceed from the most complex units and examine how these break down into less complex units and eventually into the simplest units. This is from the macroscopic toward the microscopic level. This method is often preferred.

    Thin section descriptions should begin with an examination of the section with the naked eye (1X magnification). Use a light talbe or hold the section up to a light source to help identify major pedological feature. Homogeneous zones can then be identified which will be examined separately under the microscope. Alternatively, it is sometimes useful to examine the block from which the thin section was obtained to see if there were any areas that were not impregnated or to examine the macrostructure. If the impregnating solution contains a dye, the macropore structure is often better illustrated on the polished block. At this stage it is sometimes useful to study the field description to see if items noted earlier can be seen in the thin section. If features observed in the field ar not present in the thin section, the thin section may not be representative of the soil material. Some micromorphologists even complete a macrmorphological description of the thin section first. This approach helps bridge the gap between the field relationships and micromorphology.

    Prior to starting to describe the thin section under the microscope, it is useful to first scan the entire section using different magnifications to see the variation in microstructure, groundmass, and pedofeatures that are present. When soil profiles have been sampled, it is also useful to quickly scan the thin sections from adjacent horizons to gain an overall view of the material.

    Begin the examination o fthe slide under the petrographic microscope with the lowest power available. On most scopes this will be 20-25X magnification. Use crossed and uncrossed polarized light to identify major feature. Within each major feature, use increased magnification to identify additional detail. Fitz Patrick (1993) lists the summary stages for describing a thin section as follows: