Environmental Engineering Reference
In-Depth Information
Clay or organic colloid
Clay or organic colloid
Mg
2+
H
+
H
+
Na
+
Soil solution
Soil solution
H
+
Mg
2+
=
+
+
K
+
H
+
NH
4
Ca
2+
Al
3+
Na
+
Ca
2+
K
+
Al
3+
NH
4
Negatively charged exchange sites on soil colloid
Fig. 3.7. Exchange of cations between soil colloids and the soil solution provides a
continuous supply of nutrients available to plants.
to plants (Fig. 3.7). The ability of a soil to hold cations in this form is
known as
cation exchange capacity
or CEC. It is one of the most important
chemical properties of soil and is usually expressed in units called milli-
equivalents per 100 g of soil (meq/100 g). The CEC of a soil might be
compared to the size of a car's fuel tank. The larger the fuel tank, the
longer the car can travel before refuelling. Likewise, the larger the CEC,
the more nutrients the soil can hold and the longer it can sustain plant
growth without replenishment. In general, the more clay or organic-
matter colloids a soil contains, the higher its CEC (Table 3.1). In practical
terms, turf on low-CEC (sandy) soil may need fertilization every two to
three weeks, whereas turf established on a high-CEC (clay loam) soil may
need fertilization only every 2 to 3 months.
Soil pH
Soil pH is another important chemical property. Mathematically, pH is
defined as the negative logarithm of the hydrogen-ion concentration. In
other words, the pH is a logarithmic expression of the concentration of
