1 The soil salinity problem
1.1 Primary cause
1.2 Secondary cause
2.1 Measurement 2.2 Classification 2.3 Crop tolerance
The soil salinity problem
Salty (saline) soils are soils that have a high salt content. The
predominant salt is normally sodium chloride (NaCl, "table salt").
This damage is an average of 2,000 hectares of irrigated land in arid and semi-arid areas daily for more than 20 years across 75 countries (each week the world loses an area larger than Manhattan)...To feed the world's anticipated nine billion people by 2050, and with little new productive land available, it's a case of all lands needed on deck.—principal author Manzoor Qadir, Assistant Director, Water and Human Development, at UN University's Canadian-based Institute for Water, Environment and Health
According to a study by UN University, about 62 million hectares (20%) of the world's irrigated lands are affected, up from 45 million hectares in the early 1990s. In the Indo-Gangetic Plain, home to over 10% of the world's population, crop yield losses for wheat, rice, sugarcane and cotton grown on salt-affected lands could be 40%, 45%, 48%, and 63%, respectively. Salty soils are a common feature and an environmental problem in irrigated lands in arid and semi-arid regions, resulting in poor or little crop production. The problems are often associated with high water tables, caused by a lack of natural subsurface drainage to the underground. Poor subsurface drainage may be caused by insufficient transport capacity of the aquifer or because water cannot exit the aquifer, for instance if the aquifer is situated in a topographical depression. Worldwide, the major factor in the development of saline soils is a lack of precipitation. Most naturally saline soils are found in (semi)arid regions and climates of the earth. Primary cause
Irrigated saline land with poor crop stand
The primary cause of man-made salinization is the salt brought in with
irrigation water. All irrigation water derived from rivers or
groundwater, however 'sweet', contains salts that remain behind in the
soil after the water has evaporated.
For example, assuming irrigation water with a low salt concentration
of 0.3 g/l (equal to 0.3 kg/m³ corresponding to an electric
conductivity of about 0.5 FdS/m) and a modest annual supply of
irrigation water of 10,000 m³/ha (almost 3 mm/day) brings
3,000 kg salt/ha each year. In the absence of sufficient natural
drainage (as in waterlogged soils) and without a proper leaching and
drainage program to remove salts, this would lead to a high soil
salinity and reduced crop yields in the long run.
Much of the water used in irrigation has a higher salt content than in
this example, which is compounded by the fact that many irrigation
projects use a far greater annual supply of water. Sugar cane, for
example, needs about 20,000 m3/ha of water per year. As a result,
irrigated areas often receive more than 3,000 kg/ha of salt per
year and some receive as much as 10,000 kg/ha/year.
The secondary cause of salinization is waterlogging in irrigated land.
The shallow water table and lack of oxygenation of the root zone reduces the yield of most crops It leads to an accumulation of salts brought in with the irrigation water as their removal through the aquifer is blocked With the upward seepage of groundwater more salts are brought into the soil and the salination is aggravated
Illustration of the influence of aquifer conditions on soil salinization in irrigated land
Region Area (106ha)
Latin America 59.4
Near and Middle East 53.1
Northern America 16.0
Spatial variation Although the principles of the processes of salinization are fairly easy to understand, it is more difficult to explain why certain parts of the land suffer from the problems and other parts do not, or to predict accurately which part of the land will fall victim. The main reason for this is the variation of natural conditions in time and space, the usually uneven distribution of the irrigation water, and the seasonal or yearly changes of agricultural practices. Only in lands with undulating topography is the prediction simple: the depressional areas will degrade the most. The preparation of salt and water balances for distinguishable sub-areas in the irrigation project, or the use of agro-hydro-salinity models, can be helpful in explaining or predicting the extent and severity of the problems. Diagnosis
The maize crop (corn) in Egypt has a salt tolerance of ECe=5.5 dS/m beyond which the yield declines 
The rice crop in Egypt has a similar salt tolerance as maize. 
Parameters of a horizontal drainage system
Parameters of a vertical drainage system
Land drainage for soil salinity control is usually by horizontal
drainage system (figure left), but vertical systems (figure right) are
The drainage system designed to evacuate salty water also lowers the
water table. To reduce the cost of the system, the lowering must be
reduced to a minimum. The highest permissible level of the water table
(or the shallowest permissible depth) depends on the irrigation and
agricultural practices and kind of crops.
In many cases a seasonal average water table depth of 0.6 to 0.8 m is
deep enough. This means that the water table may occasionally be less
than 0.6 m (say 0.2 m just after an irrigation or a rain storm). This
automatically implies that, in other occasions, the water table will
be deeper than 0.8 m (say 1.2 m). The fluctuation of the water table
helps in the breathing function of the soil while the expulsion of
carbon dioxide (CO2) produced by the plant roots and the inhalation of
fresh oxygen (O2) is promoted.
The establishing of a not too deep water table offers the additional
advantage that excessive field irrigation is discouraged, as the crop
yield would be negatively affected by the resulting elevated water
table, and irrigation water may be saved.
The statements made above on the optimum depth of the water table are
very general, because in some instances the required water table may
be still shallower than indicated (for example in rice paddies), while
in other instances it must be considerably deeper (for example in some
orchards). The establishment of the optimum depth of the water table
is in the realm of agricultural drainage criteria.
