SOIL BASICS I MACROSCALE FEATURES - Introduction to Soil Chemistry - page 10

tinctive horizon is the E/B. This designation indicates a transition horizon ,
which contains characteristics of both the overlying E, and underlying B
horizons. There are other designations of transition horizons such as AB.
Whichever letter comes first indicates that that horizon characteristic
dominates the transition horizon.
The small (lowercase) letter subordinate horizon designators are used
wherever appropriate to designate significant differences or distinctions in a
horizon (see Table 1.2). A common designation is the small letter t, which
stands for the German word ton , which means clay. This is a B horizon, where
there is a significant increase in clay when compared to the overlying horizons.
Another designation, shown in Figure 1.2, is the small letter g, which stands
for gleying , also called mottling . This is a situation where varying colors
develop under reducing conditions, which result when the soil is saturated with
water for a significant period of time. In many cases the colors are varying
shades of red or rust-colored spots. However, sometimes grays and even blues
can occur depending on the soil.
In Figure 1.4 the profile description shows an E horizon and a B horizon
with small letter hs designation and a B horizon below this with a small letter
s. The E horizon is light in color and, if light enough, might also be called an
albic horizon . The small letter h indicates an accumulation of highly decom-
posed illuvial organic matter, and the small letter s refers to the accumulation
of oxides of illuvial iron and aluminum.
Many other designators are used for various horizons and horizon con-
ditions. A complete list of these will not be given here but can be found in
any introductory soils text or topic on soil classification.
In naming soils, diagnostic horizons are used. These terms are different from
those used in a profile description such as those given in Figures 1.1-1.5.
However, these diagnostic horizons are indicated in the name given to the soil
and are not necessarily designations that the researcher doing soil chemistry,
developing analytical or instrumental procedures for soil, is likely to have
occasion to see or use. If the need arises, they are easily found in the litera-
ture (see Bibliography).

1.1.2. Horizon Development under Low-Rainfall and Desert Conditions

Under low-rainfall and desert conditions the lack of rainfall results in very dif-
ferent soil conditions. Figure 1.5 shows the profile description of an Aridisol
(an arid, desert-region soil) (see Section 1.4 for explanation of the Naming
System). All lower horizons contain a high content of sodium ions and most
also contain other soluble salts of various kinds.
In low-rainfall regions salts are not readily washed out of soil and may even
build up on the soil surface. This happens when rain dissolves salts in the soil
surface horizon and leaches them a little way into the soil. Subsequent evapo-
ration of water from the soil surface causes water and dissolved salts to move
to and be deposited on the soil surface. No salts or only the most soluble salts
are leached out of the soil, in low-rainfall regions, and the pH is typically basic.