The pH of these soils are usually high, between pH 7.5 and 10, which results in the deficiency of micro-nutrients such as Zn, Cu, or Mn.
In alkaline soils, the presence of sodium carbonate causes changes in the soil structure. There is decrease in the permeability and pore-space.
Clay and organic matter particles (soil colloids) are dispersed or deflocculated and then washed down between the soil aggregates and may eventually form a hard pan.
The dispersed humic materials give a black coating on the soil giving a black soil surface hence the term “black alkaline soils”.
The surfaces of alkaline (sodic) soils dry out into large massive hard clay pan always difficult to cultivate-being plastic when wet and forms hard compacted clods when dry.
Saline soils develop under conditions of low rainfall, but can also develop under poorly drainaged area, unusual topography whereby salts accumulate in the depression and saline areas are left behind after evaporation.
In most alkaline soils the activity of Ca is high. Under this high pH condition, there is precipitation of relatively insoluble dicalcium phosphate Ca(H2PO4)2 and other basic calcium phosphate such as carbonates apatite Ca10 CO3 (HPO4)6 and hydroxylapatite Ca10 (OH)2 (HPO4)6.
Soil alkalinity affects plants due to the toxicity of the Na+, OH– and HCO3– ions. These ions and the high pH (above 8.5) usually emanate from the hydrolysis of sodium carbonate in soil:
Na2 CO3 + CO2 + H2O 2Na+ + 2HCO3–
Extreme alkalinity (pH 8.5 – 10.5 or higher) is due to the presence of Na2 CO3 while the black colouration of the soil surface is due to the dispersed humus carried upward by capillary water. Irrigation water containing excess Na+ and HCO3- often causes soil alkalinity.
In general, many nutrients become unavailable in these soils of high pH, while the unstable structure leads to low water permeability, poor aeration and unworkable silts.
