Handbook for Irrigation of Wine Grapes in South Africa

TABLE 5.9. Classification of irrigation water according to EC and SAR as proposed by Richards (1954).

Class Limits

Hazard Implication for use

EC (dS/m)

Salinity

C1 < 0.25 Can be used for irrigation with most crops with little like- lihood that soil salinity will develop. Some leaching is re- quired, but this occurs under normal irrigation practices, except in soils of extremely low permeability. C2 0.25 - 0.75 Medium Can be used if a moderate amount of leaching occurs. Plants with moderate salt tolerance can be grown in most cases without special practices for salinity control. C3 0.75 - 2.25 High Low

Cannot be used on soils with restricted drainage. Even with adequate drainage, special management for salinity control may be required and plants with good salt toler- ance should be selected. Very high Is not suitable for irrigation under ordinary conditions, but may be used occasionally under very special circum- stances. The soils must be permeable, drainage must be adequate, irrigation water must be applied in excess to provide considerable leaching, and very high salt tolerant crops should be used. Can be used for irrigation on almost all soils with little dan- ger of the development of harmful levels of exchangeable sodium. However, sodium-sensitive crops such as stone fruit trees may suffer injury as a result of accumulation in plant tissues when exchangeable sodium values are lower than those effective in causing deterioration of the soil physical conditions. Medium Will present an appreciable sodium hazard in fine-tex- tured soils having high cation exchange capacity, espe- cially under low-leaching conditions, unless gypsum is present in the soil. This water may be used on coarse-tex- tured or organic soils with good permeability. May produce harmful levels of exchangeable sodium in most soils and will require special soil management, i.e. good drainage, high leaching and organic matter addi- tions. Gypsiferous soils may not develop harmful levels of exchangeable sodium from such waters. Chemical amendments may be required for replacement of ex- changeable sodium, except that amendments may not be feasible with C1 waters. Very high Is generally unsatisfactory for irrigation purposes, except at low and perhaps medium salinity, where the solution of calcium from the soil or use of gypsum or other amend- ments may make the use of these waters feasible. Sodium Low High

C4 > 2.25

SAR

S1 < 10

S2 10 - 18

S3 18 - 26

S4 > 26

118 CHAPTER 5 – IRRIGATION WATER QUALITY