Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 274

Table 8.1 Common salts in saline area and their equivalent weight

Chemical
formula/
composition

Common
name

Equivalent
weight (g) Salt name Cation (+) Anion (-)

Sodium chloride NaCl Table salt 58.45 Sodium Chloride

Sodium sulfate Na 2 SO 4 Glauber's salt 71.03 Sodium Sulfate

Magnesium sulfate MgSO 4

Epsom salt 60.18 Magnesium Sulfate

Sodium bicarbonate NaHCO 3

Baking soda 84.01 Sodium Bicarbonate

Sodium carbonate Na 2 CO 3 Sal soda 53.00 Sodium Carbonate

Calcium sulfate
(with 2 molecules
of water)

CaSO 4 .2H 2 O Gypsum 86.09 Calcium Sulfate

Calcium carbonate CaCO 3 Calcite (lime) 50 Calcium Carbonate

of irrigated farmland in the United States is affected by soil salinity. Salinity from
irrigation can occur over time wherever irrigation occurs, since almost all water
(other than natural rainfall) contains some dissolved salts. When the plants use the
water, the salts are left behind in the soil and eventually begin to accumulate.

8.2.1.3 Shallow Saline Groundwater

Salinity from drylands can occur when the water table is between two to three meters
from the surface of the soil and the groundwater is saline (which is true in many
areas). The saline water is raised by capillary action and salts are concentrated on
the surface resulting from evaporation of water (salts are left behind). In some cir-
cumstances, the process is favored by land use practices allowing more rainwater to
enter the aquifer than it could accommodate. For example, the clearing of trees for
agriculture is a major reason for drylands in some areas, since deep rooting of trees
has been replaced by shallow rooting of annual crops.
There are two vectors acting on the salt-infected water, the upward pull from
evaporation and capillary action and the downward force of infiltration. Whenever
the net flow is up, a saline soil will result. Any factor that increases downward infil-
tration in a recharge area or any practice that increases evaporation and decreases
downward percolation in a discharge area will increase the potential for having a
saline soil.
Since soil salinity makes it more difficult for plants to absorb soil moisture, these
salts must be leached out of the plant root zone by applying additional water. This, in
turn, can lead to rising water tables, requiring drainage to keep the saline groundwa-
ter out of the root zone. If the water table rises too high, then natural soil evaporation
will begin to draw the salts back upward into the soil profile. The problem is accel-
erated when too much water is added too quickly due to inefficient water use such
as over-irrigation, applying more than is required for leaching, using bad estimates
of evapotranspiration and poor system design, and is also greatly increased by poor
drainage and use of saline water for irrigating agricultural crops.