Geoscience Reference
In-Depth Information
3.3.1 Agricultural Impacts
Agriculture activities are one of the major generators of salinity but also one of the
first to suffer its damage. The salinity of agricultural return flow is derived from
two principal sources: salinity of the irrigation water and the effects of added
agrochemicals (e.g., fertilizers, boron compounds) or animal waste. Drainage
water is sometimes saline and may be accompanied by elevated concentrations of
selenium, boron, arsenic, and mercury (Hern and Feltz
1998
; Beltran
1999
;
Causape et al.
2004
; Farber et al.
2004
).
Land use changes from natural vegetation to agricultural crops, and application
of irrigation water, add salts to the system (Cao et al.
2004
; Vaze et al.
2004
; Peck
and Hatton
2003
). Approximately 60 % of supplied irrigation water is consumed
by growing crops, but the salts remain in the residual solution, as they are not
consumed by evaporation and transpiration (Vengosh
2003
). As a consequence,
adequate drainage is a key factor that determines soil salinization: in arid and
semiarid areas, the natural salinity of soil is high; therefore, flushing with irrigation
water enhances the dissolution of stored salts.
Salinity contributes to significant losses of productivity in agricultural land and
may take some land entirely out of production. Plant roots absorb water from the soil
through the process of osmosis. Osmosis moves water from areas of lower salt
concentration to areas of higher concentration. Once the salt concentration gradient
is reduced, transport of water to plants is slowed; and if extreme conditions of
salinity prevail, the osmosis may stop or even change direction, causing plant
dehydration.
When high salinity is associated with high sodium content, exchangeable sodium
may replace exchangeable calcium on the soil clays. The increased level of adsorbed
Na
+
causes soil to become dispersed, with significantly reduced porosity and per-
meability (Yaalon and Yaron
1966
; Panayiotopoulos et al.
2004
; Bethune and Batey
2002
). As a result of this interaction the soil becomes impermeable. Breakdown in
soil structure, together with the associated loss of plant cover, results in greater
exposure of the soil to erosion. Sheet, rill, gully, and wind erosion are common
effects of salinity (e.g., Funakawa et al.
2000
; Spoor
1998
). This also is a major
problem for drainage of soil water and salt flush in the partially saturated zone.
Land degradation by salinization, which decreases soil fertility, is a significant
component of desertification processes. The World Bank states that soil salinization
caused by inappropriate irrigation practices affects *60 Mha, or 24 % of all irri-
gated land worldwide. In Africa, salinization accounts for 50 % of irrigated land
(Vengosh
2003
). Increasing soil salinization also is occurring in the Middle East,
Australia, China, America, and central Asia (Kang et al.
2004
; Dehaan and Taylor
2002
; Kotb et al.
2000
; Salama et al.
1999
; Spoor
1998
; Hern and Feltz
1998
).
Soil salinization is the first stage of environmental destruction caused by
salinity and is related to river and lake salinization. For example, the diversion of
the Amu Darya and Syr Darya Rivers caused significant desiccation of the Aral
Sea, but it also caused salinization of associated agricultural land (e.g., Funakawa
