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Salinity Laboratory: Determining the suitability of soils and waters for irrigation
Salinity Laboratory
The Salinity Laboratory is concerned with the assessment of the suitability of soils and waters for irrigation.
Water used for irrigation always contains dissolved inorganic salts including calcium, magnesium and sodium chlorides). When these waters are applied to soil, a large proportion of the water is transpired by plants, leaving most of the salts behind in the soil. Unless these salt levels in the soil are maintained at a reasonable level (equivalent to about a four-gram teaspoonful of salt per five-litre bucket of soil) by leaching, salts will accumulate in the root zone to the point where crops are no longer able to extract sufficient water for their needs and yields will be decreased. By analysing the level of salinity in his soil and water, a farmer can determine the risk he faces from salt build-up in his irrigated soils.
Sodicity is also measured on samples submitted for salinity analysis; sodicity is a measure of the proportion of the salinity that is made up by sodium (measured as a ratio of sodium to calcium and magnesium). Salinity reduces the ability of plants to extract water from the soil, but sodicity affects plants differently: High levels of sodium in the water will increase the level of sodium in the soil, leading to the breakdown of the soil structure, thereby reducing the rate of infiltration of water into the soil. Sodicity, therefore, reduces the quantity of water that enters the soil to become available for crop growth. Increased sodicity can also reduce the permeability of the soil, resulting in waterlogging and reduced leaching of salts. Increased sodicity can, therefore, lead to increased salinity.
Sodium (Na) and chloride (Cl) are also toxic to most crops, if concentrations are high enough. Some crops such as citrus and other woody perennials are very sensitive to these elements. In these plants Na and Cl are taken up not only via the root system but also through foliar absorption. In the case of sensitive crops, relatively low levels of sodium and chloride can, therefore, reduce yields and cause crop failure.
Also measured in this laboratory are soil texture (clay percentage) and soil organic matter (organic carbon percentage). Knowledge of the soil texture is vital to ensure the success of any irrigation scheme and will provide information on such factors as water holding capacity, drainage, cation exchange capacity (fertility) and physical structure. The organic fraction of the soil (organic matter or humus) also contributes to the cation exchange capacity and structural stability of the soil and acts as an important reserve of plant nutrients.
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Soil Fertility and Analytical Services
KwaZulu-Natal Department of Agriculture
Private Bag X9059
Pietermaritzburg
3200Soil Salinity
Jon Lawrence
Telephone: (033) 3559 465Reception office for sample delivery and enquiries:
Ruby Punwasi and Vino Naidoo
Telephone: (033) 3559 455
Facsimile: (033) 3559 454
