Soil salinity control

{{Short description|Controlling the problem of soil salinity}} {{redirect|Salinity control|removing salt from water|Desalination}} {{Tone|date=February 2020}} [[File:MUSTARD.JPG|thumb|250px|Yield of mustard ([[colza]]) and soil salinity]] '''Soil salinity control''' refers to controlling the process and progress of [[soil salinity]] to prevent [[soil degradation]] by [[salination]] and [[land reclamation|reclamation]] of already salty (saline) soils. Soil reclamation is also known as soil improvement, rehabilitation, [[Remediation of contaminated sites with cement|remediation]], recuperation, or amelioration.

The primary man-made cause of [[Soil salinity|salinization]] is [[irrigation]]. [[river|River water]] or [[groundwater]] used in irrigation contains salts, which remain in the soil after the water has [[evaporation|evaporated]].

The primary method of controlling soil salinity is to permit 10–20% of the [[irrigation]] water to [[Leaching model|leach]] the soil, so that it will be drained and discharged through an appropriate [[Drainage system (agriculture)|drainage system]]. The salt concentration of the [[Watertable control|drainage water]] is normally 5 to 10 times higher than that of the irrigation water which meant that salt export will more closely match salt import and it will not accumulate.

==Problems with soil salinity==

[[soil salinity|Salty (saline) soils]] have high [[salt]] content. The predominant salt is normally [[sodium chloride]] (NaCl, "table salt"). [[Saline soils]] are therefore also ''sodic soils'' but there may be sodic soils that are not saline, but [[alkaline soils|alkaline]].

{{quotebox|title=World Soil Salt Degradation|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''<ref name="physorg102014">{{Cite web|url=http://phys.org/news/2014-10-world-hectares-farm-soil-daily.html|title = World losing 2,000 hectares of farm soil daily to salt damage}}</ref> }} {{clear}}

According to a study by [[United Nations University|UN University]], about {{convert|62|e6ha|e3sqmi e6acre|abbr=none}}, representing 20% of the world's irrigated lands are affected, up from {{convert|45|e6ha|e3sqmi e6acre|abbr=unit}} in the early 1990s.<ref name="physorg102014"/> In the [[Indo-Gangetic Plain]], home to over 10% of the [[World population|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.<ref name="physorg102014"/>

Salty soils are a common feature and an [[Environmental impact of irrigation|environmental problem]] in [[irrigated land]]s in [[arid]] and [[semi-arid]] regions, resulting in poor or little crop production.<ref>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.</ref> The causes of salty soils are often associated with high [[water table]]s, which are 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 [[arid|(semi) arid regions]] and [[semi arid|climates]] of the earth.

===Primary cause=== [[File:Soil Salinity2.jpg|thumb|250px|Irrigated saline land with poor crop stand]] 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 yield]]s 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. [[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, with some receiving as much as 10,000 kg/ha/year.

===Secondary cause=== The secondary cause of salinization is [[Waterlogging (agriculture)|waterlogging]] in irrigated land. Irrigation causes changes to the natural [[Hydrology (agriculture)|water balance]] of irrigated lands. Large quantities of water in irrigation projects are not consumed by plants and must go somewhere. In irrigation projects, it is impossible to achieve 100% irrigation efficiency where all the irrigation water is consumed by the plants. The maximum attainable irrigation efficiency is about 70%, but usually, it is less than 60%. This means that minimum 30%, but usually more than 40% of the irrigation water is not evaporated and it must go somewhere.

Most of the water lost this way is stored underground which can change the original [[geohydrology|hydrology]] of [[aquifer|local aquifers]] considerably. Many aquifers cannot absorb and transport these quantities of water, and so the [[water table]] rises leading to waterlogging.

Waterlogging causes three problems: * The shallow water table and lack of [[Oxygenation (environmental)|oxygenation]] of the [[Root|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.

Aquifer conditions in irrigated land and the groundwater flow have an important role in soil salinization,<ref name="Balances">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 : [http://www.waterlog.info/faqs.htm], or directly as PDF : [http://www.waterlog.info/pdf/balances.pdf]</ref> as illustrated here:

<gallery caption="Illustration of the influence of aquifer conditions on soil salinization in irrigated land" perrow="2" widths="200" heights="100"> File:Salinization1.PNG|Soil salinization in the lower parts of undulating land with a good aquifer File:Salinization2.PNG|Soil salinization in the unirrigated parts of flat land with a good aquifer File:Salinization3.PNG|Soil salinization in irrigated flat land without an aquifer File:Salinization4.gif|Soil salinization in a coastal delta from irrigation higher up </gallery>

===Salt affected area=== Normally, the salinization of [[agricultural land]] affects a considerable area of 20% to 30% in irrigation projects. When the agriculture in such a fraction of the land is abandoned, a new salt and [[Hydrology (agriculture)|water balance]] is attained, a new equilibrium is reached and the situation becomes stable.

