Argillic, Kandic, and Natric Horizons - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 87

11.2 Historical Overview of Clay-Enriched Horizons

That fine soil particles moved through the soil profile was recognized as early as the
late 1800s (King 1895 ; Sibirtsev 1900 ). The importance of clay was stressed by
Hilgard ( 1906 ), who reviewed the physicochemical properties in relation to soil
development and plant growth. At that time, no size boundary for these fine
particles was set and the fine soil particles were often referred to as colloids.
It was probably at the First International Congress of Soil Science in 1927 that
the size limit for clay was set at 2

m.
Merrill ( 1906 ) observed that in soils of humid regions, colloidal particles
became partially diffused in the rainwater, percolated through the soil, and accu-
mulated in the subsoil. He found that almost without exception, the subsoils of
humid regions have much more clay than the corresponding surface soils. As a
result the subsoils are more compact, heavier, and less permeable. He also observed
that clay eluviation was sometimes accompanied by CaCO 3 leaching which could
result in the formation of a hardpan. Merrill ( 1906 ) distinguished between soils of
the humid regions where clay eluviation takes place and soils of the drier regions
where such processes are absent. This climatic distinction on percolating water and
its effect on movement of soil particles was further developed by C.F. Marbut and
resulted in the distinction between pedalfers and pedocals (Marbut 1927 ).
Wolfanger ( 1930 ) described pedalfers and named the A the horizon of maximum
extraction and B the horizon of concentration. He wrote: “The extraction and
concentration are brought about in part through eluviation (the mechanical transfer
of material), in part by transfer through solution and reprecipitation (chemically)
and in part by both processes. Fine grained materials, clay and silt, are mechanically
transferred from the upper to the lower horizons.”
Robinson ( 1932 ) distinguished between two types of eluviation: mechanical
eluviation in which, apart from any chemical differentiation, the finer fractions of
the mineral portion of the soil are washed down to lower levels, and chemical
eluviation in which decomposition occurs and certain products thus liberated are
translocated in true or colloidal solution to be deposited in other horizons. Mechan-
ical eluviation results in the development of a texture profile characterized by a
light-textured A horizon and a heavy-textured B horizon enriched by the finer
material from the A horizon, and such soils are common in southeastern USA
(Robinson 1932 ).
One of the first descriptions of the argillic horizon was by Joffe ( 1936 ). He also
considered the B as a horizon that is gaining instead of losing as with the A horizon.
The B horizon is therefore known as the horizon of illuviation (washing in) or
horizon of accumulation. Joffe recognized that the fine particles were mechanically
carried from the A to the B horizon and that it will result in a more compact horizon.
The B was named an illuvial horizon and Joffe also postulated the idea of new clay
formations in the B horizon which enhances the differences in clay content between
the A and B horizon.
The eluvial and illuvial horizon model was well developed in the first half of the
twentieth century. The migration processes were well understood and the concepts

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