Soil salinity control
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Soil salinity control refers to controlling the process and progress of
The primary man-made cause of salinization is irrigation. River water or groundwater used in irrigation contains salts, which remain in the soil after the water has evaporated.
The primary method of controlling soil salinity is to permit 10–20% of the
Problems with soil salinity
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[1]
According to a study by UN University, about 62 million hectares (240 thousand square miles; 150 million acres), representing 20% of the world's irrigated lands are affected, up from 45 million ha (170 thousand sq mi; 110 million acres) in the early 1990s.[1] 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.[1]
Salty soils are a common feature and an
Worldwide, the major factor in the development of saline soils is a lack of
Primary cause
Man-made salinization is primarily caused by salt found in irrigation water. All irrigation water derived from rivers or groundwater, regardless of water purity, 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. With the absence of sufficient natural drainage (as in waterlogged soils), and proper leaching and drainage program to remove salts, this would lead to high soil salinity and reduced crop yields in the long run.
Much of the water used in irrigation has a higher salt content than 0.3 g/L, compounded by irrigation projects using a far greater annual supply of water.
Secondary cause
The secondary cause of salinization is
Most of the water lost this way is stored underground which can change the original
Waterlogging causes three problems:
- The shallow water table and lack of oxygenation of the root zonereduces 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.
Aquifer conditions in irrigated land and the groundwater flow have an important role in soil salinization,[3] as illustrated here:
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Soil salinization in the lower parts of undulating land with a good aquifer
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Soil salinization in the unirrigated parts of flat land with a good aquifer
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Soil salinization in irrigated flat land without an aquifer
-
Soil salinization in a coastal delta from irrigation higher up
Salt affected area
Normally, the salinization of
In India alone, thousands of square kilometers have been severely salinized. China and Pakistan do not lag far behind (perhaps China has even more salt affected land than India). A regional distribution of the 3,230,000 km2 of saline land worldwide is shown in the following table derived from the FAO/UNESCO Soil Map of the World.[4]
Region | Area (106ha) |
---|---|
Australia | 84.7 |
Africa | 69.5 |
Latin America | 59.4 |
Near and Middle East | 53.1 |
Europe | 20.7 |
Asia and Far East | 19.5 |
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
The preparation of salt and water balances[3] for distinguishable sub-areas in the irrigation project, or the use of agro-hydro-salinity models,[5] can be helpful in explaining or predicting the extent and severity of the problems.
Diagnosis
Measurement
Soil salinity is measured as the salt
The standard for the determination of soil salinity is from an extract of a saturated paste of the soil, and the EC is then written as ECe. The extract is obtained by centrifugation. The salinity can more easily be measured, without centrifugation, in a 2:1 or 5:1 water:soil mixture (in terms of g water per g dry soil) than from a saturated paste. The relation between ECe and EC2:1 is about 4, hence: ECe = 4EC1:2.[8]
Classification
Soils are considered saline when the ECe > 4.[9] When 4 < ECe < 8, the soil is called slightly saline, when 8 < ECe < 16 it is called (moderately) saline, and when ECe > 16 severely saline.
Crop tolerance
Sensitive crops lose their vigor already in slightly saline soils; most crops are negatively affected by (moderately) saline soils, and only salinity resistant crops thrive in severely saline soils. The University of Wyoming[10] and the Government of Alberta[11] report data on the salt tolerance of plants.
Principles of salinity control
Drainage is the primary method of controlling soil salinity. The system should permit a small fraction of the irrigation water (about 10 to 20 percent, the drainage or leaching fraction) to be drained and discharged out of the irrigation project.[12]
In irrigated areas where salinity is stable, the salt concentration of the drainage water is normally 5 to 10 times higher than that of the irrigation water. Salt export matches salt import and salt will not accumulate.
When reclaiming already salinized soils, the salt concentration of the drainage water will initially be much higher than that of the irrigation water (for example 50 times higher). Salt export will greatly exceed salt import, so that with the same drainage fraction a rapid desalinization occurs. After one or two years, the soil salinity is decreased so much, that the salinity of the drainage water has come down to a normal value and a new, favorable, equilibrium is reached.
In regions with pronounced dry and wet seasons, the drainage system may be operated in the wet season only, and closed during the dry season. This practice of checked or controlled drainage saves irrigation water.
The discharge of salty drainage water may pose environmental problems to downstream areas. The environmental hazards must be considered very carefully and, if necessary mitigating measures must be taken. If possible, the drainage must be limited to wet seasons only, when the salty effluent inflicts the least harm.
Drainage systems
Land drainage for soil salinity control is usually by horizontal drainage system (figure left), but vertical systems (figure right) are also employed.