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 [[Food and Agriculture Organization|FAO]]/[[UNESCO]] Soil Map of the World.<ref>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.</ref>

{| class="wikitable" ! Region || Area (106ha)  |- | [[Australia (continent)|Australia]] || 84.7 |- | [[Africa]] || 69.5 |- | [[Latin America]] || 59.4 |- | [[Near East|Near]] and [[Middle East]] || 53.1 |- | [[Europe]] || 20.7 |- | [[Asia]] and [[Far East]] || 19.5 |- | [[Northern America]] || 16.0 |}

[[File:GOHANA2.jpg|thumb|250px|Spatial variation of soil salinity]]

===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<ref name="Balances" /> for distinguishable sub-areas in the [[irrigation]] project, or the use of agro-hydro-salinity models,<ref name="Snellen">[http://www.waterlog.info/pdf/toolsalt.pdf 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.</ref> can be helpful in explaining or predicting the extent and severity of the problems.

==Diagnosis== [[File:maize egypt.png|thumb|250px|The maize crop (corn) in Egypt has a salt tolerance of ECe=5.5 dS/m beyond which the yield declines.<ref>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.</ref>]] [[File:rice egypt.png|thumb|250px|The rice crop in Egypt has a similar salt tolerance as maize.<ref>On line collection of salt tolerance data of agricultural crops from measurements in farmers' fields [https://www.waterlog.info/croptol.htm]</ref> ]]

===Measurement=== Soil salinity is measured as the salt [[concentration]] of the soil [[Solution (chemistry)|solution]] in tems of g/L or [[electric conductivity]] (EC) in [[Siemens (unit)|dS/m]]. The relation between these two units is about 5/3: y g/L => 5y/3 dS/m. [[Seawater]] may have a salt concentration of 30 g/L (3%) and an EC of 50 dS/m.

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.<ref>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: [http://www.waterlog.info/pdf/salinity.pdf]</ref>

===Classification=== Soils are considered saline when the ECe > 4.<ref>L.A.Richards (Ed.), 1954. Diagnosis and improvement of saline and alkali soils. USDA Agricultural Handbook 60. [https://www.ars.usda.gov/pacific-west-area/riverside-ca/agricultural-water-efficiency-and-salinity-research-unit/docs/publications/handbook-no-60/ On internet]</ref> 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]]<ref name="Alan">Alan D. Blaylock, 1994, ''Soil Salinity and Salt tolerance of Horticultural and Landscape Plants. [http://www.wyomingextension.org/agpubs/pubs/WY988.PDF]''</ref> and the [[Government of Alberta]]<ref name="Albert">Government of Alberta, [http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex3303 ''Salt tolerance of Plants'']</ref> report data on the [[Halotolerance|salt tolerance]] of plants.

==Principles of salinity control== [[Drainage system (agriculture)|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.<ref name="Hoorn">J.W. van Hoorn and J.G. van Alphen (2006), Salinity control. In: H.P. Ritzema (Ed.), Drainage Principles and Applications, p. 533-600, Publication 16, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. {{ISBN|90-70754-33-9}}.</ref>

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 season|dry]] and [[wet season]]s, 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.

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]] [[root]]s 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 [[orchard]]s). The establishment of the optimum depth of the water table is in the realm of [[Watertable control|agricultural drainage criteria]].<ref>''Agricultural Drainage Criteria'', Chapter 17 in: H.P.Ritzema (2006), Drainage Principles and Applications, Publication 16, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. {{ISBN|90-70754-33-9}}. On line : [http://www.waterlog.info/pdf/chap17.pdf]</ref>

