Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 279

If the amount of salt inflow in the soil system is greater than the salt outflow (or
removal), then excess salt will build up, and adversely affect crop growth. Salt in the
soil system comes from soil mineral, irrigation water, rainwater, and capillary flux.
The salt is removed from the system by physical removal (scraping), leaching/deep
drainage water, surface drainage water, and by the plants. Mathematically, it can be
expressed as

( M s + V i C i + V rw C rw + UC u ) − ( P r + V d C d + V sd C sd + V pw C pw ) = 0 (8.1)

where

M s = amount of salt dissolved from soil mineral of a specified (or unit) soil

volume, mg
V i , V rw , U are the volume of water added to the same volume of soil by
irrigation, rainfall, and upward flux (capillary rise), respectively, liter
C i , C rw , C i are the concentration of salt in irrigation water, rainwater, and
upflow, respectively, mg/l
P r =

amount of salt removed from surface by physical means, mg
V d , V sd , V pw are the volume of water removed by deep drainage, surface
drainage, and plant uptake, respectively, liter
C d , C sd , C pw are the concentrations of salt in deep drainage, surface drainage,
and plant uptake water, respectively, mg/l.

8.3 Diagnosis and Characteristics of Saline and Sodic Soils

8.3.1 Classification and Characteristics of Salt-Affected Soils

Salt-affected soils may be divided into three groups depending on the amounts and
kinds of total soluble salts present (estimated by electrical conductivity, EC), soil
pH, and exchangeable sodium percentage (ESP) (Table 8.2 ) :

Table 8.2 Generalized classification of salt-affected soils

Criteria

Soil class EC (dS/m) ESP pH

Saline soil >2 <15 <8.5

Sodic (alkali) soil <2 >15 >8.5

Saline-sodic soil >2 >15 <8.5

Note:Normalsoil-EC<2dS/m,ESP<15

Saline soil is affected by too much total dissolved salts, sodic soil is affected by
too much sodium, and saline-sodic soils is affected by an excess of both soluble
salts and sodium. It is fairly common for both sodic and saline conditions to occur
together in many soils.