Introduction to Soil Chemistry

peds 11
As would be expected, only limited horizon development will occur and
frequently the horizons formed will be thin. However, it is possible to find soils
with significant horizon development in desert regions. In some cases this soil
developed when the particular area received more rainfall.
Salts, in addition to causing the soil to be basic, will, in some cases, have
deleterious effects on analytical procedures. Significant error can occur if a
potassium-selective electrode is used to determine potassium in a high-sodium
soil.As discussed in Chapters 5 and 8, other salts could cause inaccurate results
to be obtained when an atomic absorption analysis of a soil extract is carried
out.
1.1.3. Horizon Development in Areas between
High- and Low-Rainfall Conditions
In areas between high- and low-rainfall horizonation may be well or poorly
developed, soil pH may be either acidic or basic, and there may or may
not be salt buildup in the soil. In these areas the analyst must be aware of
this potential variation and have the reagents necessary for all of these
eventualities.
1.2. PEDS
Profile descriptions also detail the structure found in that horizon and indicate
its strength. Figure 1.6 shows a soil profile with an indication of the location
of the various structure types. Looking at Figures 1.1–1.5, an example of
each major structure type is indicated. In most cases granular and crumb structure
is expected to occur only in the top 25cm of soil. Platy structure can be
found in any horizon although it is frequently found in E horizons or in the
transition zones between the A and lower horizons. Traffic, farm or other, will
promote the formation of platy structure in the A horizon and often results in
an angular, blocky structure at the base of the A horizon. In C horizons, platy
structure is a remnant of the original character of the parent material from
which the soil is forming. Subangular and angular blocky structure is typically
found in the upper part of the B horizons and the prismatic structure, in the
lower part. However, blocky and prismatic structure can be found in any part
of the B horizons.
Peds are formed by natural aggregation of sand silt and clay particles.
Although a soil’s texture is defined by the relative proportions of sand, silt,
and clay, these components almost never act independently of each other. The
binding agents that hold peds together are clay, organic matter, particularly
microbial gums, and various inorganic ions, particularly calcium, magnesium,
iron, and aluminum. Planes of weakness occur between peds and are extremely
important in determining a soil’s characteristics because these are areas where
air, water, and roots penetrate soil easily. When observing soil peds, roots are
seen growing in the voids between structures (roots can be seen on the second