The morphological character of earth material in soil profiles is popularly known as soil structure. It is well understood in general terms by practicing soil scientists, but is not well understood in scientific terms. In soil physics, it is a popular and important subject but "no one seems to now what it is" (Letey, 1991). The principal parts of soil structure are size, shape, and arrangement of the solid materials and voids. These features have many attributes and are affected by many variables, which renders any rigorous treatment of the subject complex and impractical (Brewer and Sleeman, 1960). However, descriptions and useful interpretations of soil structure can be made at any scale or "level of organization" (Brewer, 1964)
.The variations in the spectrum of morphological properties of soils appear to form a continuum and have no natural limits within the spectrum which can serve to distinguish classes. When one examines the standard methods of soil morphology studies, it is evident that only parts of the spectrum are being described or used in some way. Micromorphologists are drawn to the small features; field studies give priority to the largest features. As a consequence, features in the mid-range from about 0.1 to 5 mm are inadvertently overlooked except for pores. These mesoscale features are obviously seen but a discussion about them is rarely developed. A logical reason for this is soil structures in the meso-range may appear to be more or less extensions of the micro or macro range and describing mesoscale features may seem unnecessary. Micromorphological studies commonly note prominent mesoscale features but are limited to a thin-section or two-dimensional (2D) perspective while field studies are based on a three-dimensional (3D) perspective. The two perspectives seem to attract attention to the end members and not to the mesoscale features.
By default it would appear that the meso-features are non-existing or non-significant because they are not described. The most obvious prejudice against mesoscale features is seen in the "standard" field descriptions of soil types. The order of importance based on size stands out. The largest features are described and sometimes noted as "parting to" a smaller feature (Soil Survey Division Staff, 1993). Most large soil structural units (peds) continue to disaggregate with handling into a descending order of sizes, which goes beyond the two "levels" described in a standard description, i.e., soil features are within soil features for more than two levels of size classes. The significance of this phenomena is not so much that soil structural units have more than two levels of hierarchical relationships, but that soil structure has multiple levels of size and morphological organization in complex patterns.
This characteristic is a clear indicator of the pedogenic origin of the organization of the features. Pedogenic organization can be characterized by disorder compared to the fabric-structure of original geologic deposits. In general, the pedogenic character that is commonly seen in a hand-size specimen is a mixture of morphological forms, sizes, and colors. Geogenic features by comparison are simple and often symmetrical or repetitive, such as graded bedding and bedding planes. Also, they usually have a uniform color or simple gradations. Commonly, a distinction is clear at the mesoscale level; pedogenic fabric is complex and porous compared to a geogenic fabric which is usually non-porous and either layered or uniform ("massive"). In the profile sense, geogenic morphological forms do not have a vertical trend that is in harmony with the ABC sequence of soil horizons. When in doubt, chemical or mineralogical analysis will usually distinguish a pedogenic trend from a geogenic trend in most cases. The distinction between geogenic and pedogenic morphological features in field studies is most reliably determined by features in the mesoscale range because the processes operating produce the most meaningful physical characteristics in this range. Also, meso-features are better preserved and more accessible compared to larger features. The mesomorphology becomes a critical factor in the study of geosols and other buried features because the paleo-landscape relationships are not as accessible as they are in the study of present land surface soils.
In general, a conclusion that can be drawn from the common style of soil descriptions is that much detail is seen, but only selected parts are recognized and described. However, this does not seem to cause any problems in the study of modern soils. The risk of misinterpreting modern, "living" soils at the present ground surface is small because the selected parts are rarely out of harmony with the undescribed parts. Then, it logically follows that observing a part of the spectrum of the morphological characteristics in any soil profile is sufficient to identify a sequence of "layers" as horizons of a soil profile. This has fundamental significance for paleopedological work. Recognizing a "weathered zone" as a paleosol is often a challenging exercise. Many features of a paleosol have to be confirmed before the zone in question is generally accepted as a paleosol, particularly buried paleosols (geosols). Many morphological features of geosols are lost upon burial. The larger and more delicate features are compressed and commonly loose their original character (Follmer, 1979). Unstained or uncoated structural forms are healed together (welded) and become more massive. With increasing depth of burial, pedogenic features are progressively lost. Small features persist the longest, particularly the peds with coatings or indurated character. Soil structures have even been noted to persist into low grade metamorphic environments (Retallack, 1990).
Small pedogenic features in the mesoscale range from about 0.1-5 mm are often the first evidence of a geosol observed in many situations. These observations are often in discontinuous cores or limited access exposures. When the standard macro features are not preserved or recognized, then a closer look for the mesoscale features can provide a basis for pursuing further morphological evidence. Commonly geosols (buried paleosols of known stratigraphic relationships) are not recognized if key macroscale evidence is not observed. Paleosols do not have a predetermined existence as land surface soils do. Soils are sometimes proclaimed to be soils because they are at the land surface. The existence of buried soils (geosols, the pedostratigraphic units, and paleosols, the pedogenic profiles) must be based on other arguments.