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.[13]
Soil leaching
The
- Infiltration of rain and irrigation water (Irr) into the soil through the soil surface (Inf) :
- Inf = Rain + Irr
- Evaporation of soil water through plants and directly into the air through the soil surface (Evap)
- Percolation of water from the unsaturated zone soil into the groundwater through the watertable (Perc)
- Capillary rise of groundwatermoving by capillary suction forces into the unsaturated zone (Cap)
In
- 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
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
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)[12]
With this the irrigation and drainage requirements for salinity control can be computed too.
In irrigation projects in
The desired soil salinity level Cd depends on the crop tolerance to salt. The University of Wyoming,[10] US, and the Government of Alberta,[11] Canada, report crop tolerance data.
Strip cropping: an alternative
In irrigated lands with scarce water resources suffering from drainage (high water table) and soil salinity problems,
Owing to the water application in the irrigated strips they have a higher
In the unirrigated (sacrificial) strips the soil is dry and the groundwater comes up by
Soil salinity models
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The majority of the computer models available for water and solute transport in the soil (e.g. SWAP,[17] DrainMod-S,[18] UnSatChem,[19] and Hydrus[20]) 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, dispersity, 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, database 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,[5] 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,[21][22] using the SaltMod principles, helps in analyzing leaching experiments in which the soil salinity was monitored in various root zone layers while the model will optimize the value of the leaching efficiency of each layer so that a fit is obtained of observed with simulated soil salinity values.
Spatial variations owing to variations in topography can be simulated and predicted using salinity cum groundwater models, like SahysMod.
See also
- Alkali soils– Soil type with pH > 8.5
- Biosalinity – Use of salty water for irrigation
- Crop tolerance to seawater – Crop tolerance to seawater is the ability of an agricultural crop to withstand the high salinity induced by irrigation with seawater.
- Desalination – Removal of salts from water
- Halophyte – Salt-tolerant plant
- Halotolerance – Adaptation to high salinity
- Salt tolerance of crops
- Sodium in biology – Use of Sodium by organisms
References
- ^ a b c "World losing 2,000 hectares of farm soil daily to salt damage".
- ^ I.P. Abrol, J.S.P Yadav, and F. Massoud 1988. Salt affected soils and their management, Food and Agricultural Organization of the United Nations (FAO), Soils Bulletin 39.
- ^ a b c ILRI, 2003. Drainage for Agriculture: Drainage and hydrology/salinity - water and salt balances. Lecture notes International Course on Land Drainage, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. Download from page : [1], or directly as PDF : [2]
- ^ R.Brinkman, 1980. Saline and sodic soils. In: Land reclamation and water management, p. 62-68. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands.
- ^ a b SaltMod: a tool for interweaving of irrigation and drainage for salinity control. In: W.B.Snellen (ed.), Towards integration of irrigation, and drainage management. ILRI Special report, p. 41-43. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands.
- ^ H.J. Nijland and S. El Guindy, Crop yields, watertable depth and soil salinity in the Nile Delta, Egypt. In: Annual report 1983. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands.
- ^ On line collection of salt tolerance data of agricultural crops from measurements in farmers' fields [3]
- ^ ILRI, 2003, This paper discusses soil salinity. Lecture notes, International Course on Land Drainage International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. On line: [4]
- ^ L.A.Richards (Ed.), 1954. Diagnosis and improvement of saline and alkali soils. USDA Agricultural Handbook 60. On internet
- ^ a b Alan D. Blaylock, 1994, Soil Salinity and Salt tolerance of Horticultural and Landscape Plants. [5]
- ^ a b Government of Alberta, Salt tolerance of Plants
- ^ ISBN 90-70754-33-9.
- ISBN 90-70754-33-9. On line : [6]
- ^ R.J.Oosterbaan and M.A.Senna, 1990. Using SaltMod to predict drainage and salinity control in the Nile delta. In: Annual Report 1989, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands, p. 63-74. See Case Study Egypt in the SaltMod manual : [7]
- ^ E.A. Vanegas Chacon, 1990. Using SaltMod to predict desalinization in the Leziria Grande Polder, Portugal. Thesis. Wageningen Agricultural University, The Netherlands
- ^ ILRI, 2000. Irrigation, groundwater, drainage and soil salinity control in the alluvial fan of Garmsar. Consultancy assignment to the Food and Agriculture Organization (FAO) of the United Nations, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. On line: [8]
- ^ SWAP model
- ^ DrainMod-S model Archived 2008-10-25 at the Wayback Machine
- ^ UnSatChem model
- ^ Hydrus model
- ^ LeachMod
- ^ Reclamation of a Coastal Saline Vertisol by Irrigated Rice Cropping, Interpretation of the data with a Salt Leaching Model. In: International Journal of Environmental Science, April 2019. On line: [9]
External links
- Food and Agriculture Organization of the United Nations on soil salinity
- US Salinity Laboratory at Riverside, California