SEDIMENTS AND SOILS - Archaeological Chemistry - page 222

properties of the soil: it regulates, for example, the flow and the retention of
water and the thermal characteristics ( thermal capacity and thermal conductiv-
ity ) of the soil. Depending on the size of the main constituent particles, it is
customary to classify soils as clayey , silty , and sandy , or loamy , as listed in
Table 52; the latter type is a mixture of the first three types listed.

The pH of Soils. The pH of soils (see Textbox 48) varies widely with loca-
tion and climate; the local climate of a region usually provides a clue as to
the pH of the soil. In high-rainfall regions, for example, soils are generally
acid. Alkaline soils, in contrast, typically occur in low-rainfall areas; the
actual value of the pH varies over a wide range, from below 5 in very acid

TABLE 52 Soil Particle Size and Soil Types

Soil type Texture Particle size (mm)

Clayey Fine below 0.002

Silty Moderately fine 0.002-0.05

Sandy Coarse over 0.1

Loamy Mixture below 0.002-over 0.1

TEXTBOX 48
ACIDS AND BASES: THE pH OF SOLUTIONS

Most of the water on the surface of the earth occurs as molecules made
up of two atoms of hydrogen bonded (joined) to one of oxygen, and the
formula for water is H 2 O (see Textbox 72). A very small percentage of the
water molecules are ionized , however; that is, they are dissociated (sepa-
rated) into two electrically charged particles, known as ions : a positively
charged hydrogen cation (an atom of hydrogen bearing a positive electric
charge, H + ) and a negatively charged hydroxyl anion (composed of a hydro-
gen atom joined to one of oxygen, OH − ):

H + (OH) − H 2 O = +

water hydrogen hydroxyl

molecule cation anion