My Approach. Because the standard field method of identifying a soil profile is based on macro features, I use mesoscale features to screen for pedogenic characteristics of suspected buried soils when macro features are not accessible. When a pedo-pattern is suspected, I apply a pedo-theory to select where to go to find macro or other supporting evidence. In general, I begin a search for spatial patterns of vertical profile characteristics and lateral catena characteristics. Theory and experience predicts that (1) pedogenic structure coarsens downward and trends towards simplicity in relation to soil horizons, and (2) color patterns and morphological style varies laterally in relation to original land-surface slope. Also, a repeat pattern is commonly observed if the lateral distribution is pursued. When features are found in harmony with pedo-theory, then a basis for interpreting a pedogenic profile or catena is established. Then laboratory studies can confirm the interpretations. However, in mapping exercises, repeated observations of predictable relationships are often sufficient to confirm an interpretation.
In field and core studies of buried soils I have found that meso-structures, including bio-pores and patterns, are more useful and reliable than macro features. Some aspects of the meso-features stand out, such as what I informally have described as "lumpy-bumpy fabric at the mm-scale". Because these features are not just simple scale differences, a unique name for this class of features became desirable. After a long search for a name that would avoid confusion with other contexts, I chose the term klumpen for this class and treated it as one level of soil structure. Klumpen is an old Anglo-Saxon word for lump or cluster that evolved into modern German and English as similar words meaning lump, clump, cluster, or aggregate. Klumpen is a three-dimensional aspect and should not be confused the with two-dimensional characteristics, although the relationships are clearly related. Klumpen is intended to be a field descriptive term and not be dependent on thin section studies. In field studies, the emphasis should be placed on the actual features seen.
The selected size range of klumpen is from 0.1 to 5 mm; the range that is seen well under a 10x hand lens. The lower limit is the limit one can see with the naked eye. The upper limit is set at 5 mm because the features above this is limit are adequately described by standard macro descriptions. However, the features above this limit, which in the general sense are peds, contain klumpen features within the peds, which are expressed on the surface of the peds. The mesoscale range seems to have more forms that are distinguishable than those commonly found in the macro range. They are commonly noted in thin section descriptions if they have a recognizable 2-D expression, such as circular features interpreted to be spherical, etc. The important klumpen features are 3-D in appearance and are not recognized in thin sections, such as hackly, conchoidal, lumpy surfaces, etc. The fundamental nature of aggregates is best studied at this level. The most common observation of klumpen appears as "welded granular" aggregates with root channel (bio) pores. Other common forms result from broken surfaces producing edges, meso-blocks and a hackly appearance. Conchoidal, smooth surfaces are recognized as fracture faces and are not actually a klumpen feature. They reflect the uniform character of the material, i.e., conchoidal fractures indicate the lack of pedogenic structure. Many other klumpen shapes have been observed but discussing them here goes beyond the scope of this proposition. In general, the terms used in micromorphology and petrography can be applied to the shapes and interpretation of klumpen features.
Organization of Klumpen Classes: Classes of Grade, Size, Shape, and Pores are erected in parallel with the normal concept of macro structure (Figure 1). The same concept of weak, moderate, and strong is applied. The size classes of <1 mm, 1-3 mm, and 3-5 mm are arbitrary. The shape and pore classes are borrowed from other guidelines as needed. Most of the time I have been able to find a word to describe the form I see, but some forms defy description. Some of these forms and relationships are clearly pedogenic and some are not. If a pedogenic context is established and the klumpen features are given more attention, then the recognition of buried soils (paleosols and geosols) becomes a little easier. [Revised 9/1995]
Grade (expression) Size (mm) Shape Pores Class weak mod str fine med coarse macro 1 2 3 <5 5-10 10-20 Same Same klumpen 1 2 3 <1 1-3 3-5
Figure 1.Terms and limits used to describe klumpen (mesoscale) features compared to generalized macroscale terms for soil structure.
Brewer, R. 1964. Fabric and mineral analysis of soils. John Wiley and sons, New York.
Brewer, R. and J. R. Sleeman. 1960. Soil structure and fabric: their definition and description. Journal of Soil Science 11 (1): 172-185.
Follmer, L. R. 1979. Wisconsinan, Sangamonian, and Illinoian stratigraphy in central Illinois. Illinois State Geological Survey Guidebook 13, p. 129-134.
Letey, J. 1991. The study of soil structure: scient or art. Australian Journal of Soil Research 29 (6):699-707.
Retallack, G. J. 1990. Soils of the past: an introduction to paleopedology. Unwin Hyman, Boston. Soil Survey Division Staff. 1993. Soil survey manual U. S. Department of Agriculture, U. S. Government Printing Office, Washington D. C.