Engineering properties of peat and organic soils - Geotechnics of Organic Soils and Peat - page 59

Table 3.6 Typical specific gravity of various soils and minerals.

Soil type/mineral Specific gravity

Non-clay
Quartz 2.65

Mica 2.76-3.20

Gypsum 2.32

Clay minerals
Kaolinite 2.62-2.66

Montmorillonite 2.75-2.78

Illite 2.60-2.96

Peat
Bog peat 1.40-1.60

Fen peat 1.80

West Malaysia peat 1.38-1.70

Samarahan peat 1.07-1.63

Central Kalimantan peat 1.50-1.77

in units of specific adsorbed charge (Chapman, 1965; Sposito, 2008). Exchangeable
cations must be removed from the soil to be detected and measured (Conklin, 2005).
The unit of CEC is either cmol kg − 1 or meq per 100 g soil. The determination of CEC
gives an indication of the total amount of negative charges of the soil.
The CEC value depends on the pH and the technique which is used (Conklin,
2005). A realistic CEC value is that determined at the field pH value. This is difficult
because the soil pH varies greatly.
The ease of cation replacement depends on the valence, ion size and ion relative
amount. The range of soil colloid CEC is from a mean minimum about 4 for Al or
Fe oxides to a mean maximum about 200 cmol kg − 1 of colloid for humus (Fuchsman,
1986). Thus, humus has the highest CEC in comparison with minerals like kaolinite,
montmorillonite, smectite and even vermiculite. Soil organic matter can contribute
a significant fraction of CEC even in clay soils with low amount of organic matter
(Magdoff and Weil, 2005).
Unlike clay minerals, organic matter does not have a fixed capacity for binding
exchangeable cations. It is now well accepted that to measure the actual CEC of a soil,
the pH must not be changed during the procedure. The main problem with measure-
ment of CEC at pH 7 is that it buffers the soil at pH 7, causing large overestimates of
CEC for acidic soils (Gillman and Sumpter, 1986; Fang and Daniels, 2006).
The cation exchange capacity (CEC) of organic soils was measured at pH 7 with
ammonium acetate method (Chapman, 1965) and by the BaCl 2 compulsive exchange
method (Gillman and Sumpter, 1986).
To measure the CEC at pH 7, about 125ml of 1M NH 4 OAc is added to 25.0 g
of soil samples in a 500mL Erlenmeyer flask shaken and allowed to stand overnight.
The soil samples are washed gently with NH 4 OAc using a Buchner funnel filtration,
followed by washing with 95% ethanol. The NH4 + is extracted by leaching the soil
with eight separate 25mL additions of 1 M KCl. The concentrations of NH 4 + -N can
be determined by an auto analyzer.