Water balance factors in the soil
The vadose zone of the soil below the soil surface and the watertable is subject to four main hydrological inflow and outflow factors:
Infiltration of rain and irrigation water (Irr) into the soil through
the soil surface (Inf) :
Inf = Rain + Irr
In steady state (i.e. the amount of water stored in the unsaturated zone does not change in the long run) the water balance of the unsaturated zone reads: Inflow = Outflow, thus:
Inf + Cap = Evap + Perc or : Irr + Rain + Cap = Evap + Perc
and the salt balance is
Irr.Ci + Cap.Cc = Evap.Fc.Ce + Perc.Cp + Ss
where Ci is the salt concentration of the irrigation water, Cc is the salt concentration of the capillary rise, equal to the salt concentration of the upper part of the groundwater body, Fc is the fraction of the total evaporation transpired by plants, Ce is the salt concentration of the water taken up by the plant roots, Cp is the salt concentration of the percolation water, and Ss is the increase of salt storage in the unsaturated soil. This assumes that the rainfall contains no salts. Only along the coast this may not be true. Further it is assumed that no runoff or surface drainage occurs. The amount of removed by plants (Evap.Fc.Ce) is usually negligibly small: Evap.Fc.Ce = 0
Leaching curves, calibrating leaching efficiency
The salt concentration Cp can be taken as a part of the salt concentration of the soil in the unsaturated zone (Cu) giving: Cp=Le.Cu, where Le is the leaching efficiency. The leaching efficiency is often in the order of 0.7 to 0.8, but in poorly structured, heavy clay soils it may be less. In the Leziria Grande polder in the delta of the Tagus river in Portugal it was found that the leaching efficiency was only 0.15. Assuming that one wishes to avoid the soil salinity to increase and maintain the soil salinity Cu at a desired level Cd we have: Ss = 0, Cu = Cd and Cp = Le.Cd. Hence the salt balance can be simplified to:
Perc.Le.Cd = Irr.Ci + Cap.Cc
Setting the amount percolation water required to fulfill this salt balance equal to Lr (the leaching requirement) it is found that:
Lr = (Irr.Ci + Cap.Cc) / Le.Cd .
Substituting herein Irr = Evap + Perc − Rain − Cap and re-arranging gives :
Lr = [ (Evap−Rain).Ci + Cap(Cc−Ci) ] / (Le.Cd − Ci) 
With this the irrigation and drainage requirements for salinity control can be computed too. In irrigation projects in (semi)arid zones and climates it is important to check the leaching requirement, whereby the field irrigation efficiency (indicating the fraction of irrigation water percolating to the underground) is to be taken into account. The desired soil salinity level Cd depends on the crop tolerance to salt. The University of Wyoming, USA, and the Government of Alberta, Canada, report crop tolerance data. Strip cropping: an alternative
Hydrological principles of strip cropping to control the depth of the water table and the soil salinity
In irrigated lands with scarce water resources suffering from drainage
(high water table) and soil salinity problems, strip cropping is
sometimes practiced with strips of land where every other strip is
irrigated while the strips in between are left permanently fallow.
Owing to the water application in the irrigated strips they have a
higher watertable which induces flow of groundwater to the unirrigated
strips. This flow functions as subsurface drainage for the irrigated
strips, whereby the water table is maintained at a not-too-shallow
depth, leaching of the soil is possible, and the soil salinity can be
controlled at an acceptably low level.
In the unirrigated (sacrificial) strips the soil is dry and the
groundwater comes up by capillary rise and evaporates leaving the
salts behind, so that here the soil salinizes. Nevertheless, they can
have some use for livestock, sowing salinity resistant grasses or
weeds. Moreover, useful salt resistant trees can be planted like
This section needs expansion. You can help by adding to it. (October 2007)
The majority of the computer models available for water and solute
transport in the soil (e.g. SWAP, DrainMod-S, UnSatChem,
and Hydrus  ) are based on Richard's differential equation for the
movement of water in unsaturated soil in combination with Fick's
differential convection–diffusion equation for advection and
dispersion of salts.
The models require input of soil characteristics like the relations
between variable unsaturated soil moisture content, water tension,
water retention curve, unsaturated hydraulic conductivity,
dispersivity and diffusivity. These relations vary to a great extent
from place to place and from time to time and are not easy to measure.
Further, the models are difficult to calibrate under farmer's field
conditions because the soil salinity here is spatially very variable.
The models use short time steps and need at least a daily, if not an
hourly, data base of hydrological phenomena. Altogether this makes
model application to a fairly large project the job of a team of
specialists with ample facilities.
Simpler models, like SaltMod, based on monthly or seasonal water
and soil balances and an empirical capillary rise function, are also
available. They are useful for long-term salinity predictions in
relation to irrigation and drainage practices.
LeachMod, using the
^ a b c
^ I.P. Abrol, J.S.P Yadav, and F. Massoud 1988.
Food and Agriculture Organization of the United Nations on soil
US Salinity Laboratory at Riverside, California
Website on soil salinity and waterlogging :  Articles on soil salinity and waterlogging :  Frequently asked questions on soil salinity and waterlogging :  Reports and case studies on soil salinity and waterlogging :  Free software on soil salinity and waterlogging : 
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