Geoscience Reference
In-Depth Information
discontinued for whatever reason, a further set of ge-
omorphological changes can be expected. Lesschen,
Cammeraat and Nieman (2008) highlight this issue in the
Mediterranean basin and note how abandoned fields in
semi-arid areas are more vulnerable to gully erosion due
to the formation of soil crusts with low infiltration rates
and enhanced runoff. Their work in southeastern Spain
demonstrates the susceptibility to greater erosion of aban-
doned terraces that deteriorate due to lack of maintenance.
soil's suitability as a growing medium. Plant growth is
also directly impaired by salinisation through its effects
on osmotic pressures and via direct toxicity. Contact be-
tween a solution containing large amounts of dissolved
salts and a plant cell causes the cell's protoplasmic lin-
ing to shrink due to the osmotic movement of water from
the cell to the more concentrated soil solution. As a re-
sult the cell collapses and the plant succumbs. High salt
concentrations are also toxic to many plants. The toxicity
effect varies according to the type of salt and the species
of plant, but is especially potent during the seedling stage
of all plants. Indeed, some salts, such as boron, are highly
toxic to many crops when present in a soil solution at
concentrations of only a few parts per million (Bingham,
Rhoades and Keren, 1985).
The salt tolerance of a plant is not an exact value since
it depends on factors such as soil fertility, the salt distribu-
tion in the soil profile, irrigation methods and climate, as
well as biological factors including the stage of growth,
plant variety and rootstock. Nonetheless, different crops
are susceptible to different concentrations and forms of
salinity (Maas, 1990). Any given crop has a threshold
limit of tolerance to soil salinity beyond which yields will
decrease linearly per unit increase in salinity. Table 22.2
22.2.2
Irrigated agriculture
The practice of irrigated agriculture also has a long his-
tory, particularly on some of the major rivers that flow
through the deserts of North Africa, the Middle East and
Southwest Asia. Irrigation can affect geomorphology in
a number of ways, both directly and indirectly. The ex-
cessive use of groundwater, for example, can lead to land
subsidence and declines in streamflow, as well as the re-
duction or complete loss of vegetation, with a range of
geomorphological implications, as Zektser, Loaiciga and
Wolf (2005) outline using case studies from aquifers in
the southwestern USA. Soil piping may also occur on ir-
rigated fields, resulting in the loss of much water and soil
through the pipe networks (e.g. Garcıa-Ruiz and Lasanta,
1995). Perhaps the most widespread and most serious geo-
morphological impact of irrigated agriculture arises when
previously productive soil becomes saline as a result of
poor land management, so-called
secondary
salinisation.
Soil salinity problems affected farmers in Mesopotamia
4000 years ago (Jacobsen and Adams, 1958) and con-
tinue to be particularly prevalent in, although not solely
confined to, drylands today. The issue receives much at-
tention largely because major agricultural crops have a
low salt tolerance. Secondary salinisation is most com-
monly associated with poorly managed irrigation schemes
and occurs in four main ways: water leakage from sup-
ply canals, overapplication of water, poor drainage and
insufficient application of water to leach salts away. One
estimate suggested that nearly 50 % of all the irrigated
land in arid and semi-arid regions is affected to some
extent by secondary salinisation (Abrol, Yadav and Mas-
soud, 1988) and the process is widely regarded as irrigated
agriculture's most significant environmental problem
(Ghassemi, Jakeman and Nix, 1995; Gardner, 1997; Mid-
dleton and van Lynden, 2000).
The impact of salinisation on crop yields acts indi-
rectly via effects on the soil and through direct effects
on the plants themselves (Rhoades, 1990; Tanji, 1990).
Salt accumulation reduces soil pore space and the capa-
bility of holding soil air, moisture and nutrients, resulting
Table 22.2
Salt tolerance of selected crops with respect to
the electrical conductivity of the saturated-soil extract.
Yield decline (%)
per 1000
µ
S/cm
increase in salinity
above threshold
Threshold
(
µ
S/cm)
Crop
Sensitive
Apricot
1600
24.0
Carrot
1000
14.0
Grapefruit
1800
16.0
Onion
1200
16.0
Moderately sensitive
Alfalfa
2000
7.3
Cucumber
2500
13.0
Sugarcane
1700
5.9
Tomato
2500
9.9
Moderately tolerant
Barley (forage)
6000
7.1
Sorghum
6800
16.0
Soybean
5000
20.0
Tolerant
Barley (grain)
8000
5.0
Cotton
7700
5.2
Sugar beet
7000
5.9
