Soil Salinity

Salts accumulate in soil due to poor drainage and irrigation using saline water. This is usually a slow process and affects the fertility of the soil and eventually also the physical properties and soil structure.

Approximately 15% of arable land in South Africa is affected by soil acidity (total arable land is 12.9 million hectares).

Saline Soils

• White brak or salty soils.
• Low Na to Ca + Mg + K ratio.
• High electro-conductivity (+150 ms/m).
• Low sodium absorption ratio (SAR less than 15).
• Normally, little or few adverse effects on soil.
• Retard crop growth (water absorption).
• Reclaim by drainage.

Sodic Soils

• Black brak or black alkaline
• High Na to Ca + Mg + K ratio
• High electro-conductivity (+300 ms/m)
• High sodium absorption ratio (SAR more than 15)
• Soil deterioration (structure, crusting, infiltration)
• Crops wilt and scorch

High alkalinity is caused by over-irrigation of poorly-drained soil, salty irrigation water or a rising water table. Damaged soil can be reclaimed by artificial drainage (if soils are drainable). Apply 5-10 tons/ha of gypsum, and 1 000mm irrigation to leach out sodium. Approximately 18 200 hectares of the cultivated land in South Africa is affected by soil alkalinity.

It is important to note the following:

• In acid soils, manganese (Mn) can be toxic to cabbages and tomatoes.
• Boron (B) can become toxic at high pH.
• Molybdenum (Mo) can make copper (Cu) unavailable to plants.
• In acid soils, Mg, P and Mo could be deficient.