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Literature review
Macronutrients can be divided into two more groups; primary and secondary
nutrients. The primary nutrients are nitrogen (N), phosphorus (P), and potassium (K).
These major nutrients usually are lacking from the soil first because plants use large
amounts of these elements. The secondary nutrients are calcium (Ca), magnesium
(Mg), and sulphur (S). There are usually enough of these nutrients in the soil.
Micronutrients are those elements essential for plant growth which are needed in
only very small quantities. These are boron (B), copper (Cu), iron (Fe), chloride (Cl),
manganese (Mn), molybdenum (Mo) and zinc (Zn) (MARSCHNER, 1995).
2.6.4 pH
The abbreviation for pH comes from the French term pouvoir hydrog ne or
“hydrogen power” (DONAHUE et al., 1983). This is because the amount of hydrogen
ions is the variable measured by instruments used to determine the pH of a solution.
The pH of a soil depends on the original weathered rock content and the C/N ratio of
the surrounding biota and decomposing organic matter (LECLERC, 2003). The
occurrence of ions of Ca and Mg in a soil is often correlated with a increase in pH
(pH of 7.5 to 8.5) (DONAHUE et al., 1983; LECLERC, 2003). This is also correlated
with a decrease in the concentrations of K and Na (DONAHUE et al., 1983). A very
acidic soil is usually a result of extensive leaching, a high proportion of sesquioxide
and kaolinite, slow microbial activity and a low concentration of exchangeable basic
cations (DONAHUE et al., 1983).
The pH of the nutrient solution affects the solubility and availability of nutritional
elements (DONAHUE et al., 1983). Most minerals are more soluble in acidic soils
(DONAHUE et al., 1983). Soluble forms of aluminium and manganese are commonly
found in soils with a high acidity (pH of 4 to 5) (DONAHUE et al., 1983). A high acidity
also negatively affects many nitrogen fixing bacteria (DONAHUE et al., 1983).
A highly basic soil can also affect plant growth negatively (DONAHUE et al., 1983).
Examples of such soils include soils that have a high calcium content and soil which
have not been leached (DONAHUE et al., 1983). These are common in low rainfall
areas such as Namaqualand (DONAHUE et al., 1983)
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