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Because processes that causes the production of H + ions contributes to acidity,
anything that reduces the activity of H + ions in the soil solution would thus neutralize
soil acidity. Sources of such alkalinity include low rainfall, resulting in less leaching of
non-acid cations, increased sodium levels and plant uptake of nutrients. An effective
neutralizer of acidity is calcium. Each mole of CaCO3 neutralizes 2 moles soil acid. It
is the carbonate in CaCO3 that acts to neutralize soil acidity. Other neutralizers
include magnesium carbonate, calcium oxide, calcium hydroxide and wood ash. An
acid soil with a high cation exchange capacity needs a greater amount of limestone
than a low cation exchange capacity soil of the same pH, because of the much
greater number of reserve H + ions held in the soil with the high cation exchange
capacity (HEWITT & SMITH, 1974).
2.6.5 Salinity
A saline soil is defined as a nonalkali soil containing soluble salts in quantities that
interfere with the growth of common crop plants (GAUCH, 1972). Soil is generally
seen as saline if it contains more than 0.1% soluble salts (GAUCH, 1972).
2.6.6 Cation and anion exchange capacity and surface charges
The cation exchange capacity of soil can be defined as the sum of positive charges
of the absorbed cations that a soil can absorb at a specific pH (MARSCHNER, 1995).
The more clay and organic matter a soil contains, the higher its cation exchange
capacity and the stronger the cations are held.
2.6.7 Soils of Namaqualand
The soils of Namaqualand are very diverse. The occurrence of a impenetrable
hardpan layer in most plain or valley landscapes is however a common feature of
this area (DESMET, 2007). The soils of Namaqualand can be broadly classified into
3 groups according to DESMET (2007).
These are the weakly structured grey, yellow or red medium sands of the Sandveld
and Bushmanland, the shallow, undifferentiated and free-draining red and yellow,
variably grained, sandy to loamy soils of the Kamiesberg and Richtersveld ranges
and the red, base-rich, granite-derived colluvial soils rich in clay of the inland margin
of the coastal plain below the Hardeveld range (DESMET, 2007).
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