The Terminology of Salinity
Salinity in based on the presence of charged ions, which can have either a positive charge –cations- or a negative charge –anions-. These ions can be toxic to plants, depending on the plant and the concentration. Milligrams per liter -mg/L- is the typical unit of measure for ions. Examples of common ions affecting salinity:
cations: calcium -Ca++-, sodium -Na+-, magnesium -Mg++-
anions: chloride -Cl--, bicarbonate -HCO3-
Monovalent ions have an ionic charge of 1; divalent ions have an ionic charge of 2.
Boron –B- can be toxic to plants, but does not contribute to salinity. Boron is measured in parts per million -ppm-.
Electrical Conductivity –EC-
The ions bind together in soil, based on their opposing charges, but these bonds are broken in water. The resulting presence of charged ions controls the electrical conductivity of the water. EC is a measure used to determine total salinity of water. Deciseimens/meter -dS/m- is the unit of measure for EC. ECe= EC for a soil-water . ECw = EC for irrigation water.
SAR is measured in irrigation water to evaluate sodium levels, called sodicity. It is the ratio of soluble cations: Na: (Ca+ Mg)1/ and is more useful than sodium values alone, because more sodium can be tolerated when calcium is present.
1/The formula: Na+/sqrt(Ca+++Mg++)/2