==Soil leaching== [[File:SoilLeaching2.jpg|thumb|250px|Water balance factors in the soil]] The [[vadose zone]] of the [[soil]] below the soil surface and the [[watertable|water table]] is subject to four main [[hydrology|hydrological]] inflow and outflow factors:<ref name="Balances" /> *[[Infiltration (hydrology)|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 [[groundwater]] moving by capillary suction forces into the unsaturated zone (Cap) In [[steady state]] (i.e. the amount of water stored in the unsaturated zone does not change in the long run) the [[Hydrology (agriculture)|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 [[Surface runoff|runoff]] or surface drainage occurs. The amount of removed by plants (Evap.Fc.Ce) is usually negligibly small: Evap.Fc.Ce = 0 [[File:SALTMOD4.JPG|thumb|250px|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 model|leaching efficiency]]. The leaching efficiency is often in the order of 0.7 to 0.8,<ref>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 : [http://www.waterlog.info/pdf/saltmod.pdf]</ref> but in poorly [[soil structure|structured]], heavy [[clay]] soils it may be less. In the Leziria Grande [[polder]] in the delta of the [[Tagus|Tagus river]] in [[Portugal]] it was found that the leaching efficiency was only 0.15.<ref>E.A. Vanegas Chacon, 1990. Using SaltMod to predict desalinization in the Leziria Grande Polder, Portugal. Thesis. Wageningen Agricultural University, The Netherlands</ref> 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)<ref name="Hoorn" /> With this the irrigation and drainage requirements for salinity control can be computed too. In irrigation projects in [[arid|(semi)arid zones]] and [[semi arid|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,<ref name="Alan"/> US, and the Government of Alberta,<ref name= "Albert" /> Canada, report crop tolerance data.

==Strip cropping: an alternative== [[File:StripCropping.jpg|thumb|250px|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]].<ref>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. Online: [http://www.waterlog.info/pdf/garmsar.pdf]</ref>

Owing to the water application in the irrigated strips they have a higher [[watertable|water table]] which induces [[Groundwater|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 (agriculture)|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 [[weed]]s. Moreover, useful salt resistant trees can be planted like [[Casuarina]], [[Eucalyptus]], or [[Atriplex]], keeping in mind that the trees have deep rooting systems and the salinity of the wet [[subsoil]] is less than of the [[topsoil]]. In these ways [[wind erosion]] can be controlled. The unirrigated strips can also be used for [[salt]] harvesting.{{Needs citation|date=May 2023}}

==Soil salinity models== {{Expand section|date=October 2007}} [[File:File-Saltmod8 (2).JPG|thumb|250px|SaltMod components]] The majority of the computer models available for water and solute transport in the soil (e.g. SWAP,<ref>[http://www.swap.alterra.nl/ SWAP model]</ref> DrainMod-S,<ref>[http://www.bae.ncsu.edu/soil_water/drainmod/models.html DrainMod-S model] {{webarchive|url=https://web.archive.org/web/20081025223837/http://www.bae.ncsu.edu/soil_water/drainmod/models.html |date=2008-10-25 }}</ref> UnSatChem,<ref>[https://www.ars.usda.gov/pacific-west-area/riverside-ca/agricultural-water-efficiency-and-salinity-research-unit/docs/model/unsatchem-model/ UnSatChem model]</ref> and [[Hydrus (software)|Hydrus]]<ref>[http://www.pc-progress.com/en/default.aspx?hydrus-3d Hydrus model]</ref>) 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 [[Fick's laws of diffusion|dispersion]] of salts.

The models require the input of soil characteristics like the relations between variable unsaturated [[soil moisture]] content, water tension, [[water retention curve]], unsaturated [[hydraulic conductivity]], [[dispersity]], and [[Mass diffusivity|diffusivity]]. These relations vary greatly from place to place and time to time and are not easy to measure. Further, the models are complicated to [[calibration|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 hourly, database of [[hydrology|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]],<ref name="Snellen" /> 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,<ref>[http://www.waterlog.info/leachmod.htm LeachMod]</ref><ref>''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: [https://www.iaras.org/iaras/filedownloads/ijes/2019/008-0006(2019).pdf]</ref> 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 model]]s, like [[SahysMod]].

== See also ==

* {{annotated link|Alkali soils}} * {{annotated link|Biosalinity}} * {{annotated link|Crop tolerance to seawater}} * {{annotated link|Desalination}} * {{annotated link|Halophyte}} * {{annotated link|Halotolerance}} * {{annotated link|Salt tolerance of crops}} * {{annotated link|Sodium in biology}}

==References== {{Reflist|2}}

==External links== * [https://www.fao.org/3/R4082E/r4082e08.htm Food and Agriculture Organization of the United Nations] on soil salinity * [https://www.ars.usda.gov/pacific-west-area/riverside-ca/agricultural-water-efficiency-and-salinity-research-unit/ US Salinity Laboratory] at Riverside, California

{{Agricultural water management}}{{Soil science topics}}{{DEFAULTSORT:Soil Salinity Control}} [[Category:Soil]] [[Category:Soil science]] [[Category:Environmental soil science]] [[Category:Agricultural soil science]]